
==== Front
J Anim Sci
J Anim Sci
jansci
Journal of Animal Science
0021-8812
1525-3163
Oxford University Press US

38995118
10.1093/jas/skae191
skae191
Animal Health and Well Being
AcademicSubjects/SCI00960
Evaluation of the capacity and effective angle of thalamic damage for 2 commercially available captive bolt tool types on cadaver heads from sows >200 kg body weight
https://orcid.org/0000-0003-4997-997X
Anderson Karly N Department of Animal Science, University of Nebraska – Lincoln, Lincoln, NE 68583, USA

Woiwode Ruth Department of Animal Science, University of Nebraska – Lincoln, Lincoln, NE 68583, USA

https://orcid.org/0000-0002-1623-8959
Kirk Ashlynn A Department of Animal and Food Science, University of Wisconsin – River Falls, River Falls, WI 54022, USA

Berger Jennifer Abbyland Pork Pack, Curtiss, WI 54422, USA

Reyes Arquimides A Department of Animal and Food Science, University of Wisconsin – River Falls, River Falls, WI 54022, USA

https://orcid.org/0000-0002-7145-0960
Zhitnitskiy Perle E Department of Veterinary Population Medicine, College of Veterinary Medicine, University of Minnesota, St. Paul, MN 55108, USA

https://orcid.org/0000-0001-8712-7298
Vogel Kurt D Department of Animal and Food Science, University of Wisconsin – River Falls, River Falls, WI 54022, USA

Corresponding author: kurt.vogel@uwrf.edu
2024
12 7 2024
12 7 2024
102 skae19120 4 2024
11 7 2024
05 9 2024
© The Author(s) 2024. Published by Oxford University Press on behalf of the American Society of Animal Science.
2024
https://creativecommons.org/licenses/by-nc/4.0/ This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial License (https://creativecommons.org/licenses/by-nc/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site—for further information please contact journals.permissions@oup.com.

Abstract

This study evaluated the ability of 2 penetrating captive bolt (PCB) types (PISTOL, INLINE) to reach and disrupt the thalamus when applied in 2 placements (FRONTAL, BEHIND EAR) to chilled cadaver heads (N = 60) from sows >200 kg. Heads were randomly distributed across 6 treatments (n = 10): FRONTAL-INLINE, FRONTAL-PISTOL, FRONTAL-NO SHOT, BEHIND EAR-INLINE, BEHIND EAR-PISTOL, and BEHIND EAR-NO SHOT. The FRONTAL shot was placed 3.5 cm superior to the optic orbits at the midline; the BEHIND EAR shot was placed directly caudal to the pinna of the ear on the same plane as the eyes and targeting the middle of the opposite eye. For INLINE treatments, a Jarvis PAS—Type C 0.25R Super Heavy Duty PCB with a Long Bolt and 6.0 GR power loads was used. For PISTOL treatments, a Jarvis PAS—Type P 0.25R Pistol PCB with a Long Stunning Rod Nosepiece Assembly and 3.5 GR power loads was used. Heads were split along the bolt with a band saw. Tissue depth measurements are reported as Mean ± SE followed by 97.5% one-sided upper reference limit (URL). Total tissue thickness was less (P < 0.0001) at the FRONTAL (56.31 ± 1.76 mm; URL: 73.17 mm) than the BEHIND EAR placement (95.52 ± 3.30 mm; URL: 126.53 mm). Thalamic depth was less (P < 0.0001) at the FRONTAL (78.31 ± 1.32 mm; URL: 88.19 mm) than the BEHIND EAR placement (111.86 ± 3.22 mm; URL: 135.99 mm). The effective angle was greater (P < 0.0001) at the FRONTAL (4.72 ± 0.20°) than the BEHIND EAR placement (3.22 ± 0.17°). Potential for bolt-brain contact was not different (P = 1.0000) between FRONTAL-INLINE (10/10, 100% ± 0.01%), FRONTAL-PISTOL (10/10, 100% ± 0.01%), BEHIND EAR-INLINE (9/10, 90% ± 9.49%), and BEHIND EAR-PISTOL (10/10, 100% ± 0.01%); brain damage (P = 0.5577) between FRONTAL-INLINE (9/9, 100% ± 0.02%), FRONTAL-PISTOL (10/10, 100% ± 0.02%), BEHIND EAR-INLINE (4/10, 40% ± 15.49%), and BEHIND EAR-PISTOL (1/10, 10% ± 9.49%); potential for bolt-thalamus contact (P = 0.0683) for FRONTAL-INLINE (2/10, 20% ± 12.65%), FRONTAL-PISTOL (8/10, 80% ± 12.65%), BEHIND EAR-INLINE (7/9, 77.78% ± 13.86%), and BEHIND EAR-PISTOL (9/9, 100% ± 0.02%); or thalamic damage (P = 0.8041) for FRONTAL-INLINE (1/10, 10% ± 9.49%), FRONTAL-PISTOL (1/10, 10% ± 9.49%), BEHIND EAR-INLINE (2/8, 25% ± 15.31%), and BEHIND EAR-PISTOL (0/9, 0% ± 0.00%). The FRONTAL placement with an INLINE PCB may present the least risk of failure for the PCB euthanasia of mature sows >200 kg body weight due to less total tissue thickness and thalamic depth, greater effective angle, and prevalent brain damage.

The frontal placement, when used with an inline free-flight penetrating captive bolt device, may be more reliable than other placement and device combinations for the euthanasia and stunning of sows >200 kg BW due to less total tissue thickness and thalamic depth, more room for error with positioning the PCB, and prevalent brain damage.

captive bolt
euthanasia
sow
stunning
swine
welfare
==== Body
pmcIntroduction

Euthanasia is a key component of safeguarding animal welfare in swine production. Methods of euthanasia must be reliable and effective in order to ensure a “good death” (National Pork Board and American Association of Swine Veterinarians (NPB and AASV), 2016); American Veterinary Medical Association (AVMA), 2020). One euthanasia method approved for grow-finish and breeding swine is penetrating captive bolt (PCB; NPB and AASV, 2016; AVMA, 2020; World Organization for Animal Health [WOAH, 2022]). The AVMA (2020), NPB and AASV (2016), and North American Meat Institute (NAMI, 2021) describe that the PCB placement for swine should be on the front of the animal’s head; this is supported by international guidance documents (Humane Slaughter Association (HSA), 2016; European Food Safety Authority (EFSA), 2004, 2020). In recent surveys of swine caretakers (Simpson et al., 2020) and veterinarians (Edwards-Callaway et al., 2020), penetrating captive bolt was described as the most common method of euthanasia for sows.

Woods (2012) evaluated a pistol-type PCB as a single-step method of euthanasia for swine and found that a frontal application was not completely effective for sows > 200 kg body weight (BW), indicated by 6.67% (2 of the 30) of sows requiring a second PCB application on-farm. More recently, Kramer et al. (2021) found that a pistol-type PCB applied in a frontal location was not 100% effective in achieving death, defined by cessation of cardiac and respiratory activity, for sows >200 kg BW. The same study also reported that an inline-type PCB applied in a frontal location was 100% (10 of the 10) effective in achieving death, as previously defined, for sows >200 kg BW. Additionally, Kramer et al. (2022) reported 100% efficacy in achieving death for an additional 10 sows >200 kg BW, using the same frontal PCB application with an inline device as their earlier work (Kramer et al., 2021).

An alternative behind ear placement has been described (AVMA, 2013); however, the AVMA (2020) has since specified that this placement is only intended for use with gunshot and not PCB. The NPB and AASV (2016) also describe the behind-ear placement as an alternative to the frontal placement for use with gunshot. The behind-ear PCB placement has been evaluated for market hogs (Anderson et al., 2019), as well as mature sows and boars (Anderson et al., 2021a, 2024; Kramer et al., 2021). Both Anderson et al. (2021a, 2024) and Kramer et al. (2021) concluded that additional investigation related to the BEHIND EAR placement for mature swine was necessary.

The goal of PCB euthanasia is to cause immediate insensibility through severe damage to the brain, through direct penetration by the bolt, bone fragmentation, or concussive forces (Terlouw et al., 2016). In other words, the goal of a PCB application is to cause an immediate loss of sensibility through widespread brain damage. Terlouw et al. (2016) described that the PCB should aim to damage either the reticular formation or ascending reticular activating system (ARAS) because they are associated with the presence or absence of sensibility. As such, the thalamus is of particular interest because it houses the ARAS which connects the reticular formation, in the brainstem, to the cerebral cortex (Terlouw et al., 2016; Terlouw and Le Neindre, 2024). In humans, lesions of the ARAS are associated with impaired consciousness associated with chronic traumatic brain injury (TBI; Jang and Kwon, 2020). Therefore, when the bolt, or resulting bone fragment, disrupts the thalamus insensibility should result (Terlouw et al., 2016). It should be noted that physical thalamic damage is not a requirement for an immediate insensibility, as the concussive impact of the bolt on the skull and diffuse brain damage also contribute to the loss of sensibility (Terlouw et al., 2016; Terlouw and Le Neindre, 2024).

The objectives of this study were to (1) evaluate the capacity of 2 commercially available captive bolt types (pistol-retracting, inline-free flight) to reach and disrupt the thalamus when applied in 2 placements (FRONTAL, BEHIND EAR) to cadaver heads from sows >200 kg; and (2) to identify and describe the optimal PCB placement and angle based on potential for thalamic damage, using external landmarks for the FRONTAL and BEHIND EAR PCB placements. Our hypothesis was that differences in the ability of PCB types to reach and disrupt the thalamus at the FRONTAL and BEHIND EAR placement would not be detected. Additionally, we hypothesized that the effective angle for PCB placement would not differ between placements.

Materials and Methods

Animal use protocol

It was not necessary to submit an animal use protocol to the University of Wisconsin—River Falls Institutional Animal Care and Use Committee because live animals were not directly manipulated in this study. The sows from which the heads were obtained were slaughtered at a commercial slaughter establishment under inspection by the United States Department of Agriculture Food Safety and Inspection Service (USDA FSIS) in accordance with the Humane Methods of Slaughter Act (7 USC 1901; United States House of Representatives Office of the Law Revision Counsel, 2024) and the regulations that enforce it (9 CFR 313; United States Electronic Code of Federal Regulations, 2024). The exemption from Institutional Animal Care and Use Committee approval followed the precedent set by Anderson et al. (2019, 2021a, b, 2024) and Hamilton et al. (2023).

Description of cadaver heads

To avoid the use of live animals in this study all data were collected using cadaver heads, as described by Anderson et al. (2021a, 2024) and Hamilton et al. (2023). Cadaver heads were obtained across 5 collection days from sows (N = 60; average BW: 225.36 ± 21.10 kg, mean ± SD) that were electrically stunned and slaughtered at a regional commercial processing facility. All heads were collected from cull sows; however, the age and parity of each source sow were unavailable. The sample size was determined via a power calculation using the POWER procedure of SAS 9.4 (SAS Institute, Inc., Cary, NC) with the following parameters: detectable difference of 12.7, SD of 9.893, and power of 0.8, as described by Anderson et al. (2024). The detectable difference was derived from cranial thickness measurements of boars >200 kg BW in the FRONTAL and temporal PCB placements, and the SD value was determined from SE values reported for the same measurements (Anderson et al., 2021a).

All heads had the skin on, with intact jowls, and were not scalded. Approximately 6 cm of neck tissue was left on each head, beyond where the head would normally be separated from the carcass, to prevent tissue distortion at the PCB application sites. The estimated live weight for each source sow was calculated from the hot carcass weight of the skinned and a 58% dressing percentage, the slaughter establishment’s average dressing percentage (Anderson et al., 2021a, 2024). To account for the additional neck tissue, 4.535 kg were added to the weight of each sow (formula: estimated BW = hot carcass weight (kg)/0.58 + 4.535 kg; Anderson et al., 2021a, 2024). Once separated from their respective carcasses, heads were placed into large storage totes with secured lids (Sterilite 1466 – 27 Gallon Industrial Tote, Sterilite Corp., Townsend, MA) with 2 heads per tote, and stored at ambient temperature for approximately 4 h postmortem prior to unrefrigerated transport (distance traveled: 188 km; duration: approximately 2 h) to the University of Wisconsin—River Falls Meat Science Laboratory and Animal Welfare Lab, as described by Anderson et al. (2021a, 2024). Upon arrival at the lab, all heads were stored in the storage totes with the lids slightly offset inside a walk-in cooler for approximately 61 h at 2 to 4 °C prior to head processing to improve the rigidity of brain tissue (Anderson et al., 2021a, 2024). Following this refrigeration period, each head was removed from the cooler and received 1 of the 6 PCB placement treatments. After each head was split on the band saw, the mean temperature of the exposed internal brain surface was 8.97 ± 1.07 °C (Mean ± SD) and did not differ between treatments (Table 1). Data collection for this project occurred concurrently with another project that was focused on captive bolt use for sows and boars that weighed less than 200 kg BW (Anderson et al., 2024). As a result, between 19 and 30 heads were obtained for each head processing date, but only heads from sows with an estimated BW > 200 kg were enrolled in this study. Six to eighteen heads per processing date were included in this study.

Table 1. Source animal and cadaver head characteristics from mature sows (BW > 200 kg) assigned to 6 treatments1 and sectioned by band saw following the plane of bolt entry or theoretical bolt entry

Dependent variable	Treatment	
FRONTAL-INLINE
(n = 10)	FRONTAL-PISTOL
(n = 10)	FRONTAL-NO SHOT
(n = 10)	BEHIND EAR-INLINE
(n = 10)	BEHIND EAR-PISTOL
(n = 10)	BEHIND EAR-NO SHOT
(n = 10)	
LS means	SE	LS means	SE	LS means	SE	LS means	SE	LS means	SE	LS means	SE	
Brain temp2, °C	9.27	0.24	9.13	0.29	9.48	0.43	8.48	0.28	8.88	0.41	8.61	0.35	
Estimated BW, kg	227.58	8.06	220.46	4.58	235.98	7.20	222.89	8.68	218.31	4.01	226.95	6.35	
Head weight, kg	20.23	1.38	19.57	1.03	20.08	1.03	17.80	0.72	19.17	0.55	19.24	1.25	
Distance between optic orbits, cm	14.73	0.49	14.83	0.34	16.31	0.60	14.61	0.52	15.92	0.38	15.08	0.72	
Snout to poll distance, cm	30.50	0.59	29.44	0.54	30.52	0.46	29.09	0.57	30.72	0.64	29.50	0.74	
Maximum deflection distance, cm	4.06	0.34	3.92	0.46	4.14	0.30	4.15	0.27	4.57	0.33	3.54	0.33	
1Placements: FRONTAL—Medial bolt entry approximately 3.5 cm superior to the optic orbits at midline perpendicular with the external surface of the head; BEHIND EAR—Bolt entry directly caudal to the pinna of the ear on the same plane as the eyes and targeting the middle of the opposite eye. PCB Types: INLINE—Inline, free-flight PCB; PISTOL—pistol, self-retracting PCB; SPLIT ONLY—no PCB application.

2Sample size for Brain Temp, °C: FRONTAL INLINE, n = 10; FRONTAL PISTOL, n = 9; FRONTAL NO SHOT, n = 9; BEHIND EAR INLINE, n = 9, BEHIND EAR PISTOL, n = 10, BEHIND EAR NO SHOT, n = 10.

a,b,cSuperscripts that differ within a row identify significant differences between means within dependent variables across placements (P ≤ 0.05).

Description of PCB devices, placement, and treatment assignment

Two PCB devices (Figure 1) were used in this study: an INLINE type (Jarvis Model PAS—Type C 0.25R Caliber Captive Bolt; Order #: 4144059, Jarvis Corp., Middletown, CT) and a PISTOL type (Jarvis Model PAS—Type P 0.25R Caliber Captive Bolt Pistol; Order #: 4144035, Jarvis Corp.). Jarvis Red Powder Cartridges (0.25R Caliber, 6.0 GR; Order #: 1176025, Jarvis Corp.) were used with the INLINE PCB, as described by Kramer et al. (2021, 2022). Jarvis Orange Powder Cartridges (0.25R Caliber, 3.5 GR; Order #: 1176019, Jarvis Corp.) and the Long Stunning Rod Nosepiece Assembly (Order #: 3116605, Jarvis Corp.) were used with the PISTOL PCB, as described by Anderson et al. (2021a, 2024). The red and orange powder cartridges used with the INLINE and PISTOL PCBs, respectively, are the highest powder cartridges approved by the manufacturer for repeated use with those PCB devices. The cartridges used with each PCB were selected based on manufacturer recommendations for mature sows. The predicted bolt travel distance for the INLINE PCB device used in our study, with the 6.0 GR cartridge was 114.3 mm and the expected bolt velocity was 66.14 m/s (J. Evans, Jarvis Corp., Middletown, CT, personal communication). The predicted bolt travel distance for the PISTOL PCB device, with the stunning rod and cartridge combination used in our study, was 76.2 mm and the expected bolt velocity was 51.82 m/s (M. Abdul, Jarvis Corp., Middletown, CT, personal communication). It should be noted that the expected bolt velocity, as well as predicted bolt travel distance, will differ for each of these PCB devices when different strength cartridges are used.

Figure 1. Penetrating captive bolt (PCB) devices used in this study. INLINE (Free-flight)—Jarvis Model PAS—Type C 0.25R Caliber Captive Bolt (Order #: 4144059, Jarvis Corp., Middletown, CT) was used with Jarvis Red Powder Cartridges (0.25R Caliber, 6.0 GR; Order #: 1176025, Jarvis Corp.). PISTOL (Self-retracting) –Jarvis Model PAS—Type P 0.25R Caliber Captive Bolt Pistol (Order #: 4144035, Jarvis Corp) was used with the Long Stunning Rod Nosepiece Assembly (Order #: 3116605, Jarvis Corp.) and Jarvis Orange Powder Cartridges (0.25R Caliber, 3.5 GR; Order #: 1176019, Jarvis Corp.).

Two PCB placements were utilized in this study (Figure 2): FRONTAL—shot placed 3.5 cm superior to the optic orbits at the midline (Woods et al., 2010), and BEHIND EAR—shot placed directly caudal to the pinna of the ear on the same plane as the eyes and targeting the middle of the opposite eye (AVMA, 2013; Anderson et al., 2019). Within each placement, each head was allocated to an INLINE, PISTOL, or NO SHOT PCB treatment. As a result, there were 6 possible treatments: FRONTAL-INLINE, FRONTAL-PISTOL, FRONTAL-NO SHOT, BEHIND EAR-INLINE, BEHIND EAR-PISTOL, and BEHIND EAR-NO SHOT. The heads for this study were received earlier than anticipated, and as a result, the PCB equipment was not prepared for use and only treatments that did not include a PCB application (FRONTAL-NO SHOT, BEHIND EAR-NO SHOT) were assigned on the first head processing day. For the first head processing day, each head was assigned to its respective treatment through sorting by estimated BW, then randomly allocating the 2 treatments to each pair of heads, working from lightest to heaviest, using a random number generator in Excel (Microsoft Corp., Spokane, WA). For subsequent head processing days, each head was assigned to its respective treatment by sorting the sample group by estimated BW, then randomly allocating the 6 treatments, working from lightest to heaviest, using a random number generator in Excel (Microsoft Corp., Spokane, WA). Once 10 heads were allocated to a given treatment, that treatment was no longer assigned.

Figure 2. Penetrating captive bolt placement and type treatments utilized in this study. Placements: FRONTAL—Medial bolt entry approximately 3.5 cm superior to the optic orbits at midline perpendicular with the external surface of the head; BEHIND EAR—Bolt entry directly caudal to the pinna of the ear on the same plane as the eyes and targeting the middle of the opposite eye. PCB Types: NO SHOT—no PCB application; PISTOL—pistol, self-retracting PCB; INLINE—Inline, free-flight PCB. Illustrations by Ashlynn A. Kirk.

A PCB was applied to all heads that received the FRONTAL-INLINE, FRONTAL-PISTOL, BEHIND EAR-INLINE, and BEHIND EAR-PISTOL treatments following the PCB application process described by Anderson et al. (2021a, 2024). A custom-fabricated benchtop-mounted stainless steel brace that formed a corner was hooked around the edge of the table to prevent movement during PCB application. All heads were positioned and firmly secured in the corner during each PCB application. The muzzle of the PCB was placed firmly against the head prior to activation for each of these treatments. All PCB applications were made by a single individual who had made PCB applications on cadaver heads from swine in previous work (Anderson et al., 2021a, 2021b, 2024). Heads in the FRONTAL-NO SHOT and BEHIND EAR-NO SHOT treatments did not receive a PCB application. Instead, either the FRONTAL or BEHIND EAR placement was marked on each head with a permanent marker. Once a PCB placement was marked on the head, the treatment was obvious to any observer, so blinding was not possible; as a result, individuals performing tissue thickness measurements and head image assessment were not blinded to treatment (Anderson et al., 2024).

A Jarvis PAS Stunning System Tester (Order #: 4116001, Jarvis Corp.) was used to test the bolt speed before the first PCB application at the beginning of each sampling day and before the end of cleaning of each sampling day. The average bolt speed was calculated from 3 successive readings at each time point. The average bolt speed of the INLINE PCB on all sampling days was 66.03 ± 7.75 m/s and the average bolt speed of the PISTOL PCB on all sampling days was 48.7 ± 2.19 m/s. The expected bolt speed of the INLINE PCB device and cartridge combination used in this study was 66.14 ± 3.05 m/s (J. Evans, Jarvis Corp., Middletown, CT, personal communication). The expected bolt speed of the PISTOL PCB and cartridge combination used in this study was 51.82 ± 3.05 m/s (M. Abdul, Jarvis Corp., Middletown, CT, personal communication).

Head processing, measurements, and image collection

A band saw (Hobart 6801 Vertical Meat Band Saw, Hobart, Troy, OH) equipped with a 0.06 mm thick, 360.68 mm long blade with a 3-degree hook angle and 4 teeth/2.54 cm (Product #: C78529545, Bunzl Processor Division, Riverside, MO) was used to split each head along the bolt path or theoretical bolt path. After each split, a 150 mm digital caliper (HC Kenshin Electronic Digital Vernier Caliper, HC Kenshin, HuiChuang Technology, Fujia, China) was used to take measurements from both sides of each split head, as described by Hamilton et al. (2023) and Anderson et al. (2024).

For heads that did not receive a PCB application (FRONTAL-NO SHOT, BEHIND EAR-NO SHOT), thalamic diameter (mm) and thalamic depth (mm) were measured for both sides of each head. These measurements are depicted in Figure 3 for the FRONTAL placement and Figure 4 for the BEHIND EAR placement. Thalamic diameter refers to the diameter of the thalamus, as seen from the exposed brain of each head. Thalamic depth refers to the distance from the site of theoretical PCB application on the surface of the skin to the center of the thalamus, following the theoretical bolt path. An effective angle for thalamic contact (Figure 3—FRONTAL, Figure 4—BEHIND EAR) was calculated from these measurements (effective angle = tan−1 ((0.5*thalamic diameter)/thalamic depth)).

Figure 3. Thalamic measurements at the FRONTAL placement. Thalamic diameter—the diameter of the thalamus. Thalamic depth—the distance from the site of theoretical captive bolt application on the surface of the skin to the center of the thalamus. Effective angle was calculated from these measurements—effective angle = tan−1 ((0.5*thalamic diameter)/thalamic depth).

Figure 4. Thalamic measurements at the BEHIND EAR placement. Thalamic diameter—the diameter of the thalamus. Thalamic depth—the distance from the site of theoretical captive bolt application on the surface of the skin to the center of the thalamus. Effective angle was calculated from these measurements—effective angle = tan−1 ((0.5*thalamic diameter)/thalamic depth).

For heads that received a PCB application (FRONTAL-INLINE, FRONTAL-PISTOL, BEHIND EAR-INLINE, and BEHIND EAR-PISTOL), soft tissue thickness (mm) and cranial thickness (mm) were measured for both sides of each head. Soft tissue thickness refers to the tissue from the application site to the exterior surface of the cranium and cranial thickness refers to the exterior surface of the cranium to the interior surface of the cranium along the bolt path (Anderson et al., 2019, 2021a, 2021b, 2024; Hamilton et al., 2023). These measurements were collected on both sides of the bolt path (Anderson et al., 2019, 2021a, 2021b; Hamilton et al., 2023) and were averaged prior to statistical analysis. If the bolt itself did not reach the brain cavity, measurements were collected for the entire distance from the site of application to the interior surface of the brain in alignment with the existing bolt path, as described by Anderson et al. (2021a, 2024). Soft tissue thickness and cranial thickness measurements were not recorded if the bolt path was not aligned with the brain (Anderson et al., 2024). Total tissue thickness—the total soft tissue and cranial thickness from the site of PCB application to the interior surface of the cranium, along the bolt path—was determined by the summation of soft tissue thickness and cranial thickness measurements for each head (Anderson et al., 2019, 2021a, 2021b, 2024; Hamilton et al., 2023). These measurements are depicted in Figure 5 for the FRONTAL placement and Figure 6 for the BEHIND EAR placement.

Figure 5. Tissue depth measurements at the FRONTAL placement. Soft tissue thickness—the tissue from the application site on the surface of the skin to the exterior surface of the cranium. Cranial thickness—the tissue from the exterior surface of the cranium to the interior surface of the cranium along the bolt path.

Figure 6. Tissue depth measurements at the BEHIND EAR placement. Soft tissue thickness—the tissue from the application site on the surface of the skin to the exterior surface of the cranium. Cranial thickness—the tissue from the exterior surface of the cranium to the interior surface of the cranium along the bolt path.

After all measurements were obtained for each head, digital images were collected from both sides of each intracranial surface with a digital camera (Olympus Tough TG-6, Waterproof Camera, OM Digital Solutions Corp., Tokyo, Japan) positioned 50.8 cm directly above and perpendicular to the exposed cut surface (Anderson et al., 2021a, 2024; Hamilton et al., 2023). All digital images included a 15 cm ruler for reference. To determine brain temperature, thermal images (Model E8, FLIR Systems, Boston, MA) were also collected from both sides of each exposed intracranial surface from 50.8 cm directly above and perpendicular to the exposed cut surface. The average brain temperature for all heads was 8.97 ± 0.14 °C and there was no evidence to support a difference (P > 0.05) in brain temperature between treatments (Table 1).

Head image assessment

Digital images of each head that received a PCB application (FRONTAL-INLINE, FRONTAL-PISTOL, BEHIND EAR-INLINE, and BEHIND EAR-PISTOL) were assessed by a team of 2 observers (KNA and KDV) to determine brain contact plane (Anderson et al., 2021a, 2024), and more specifically, thalamic contact plane (Hamilton et al., 2023). The bolt path was considered to be in the plane of the brain when a bolt with unlimited travel distance would have made contact with the brain. The bolt path was considered to be in the plane of the thalamus when a bolt with unlimited travel distance would have made contact with any part of the thalamus. These measurements were recorded on a yes/no basis. If the thalamus was not located within the plane of bolt travel, the position of the bolt path relative to the thalamus was recorded (i.e., whether the bolt path was above or below the thalamus). Visually detectable brain damage and thalamic damage were also assessed from images collected at the time of head processing by the same team of 2 observers. Brain and thalamic damage were both assessed on a yes/no basis. The location of the thalamus for the FRONTAL and BEHIND EAR placements was determined from brain maps presented by Anderson et al. (2021a).

External head measurements

Head weight was recorded for each head using a calibrated benchtop scale. A flexible measuring tape (Singer 218 60 in, The Singer Company Ltd., Boston, MA) was used to measure the distance between optic orbits (cm), snout to poll distance (cm), and maximum deflection distance (cm) (Anderson et al., 2021a, 2024; Hamilton et al., 2023). These measurements were collected in duplicate by a team of 2 trained observers and averaged prior to statistical analysis. The mean interobserver coefficients of variation were 11.0%, 3.6%, and 6.1% for the distance between optic orbits, snout-to-poll distance, and maximum deflection distance, respectively. Distance between optic orbits was the distance between the medial aspects of the optic orbits. Snout-to-poll distance was the distance from the tip of the snout to the first point of contact between a taught measurement tape at the crest of the head. Maximum deflection distance was the maximum distance from a straight edge placed between the tip of the snout and the poll or first point where the straight edge touched the head when placed from the tip of the snout.

Statistical analyses

All continuous data outcomes (brain temperature, BW, head weight, distance between optic orbits, snout to poll distance, maximum deflection distance, soft tissue thickness, cranial thickness, total tissue thickness, thalamic diameter, depth to thalamus, and effective angle) were assessed for normality via the histogram statement within the UNIVARIATE procedure of SAS Enterprise Guide 7.1 (Statistical Analysis System Institute, Inc.). Brain temperature, BW, head weight, the distance between optic orbits, snout to poll distance, and maximum deflection distance were then analyzed for PCB placement-device treatment (FRONTAL-INLINE, FRONTAL-PISTOL, FRONTAL-NO SHOT, BEHIND EAR-INLINE, BEHIND EAR-PISTOL, and BEHIND EAR-NO SHOT) effects using models constructed within the MIXED procedure of SAS Enterprise Guide 7.1 (Statistical Analysis Systems Institute, Inc.). A repeated statement with grouping by treatment and the Satterthwaite method for degrees of freedom was used to account for unequal variances by treatment. Three were excluded from the brain temperature analysis because thermal images were not available, these heads were from the FRONTAL-PISTOL, FRONTAL-NO SHOT, and BEHIND EAR-INLINE treatments. Soft tissue thickness, cranial thickness, and total tissue thickness were first analyzed for PCB placement-device treatment (FRONTAL-INLINE, FRONTAL-PISTOL, BEHIND EAR-INLINE, and BEHIND EAR-PISTOL) effects using models constructed within the MIXED procedure of SAS Enterprise Guide 7.1 (Statistical Analysis System Institute, Inc.), with a repeated statement with grouping by treatment and the Satterthwaite method for degrees of freedom. However, there was no evidence to support differences between PCB devices, so these measurements were pooled for the FRONTAL and BEHIND EAR placement and the analysis was repeated with only placement effects. One head that received a BEHIND EAR-INLINE PCB application was excluded from tissue depth analyses because tissue depth measurements could not be recorded. Thalamic diameter, thalamic depth, and effective angle were analyzed for placement treatments (FRONTAL, BEHIND EAR) with models constructed in the MIXED procedure of SAS Enterprise Guide 7.1 (Statistical Analysis System Institute, Inc.) with the Satterthwaite method for degrees of freedom; a repeated statement with grouping by placement was included. Mean separation was determined using Student’s T-tests, protected by the Bonferroni-Holm adjustment for multiple comparisons when needed. Differences between means were recognized as significant when P < 0.05.

To estimate the bolt length necessary to reach the brain, and thalamus, for the majority of sows weighing more than 200 kg BW, a 97.5% one-sided reference range was computed using a t-based prediction interval for normally distributed data with unknown mean and standard deviation (Horn, 1988), along with a 90% confidence interval for the upper reference limit (URL) for soft tissue thickness, cranial thickness, total tissue thickness, and thalamic depth for the FRONTAL and BEHIND EAR PCB placements, as described by Anderson et al. (2021a, 2024). R version 4.2.2 (R Core Team, 2024) via R Studio Version 2023.03.0 (R Core Team, 2024) using the tidyverse and ggbeeswarm packages was used to calculate all URLs, 90% CIs around each URL value (URL 90% CI), and associated plots.

Brain contact plane, thalamic contact plane, brain damage, and thalamic damage were analyzed for PCB placement-device treatments (FRONTAL-INLINE, FRONTAL-PISTOL, BEHIND EAR-INLINE, and BEHIND EAR-PISTOL) using logistic regression models constructed within the GLIMMIX procedure of SAS Enterprise Guide 7.1 (Statistical Analysis System Institute, Inc.). Least squares mean and SE values were calculated on natural log scale and were back-transformed within SAS Enterprise Guide 7.1 using the ILINK statement. Pairwise differences between LS means were determined with the PDIFF option within the GLIMMIX procedure of the SAS Enterprise Guide, (Statistical Analysis System Institute, Inc.). For heads where the bolt path was not located within the plane of the thalamus, the position of the bolt path relative to the thalamus was analyzed using the methods described above.

Relationships between BW, head weight, and external head measurements (distance between optic orbits, snout to poll distance, and maximum deflection distance) were assessed for correlations with soft tissue thickness, cranial thickness, and total tissue thickness for all heads that received a PCB application (FRONTAL-INLINE, FRONTAL-PISTOL, BEHIND EAR-INLINE, and BEHIND EAR-PISTOL) via simple linear regression using the Regression procedure in SAS (Statistical Analysis System Institute, Inc.). Relationships between BW, head weight, and external head measurements (distance between optic orbits, snout to poll distance, and maximum deflection distance) were assessed for correlations with thalamic diameter, depth to thalamus, and effective angle for all heads that did not receive a PCB application (FRONTAL-NO SHOT, BEHIND EAR-NO SHOT) via simple linear regression using the Regression procedure in SAS (Statistical Analysis System Institute, Inc.). Correlations were recognized as significant when P < 0.05.

Results and Discussion

Source animal and cadaver head characteristics

Estimated BW, head weight, and external head measurements (distance between optic orbits, snout to poll distance, and maximum deflection distance) can be observed in Table 1 and are reported as mean ± SE.

There was no evidence to support a significant difference (P = 0.3811) in estimated BW between PCB placement and PCB type combinations. Estimated BW was 227.58 ± 8.06 kg for FRONTAL-INLINE heads, 220.46 ± 4.58 kg for FRONTAL-PISTOL heads, 235.98 ± 7.20 kg for FRONTAL-NO SHOT heads, 222.89 ± 8.68 kg for BEHIND EAR-INLINE heads, and 218.31 ± 4.01 kg for BEHIND EAR-PISTOL heads, and 226.95 ± 6.35 kg for BEHIND EAR-NO SHOT heads. There was also no evidence (P = 0.4328) to support a significant difference in head weight between PCB placement and PCB type combinations. Head weight was 20.23 ± 1.38 kg for FRONTAL-INLINE heads, 19.57 ± 1.03 kg for FRONTAL-PISTOL heads, 20.08 ± 1.03 kg for FRONTAL-NO SHOT heads, 17.80 ± 07.2 kg for BEHIND EAR-INLINE heads, and 19.17 ± 0.55 kg for BEHIND EAR-PISTOL heads, and 19.25 ± 1.25 kg for BEHIND EAR-NO SHOT heads. The head weights observed in the present study are similar to those observed by Anderson et al. (2024), who reported an average head weight of 17.34 kg for sows <200 kg BW. Head weight has not been reported in other studies evaluating PCB use for sows >200 kg BW with cadaver heads (Anderson et al., 2021a) or live animals (Woods, 2012; Kramer et al., 2021, 2022).

There was no evidence to support a difference in distance between optic orbits (P = 0.1186), snout-to-poll distance (P = 0.2514), or maximum deflection distance (P = 0.4344) between any PCB placement and PCB type combination. Distance between optic orbits was 14.73 ± 0.49 cm for heads that received the FRONTAL-INLINE treatment, 14.83 ± 0.34 cm for heads that received the FRONTAL-PISTOL treatment, 16.31 ± 0.60 cm for heads that received the FRONTAL-NO SHOT treatment, 14.61 ± 0.52 cm for heads that received the BEHIND EAR-INLINE treatment, and 15.92 ± 0.38 cm for heads that received the BEHIND EAR-PISTOL treatment, and 15.08 ± 0.72 cm for heads that received the BEHIND EAR-NO SHOT treatment. Snout to poll distance was 30.50 ± 0.59 cm for heads that received the FRONTAL-INLINE treatment, 29.44 ± 0.54 cm for heads that received the FRONTAL-PISTOL treatment, 30.52 ± 0.46 cm for heads that received the FRONTAL-NO SHOT treatment, 29.09 ± 0.57 cm for heads that received the BEHIND EAR-INLINE treatment, and 37.30 ± 0.64 cm for heads that received the BEHIND EAR-PISTOL treatment, and 29.50 ± 0.74 cm for heads that received the BEHIND EAR-NO SHOT treatment. Maximum deflection distance was 4.06 ± 0.34 cm for heads that received the FRONTAL-INLINE treatment, 3.92 ± 0.46 cm for heads that received the FRONTAL-PISTOL treatment, 4.14 ± 0.30 cm for heads that received the FRONTAL-NO SHOT treatment, 4.15 ± 0.27 cm for heads that received the BEHIND EAR-INLINE treatment, and 4.57 ± 0.33 cm for heads that received the BEHIND EAR-PISTOL treatment, and 3.54 ± 0.33 cm for heads that received the BEHIND EAR-NO SHOT treatment. The lack of a significant difference in maximum deflection distance between treatments suggests that “plank” and “dish” shaped profiles (Woods, 2012) were distributed evenly across PCB placement and device treatments, as described by Anderson et al. (2024). Anderson et al. (2024) reported maximum deflection distances of 3.92 and 4.17 cm for sows and boars <200 kg BW, respectively; these are similar to the maximum deflection distances observed in the present study. Hamilton et al. (2023) reported maximum deflection distances of 3.31 and 3.08 cm for physically castrated market barrows and immunocastrated boars, respectively. The similarity between maximum deflection distances reported in the present study and by Anderson et al. (2024) compared to those reported by Hamilton et al. (2023) suggests that the “dish” face shape may become more pronounced once sexual maturity is reached but may not continue to increase beyond that point. This idea is not new, as the HSA (2016) described that the “dish” face shape is characteristic of mature pigs; however, it provides supportive evidence. Future investigations to increase the understanding of how the “dish” face shape develops up to and after sexual maturity would be valuable.

Tissue and cranial measurements

Tissue depth measurements collected from heads that received a PCB application with either the INLINE or PISTOL type PCB device can be observed in Table 2 (means ± SE) and Table 3 (97.5% URL and URL 90% CI). All tissue depth measurements are reported as mean ± SE; URL; URL 90% CI. Soft tissue thickness, cranial thickness, and total tissue thickness were significantly different (P < 0.05) between the FRONTAL and BEHIND EAR placements. Less (P < 0.0001) soft tissue thickness was observed at the FRONTAL placement (mean ± SE: 7.83 ± 0.29 mm; URL: 10.59 mm; URL 90% CI: 9.78 to 11.41 mm) than the BEHIND EAR placement (56.01 ± 3.25 mm; URL: 85.53 mm; URL 90% CI: 77.40 to 95.67 mm). Cranial thickness was greater (P < 0.0001) at the FRONTAL placement (48.49 ± 1.81 mm; URL: 65.85; URL 90% CI: 60.75 to 70.92 mm) than the BEHIND EAR placement (39.51 ± 2.49 mm; URL: 62.91; URL 90% CI: 55.91 to 69.91 mm). There was less (P < 0.0001) total tissue thickness observed at the FRONTAL placement (56.31 ± 1.76 mm; URL: 73.17 mm; URL 90% CI: 68.23 to 78.11 mm) than the BEHIND EAR placement (95.52 ± 3.30 mm; URL: 126.53 mm; URL 90% CI: 117.25 to 135.81 mm). Less total tissue thickness at the FRONTAL placement compared to the BEHIND EAR placement was also observed for market hogs (Anderson et al., 2019), sows and boars >200 kg (Anderson et al., 2021a), and sows and boars <200 kg (Anderson et al., 2024). Together, these findings suggest that the FRONTAL placement may be favorable to the BEHIND EAR placement, based on less tissue for the bolt to travel through to reach the brain.

Table 2. Soft tissue thickness, cranial thickness, and total tissue thickness of cadaver heads from mature sows >200 kg body weight assigned to 2 penetrating captive bolt (PCB) placements1 and sectioned by band saw following the plane of bolt entry

Dependent variable	PLACEMENT	
FRONTAL
(n = 20)	BEHIND EAR
(n = 19)	P-value	
LS Means	SE	LS Means	SE	
Soft tissue thickness, mm	7.83	0.29	56.01	3.25	<0.0001	
Cranial thickness, mm	48.49	1.81	39.51	2.49	<0.0001	
Total tissue thickness, mm	56.31	1.76	95.52	3.30	<0.0001	
1Placements: FRONTAL—Medial bolt entry approximately 3.5 cm superior to the optic orbits at midline perpendicular with the external surface of the head; BEHIND EAR—Bolt entry directly caudal to the pinna of the ear on the same plane as the eyes and targeting the middle of the opposite eye.

Table 3. Upper 97.5% reference interval limits (URL) and associated 90% confidence intervals (URL 90% CI) for tissue depth parameters of cadaver heads from sows >200 kg body weight assigned to 2 penetrating captive bolt (PCB) placements1 and sectioned by band saw following the plane of bolt entry

Dependent variable	PCB placement	
FRONTAL	BEHIND EAR	
n	URL	URL 90% CI	n	URL	URL 90% CI	
Soft tissue thickness, mm	20	10.59	9.78 to 11.41	19	85.53	77.40 to 95.67	
Cranial thickness, mm	20	65.84	60.75 to 70.92	19	62.91	55.91 to 69.91	
Total tissue thickness, mm	20	73.17	68.23 to 78.11	19	126.53	117.25 to 135.81	
Thalamic depth, mm	10	88.19	84.49 to 91.89	10	135.99	126.95 to 145.02	
1Placements: FRONTAL—Medial bolt entry approximately 3.5 cm superior to the optic orbits at midline perpendicular with the external surface of the head; BEHIND EAR—Bolt entry directly caudal to the pinna of the ear on the same plane as the eyes and targeting the middle of the opposite eye.

The NPB and AASV (2016) and HSA (2016) describe that cranial thickness along the front of the pig’s head increases with age, as such, it is important to look at the total tissue thickness, and more specifically the cranial thickness, at the FRONTAL placement across studies. Total tissue thickness for market hogs, including immunocastrated boars, have reported total tissue thickness values ranging from 22.88 mm (Anderson et al., 2021b) to 32.02 mm (Hamilton et al., 2023) and cranial thickness measurements ranging from 16.73 mm (Anderson et al., 2021b) to 23.4 mm (Anderson et al., 2019). Total tissue thickness measurements reported for mature sows have ranged from 52.7 mm (Anderson et al., 2021a) to 56.31 mm in this study and cranial thickness measurements have ranged from 42.11 mm (Anderson et al., 2024) to 48.49 mm in this study. Total tissue thickness measurements reported for mature boars have ranged from 41.2 mm (Anderson et al., 2021a) to 54.73 mm (Anderson et al., 2024) and reported cranial thickness measurements have ranged from 34.8 mm (Anderson et al., 2021a) to 44.09 mm (Anderson et al., 2024). These data support the statements that skull thickness, and the distance the bolt must travel to reach the brain, increase with maturity. Kramer et al. (2021, 2022) also reported cranial thickness for sows and boars >200 kg BW; however, these measurements were not taken along the FRONTAL bolt path and as a result, comparisons to the present study and others cannot be made.

The URL values reported here can be used to predict whether a PCB device with a penetration depth, or bolt travel distance, would have the capacity to reach the brain for the majority (97.5%) of sows in the FRONTAL or BEHIND EAR placements (as described by Anderson et al., 2021a, 2024). All URL values, URL 90% CIs, and sample distributions for tissue thickness measurements can be viewed in Figure 7. Our findings suggest that a penetration depth of 73.17 mm would be required to reach the brain at the FRONTAL placement for 95% of sows >200 kg BW and a penetration depth of 126.53 mm would be required to reach the brain at the BEHIND EAR placement for 97.5% of sows >200 kg BW. As a reminder, the bolt travel distances for the PCB devices used in this study were 114.3 and 76.2 mm, for the INLINE and PISTOL-type devices, respectively; these values account for the compression of the buffers that line the bolt. These findings align with those of Anderson et al. (2021a, 2024) who found that the least required penetration depth to reach the brain was at the FRONTAL placement and the greatest required penetration depth to reach the brain was at the BEHIND EAR placement. These URL values are highly valuable to manufacturers of PCB devices and individuals responsible for the selection of PCB devices for the euthanasia or stunning of mature sows.

Figure 7. Upper 97.5% reference interval limits (upper reference limit [URL], black dot) and associated 90% confidence intervals (URL 90% CI, black bars), along with data distribution for tissue thickness (n = 39) and thalamic depth (n = 20) of cadaver heads from mature sows >200 kg body weight assigned to 2 penetrating captive bolt (PCB) placements and sectioned by band saw following the plane, or theoretical plane, of bolt entry.

Measurements of thalamic diameter, thalamic depth, and effective angle can be observed in Table 4 (means ± SE) for heads that did not receive a PCB application (FRONTAL-NO SHOT, BEHIND EAR-NO SHOT). To the best of our knowledge, this is the first study to report thalamic diameter, thalamic depth, or effective angle for swine. Additionally, URLs and URL 90% CI values for thalamic depth can be observed in Table 3. There was no evidence to support a difference (P = 0.5163) in exposed thalamic diameter between the FRONTAL (12.86 ± 0.40 mm) and BEHIND EAR (12.38 ± 0.60 mm) placements. Thalamic depth was less (P < 0.0001) at the FRONTAL placement (78.31 ± 1.32 mm; URL: 88.19 mm; URL 90% CI: 84.49 to 91.89 mm) than the BEHIND EAR placement (111.86 ± 3.22 mm; URL: 135.99 mm; URL 90% CI: 126.95 to 145.02 mm). Within the brain, it is important that PCB placements align with the thalamus, as direct damage to the thalamus should result in an immediate loss of sensibility (Terlouw et al., 2016). A greater (P < 0.0001) effective angle was observed at the FRONTAL placement (4.72 ± 0.02°) than at the BEHIND EAR placement (3.22 ± 0.17°). This value represents the angle that the PCB may be positioned away from perpendicular to the animal’s head for the FRONTAL or BEHIND EAR placements on one side of the bolt path. As such, there is a 9.44° range at the FRONTAL placement and a 6.44° range at the BEHIND EAR placement where the PCB can fluctuate, while the thalamus remains in the bolt path. The greater effective angle observed at the FRONTAL placement is advantageous, as it allows more room for error in the angle at which the PCB is placed on the pig’s head.

Table 4. Thalamic measurements of cadaver heads from sows >200 kg body weight assigned to 2 penetrating captive bolt (PCB) placements1 and sectioned by band saw following the plane of theoretical bolt entry

Dependent variable	PLACEMENT		
FRONTAL (n = 10)	BEHIND EAR (n = 10)	P-value	
LS means	SE	LS means	SE	
Thalamic diameter, mm	12.86	0.40	12.38	0.60	0.5163	
Thalamic depth, mm	78.31	1.32	111.86	3.22	<0.0001	
Effective angle, °	4.72	0.20	3.22	0.17	<0.0001	
1Placements: FRONTAL—Medial bolt entry approximately 3.5 cm superior to the optic orbits at midline perpendicular with the external surface of the head.

BEHIND EAR—Bolt entry directly caudal to the pinna of the ear on the same plane as the eyes and targeting the middle of the opposite eye.

The URL values for thalamic depth provide additional insight into the penetration depth, or bolt travel distance, that is needed for mature sows. Specifically, they can be used to predict whether a PCB device with a known penetration depth would have the capacity to reach the thalamus for the majority (97.5%) of sows in the FRONTAL or BEHIND EAR placements. All URL values, URL 90% CIs, and sample distributions, for thalamic depth measurements can be viewed in Figure 7. Our findings suggest that a penetration depth of 84.49 mm is necessary to reach the thalamus at the FRONTAL placement for 97.5% of sows >200 kg BW and a penetration depth of 135.99 mm is necessary to reach the thalamus at the BEHIND EAR placement for 97.5% of sows >200 kg BW. An additional 11.32 mm and an additional 10.46 mm of penetration depth are required to reach the thalamus, beyond the outer surface of the brain, for the FRONTAL and BEHIND ear placements, respectively.

Head image assessment

Brain contact plane, the position of the bolt path relative to the thalamus, brain damage, and thalamic damage results for all heads that received a PCB application (FRONTAL-INLINE, FRONTAL-PISTOL, BEHIND EAR-INLINE, and BEHIND EAR-PISTOL) can be observed in Table 5.

Table 5. Occurrence of brain contact plane, thalamic contact plane, visible brain damage, and visible thalamic damage in cadaver heads from mature sows (BW > 200 kg) assigned to 4 treatments1 and sectioned by band saw following the plane of bolt entry

		Treatment	
Dependent variable		FRONTAL-INLINE	FRONTAL-PISTOL	BEHIND EAR-INLINE	BEHIND EAR-PISTOL	
Brain contact plane	Percentage ± SE	100. 00 ± 0.01	100.00 ± 0.01	90.00 ± 9.49	100.00 ± 0.01	
Proportion	10/10	10/10	9/10	10/10	
Thalamic contact plane	Percentage ± SE	20.00 ± 12.65	80.00 ± 12.65	77.78 ± 13.86	100.00 ± 0.02	
Proportion	2/10	8/10	7/9	9/9	
No thalamic contact plane, low	Percentage ± SE	0.00 ± 0.02	0.00 ± 0.04	50.00 ± 35.36	—	
Proportion	0/8	0/2	1/2	—	
Brain damage	Percentage ± SE	100.00 ± 0.02	100.00 ± 0.02	40.00 ± 15.49	10.00 ± 9.49	
Proportion	9/9	10/10	4/10	1/10	
Thalamic damage	Percentage ± SE	10.00 ± 9.49	10.00 ± 9.49	25.00 ± 15.31	0.00 ± 0.00	
Proportion	1/10	1/10	2/8	0/9	
1Placements: FRONTAL—Medial bolt entry approximately 3.5 cm superior to the optic orbits at midline perpendicular with the external surface of the head; BEHIND EAR—Bolt entry directly caudal to the pinna of the ear on the same plane as the eyes and targeting the middle of the opposite eye. PCB Types: INLINE—Inline, free-flight PCB; PISTOL—pistol, self-retracting PCB.

There was no evidence to support a difference (P = 1.0000) in brain contact plane between treatments. The bolt path was in the plane of the brain for all heads that received a FRONTAL PCB application (FRONTAL-INLINE and FRONTAL-PISTOL: 10 of the 10, 100.00% ± 0.01%) and all heads that received a BEHIND EAR-PISTOL PCB application (100.00% ± 0.01%). The bolt path was in the plane of the brain for 9 of the 10 (90.00% ± 9.49%) heads that received a BEHIND EAR-INLINE PCB application. For one head that received a BEHIND EAR-INLINE PCB application, the bolt path was located below the brain. Anderson et al. (2021a, 2024) also observed that all sow heads that received a FRONTAL PCB application had the bolt path located within the plane of the brain. Anderson et al. (2021a) observed that the bolt path was located within the plane of the brain for most (87.2%, 34 of the 39) sow heads that received a BEHIND EAR PCB application and Anderson et al. (2024) observed the bolt path was located within the plane of the brain for all (100.0%, 10 of the 10) sow heads that received a BEHIND EAR PCB application.

There was also no evidence to support a difference (P = 0.0683) in thalamic contact plane between treatments. The bolt path was in the plane of the thalamus for 2 of the 10 (20.00% ± 12.65%) heads that received a FRONTAL-INLINE PCB application, 8 of the 10 (80.00% ± 12.65%) heads that received a FRONTAL-PISTOL PCB application, 7 of the 9 (77.78% ± 13.86%) heads that received a BEHIND EAR-INLINE PCB application, and 9 of the 9 (100.00% ± 0.02%) heads that received a BEHIND EAR-PISTOL PCB application. Although there was no evidence to support a difference in thalamic contact plane between treatments, it should be noted that the percentage of heads that received the FRONTAL-INLINE PCB treatment was over 50% less than any of the other PCB placement-treatment combinations. The reason for this is unknown and should be investigated in future studies. For the treatments where the bolt path was not located in the plane of the thalamus for all heads (FRONTAL-INLINE, FRONTAL-PISTOL, and BEHIND EAR-INLINE), there was no evidence to support a difference (P = 0.9995) in the position of the bolt path relative to the thalamus. The bolt path was above the thalamus for 8 of the 8 (100.00% ± 0.02%) heads that received a FRONTAL-INLINE PCB application where the bolt path was not in the plane of the thalamus (8 of the 10 heads). The bolt path was above the thalamus for 2 of the 2 (100.00% ± 0.04%) heads that received a FRONTAL-PISTOL PCB application where the bolt path was not in the plane of the thalamus (2 of the 10 heads). It is important to note that for both FRONTAL placement treatments, the bolt path was located above the thalamus for all heads where the bolt was not located within the plane of the thalamus. This finding indicates the need for future research to evaluate whether a frontal placement of 3.5 cm above the top of the optic orbits is the most appropriate for mature sows, or if a slightly lower placement (i.e., the recommended placement for market hogs, 2.54 cm above the top of the optic orbits) might be more appropriate. The bolt path was above the thalamus for 1 of the 2 (50.00% ± 25.36%) heads that received a BEHIND EAR-PISTOL PCB application where the bolt path was not in the plane of the thalamus (2 of the 9 heads). Hamilton et al. (2023) reported that the FRONTAL placement used in this study was in the plane of the thalamus for 76.2% (16 of the 21) of physically castrated market barrows and 90.5% (19 of the 21) of immunocastrated boars; it was also reported that the frontal placement for market hogs (2.54 cm above the optic orbits at midline) resulted in a bolt path that was in the plane of the thalamus for 100.0% (21 of the 21) of physically castrated market barrows and 90.5% (19 of the 21) of immunocastrated boars. It is possible that the increased cranial thickness of sows compared to market-weight hogs contributed to the lower levels of thalamic contact plane observed at the FRONTAL placement in the present study. Additionally, differences in external head morphology may play a role in the location of the thalamus relative to the FRONTAL PCB placement. To the best of our knowledge, this is the first study to evaluate the position of the BEHIND EAR bolt path relative to the thalamus.

There was no evidence to support a difference (P = 0.5577) in visually detectable brain damage between treatments. Brain damage was observed in all heads that received a FRONTAL PCB application (FRONTAL-INLINE: 9 of the 9, 100.00% ± 0.02%; FRONTAL-PISTOL: 10 of 10, 100.00% ± 0.02%). Brain damage was observed in 4 of the 10 (40.00% ± 15.49%) heads that received a BEHIND EAR-INLINE PCB application and 1 of the 10 (10.00% ± 9.49%) heads that received a BEHIND EAR-PISTOL PCB application. Although there was no evidence to support a significant difference in brain damage between any treatments, it should be noted that all of the heads that received a PCB application at the FRONTAL placement (19 of the 19) had some brain damage, while only 5 of the 20 heads that received a PCB application at the BEHIND EAR placement. Previous work has demonstrated a high percentage of brain damage at the FRONTAL placement, with 96.2% (25 of the 26; Anderson et al., 2021a) and 100% (10 of the 10; Anderson et al., 2024) of sow heads displaying visual detectable brain damage following a FRONTAL PCB application. Similarly, previous work has demonstrated a low percentage of brain damage at the BEHIND EAR placement, with 12.5% (5 of the 40; Anderson et al., 2021a) and 0.0% (0 of the10; Anderson et al., 2024) of sow heads displaying visually detectable brain damage following a BEHIND EAR PCB application. Both Anderson et al. (2021a, 2024) studies used the PISTOL type PCB device. Kramer et al. (2021) evaluated TBI using a 3-point scoring system based on Millar and Mills (2000) and upon the summation of TBI scores reported that there was greater brain damage that resulted from the BEHIND EAR placement than the FRONTAL placement, when a PISTOL type PCB was used. However, when an INLINE type PCB was used, Kramer et al. (2021) did not report a difference in brain damage between the FRONTAL and BEHIND EAR placements. Future studies, using a larger sample size than those used in the present study or by Kramer et al. (2021), to investigate the causative factors of brain damage (i.e., bolt length, bolt velocity, and placement) are warranted.

There was also no evidence to support a difference (P = 0.8041) in visually detectable thalamic damage between treatments. Thalamic damage was observed in 1 of the 10 (10.00% ± 0.49%) heads that received a FRONTAL-INLINE PCB application, 1 of the 10 (10.00% ± 9.49%) heads that received a FRONTAL-PISTOL PCB application, 2 of the 8 (25.00% ± 15.31%) heads that received a BEHIND EAR-INLINE PCB application, and 0 of the 9 (0.00% ± 0.00%) heads that received a BEHIND EAR-PISTOL PCB application. An immediate loss of sensibility may result when direct damage—either from the bolt itself or subsequent bone fragmentation—does not occur to the thalamus (Terlouw et al., 2016; Terlouw and Le Neindre, 2024). Other factors, including the concussive impact of the bolt on the skull and hemorrhage, also contribute to the loss of sensibility; however, using a PCB device that has the capacity for the bolt—or subsequent bone fragmentation—to reach the level of the thalamus is a good way to ensure immediate insensibility (Terlouw et al., 2016; Terlouw and Le Neindre, 2024). Kramer et al. (2021) evaluated damage to the thalamus using a TBI scoring system; however, specific thalamic damage was not reported because TBI scores were summed for various regions of the brain. Previous work by Anderson et al. (2021a, 2024) did not include an evaluation of thalamic damage; however, damage to the diencephalon, where the thalamus is located, was reported.

Relationship between extrinsic parameters and tissue thicknesses

Nonsignificant (P > 0.05) linear regression results for the FRONTAL placement can be observed in Table 6 and nonsignificant (P > 0.05) linear regression results for the BEHIND EAR placement can be observed in Table 7.

Table 6. Linear regression relationships between source animal/head characteristics and tissue measurements of cadaver heads from sows >200 kg body weight assigned to a FRONTAL penetrating captive bolt (PCB) placement1

Variables (X;Y)	R2	P-value	
Body weight, kg; soft tissue thickness, mm	0.0036	0.8019	
Body weight, kg; cranial thickness, mm	0.0210	0.5420	
Body weight, kg; total tissue thickness, mm	0.0253	0.5029	
Body weight, kg; thalamic depth, mm	0.1958	0.2003	
Body weight, kg; thalamic dimeter, mm	0.1547	0.2608	
Body weight, kg; effective angle	0.0058	0.8350	
Head weight, kg; soft tissue thickness, mm	0.0075	0.7159	
Head weight, kg; thalamic depth, mm	0.2939	0.1054	
Head weight, kg; thalamic diameter, mm	0.1547	0.2608	
Head weight, kg; effective angle	0.0058	0.8350	
Snout to poll distance, cm; soft tissue thickness, mm	0.0587	0.3032	
Snout to poll distance, cm; cranial thickness, mm	0.0416	0.3884	
Snout to poll distance, cm; total tissue thickness, mm	0.0290	0.4731	
Snout to poll distance, cm; thalamic depth, mm	0.0747	0.4449	
Snout to poll distance, cm; thalamic diameter, mm	0.0673	0.4690	
Snout to poll distance, cm; effective angle	0.1039	0.3636	
Distance between optic orbits, cm; soft tissue thickness, mm	0.0214	0.5386	
Distance between optic orbits, cm; cranial thickness, mm	0.1308	0.1171	
Distance between optic orbits, cm; total tissue thickness, mm	0.1214	0.1321	
Distance between optic orbits, cm; thalamic depth, mm	0.2529	0.1385	
Distance between optic orbits, cm; thalamic dimeter, mm	0.0417	0.5713	
Distance between optic orbits, cm; effective angle	0.1567	0.2574	
Maximum deflection distance, cm; cranial thickness, mm	0.0252	0.5039	
Maximum deflection distance, cm; total tissue thickness, mm	0.0655	0.2759	
Maximum deflection distance, cm; thalamic depth, mm	0.1287	0.3087	
Maximum deflection distance, cm; thalamic diameter, mm	0.3766	0.0592	
Maximum deflection distance, cm; effective angle	0.3437	0.0749	
1FRONTAL placement—Medial bolt entry approximately 3.5 cm superior to the optic orbits at midline perpendicular with the external surface of the head.

Table 7. Linear regression relationships between source animal/head characteristics and tissue measurements of cadaver heads from sows >200 kg body weight assigned to a BEHIND EAR penetrating captive bolt (PCB) placement1

Variables (X;Y)	R2	P-value	
Body weight, kg; soft tissue thickness, mm	0.0107	0.6735	
Body weight, kg; cranial thickness, mm	0.0049	0.7757	
Body weight, kg; total tissue thickness, mm	0.0024	0.8422	
Body weight, kg; thalamic depth, mm	0.0543	0.5170	
Body weight, kg; thalamic diameter, mm	0.0522	0.5256	
Body weight, kg; effective angle	0.0825	0.4210	
Head weight, kg; soft tissue thickness, mm	0.0110	0.6696	
Head weight, kg; cranial thickness, mm	0.0438	0.3900	
Head weight, kg; total tissue thickness, mm	0.0681	0.2805	
Head weight, kg; thalamic depth, mm	0.2127	0.1798	
Head weight, kg; thalamic diameter, mm	0.0522	0.5256	
Head weight, kg; effective angle	0.0825	0.4210	
Snout to poll distance, cm; soft tissue thickness, mm	0.0948	0.1997	
Snout to poll distance, cm; cranial thickness, mm	0.0337	0.4518	
Snout to poll distance, cm; total tissue thickness, mm	0.1950	0.0584	
Snout to poll distance, cm; thalamic depth, mm	0.0496	0.5364	
Snout to poll distance, cm; thalamic diameter, mm	0.0854	0.4126	
Snout to poll distance, cm; effective angle	0.1189	0.3291	
Distance between optic orbits, cm; soft tissue thickness, mm	0.0103	0.6786	
Distance between optic orbits, cm; cranial thickness, mm	0.1031	0.1801	
Distance between optic orbits, cm; total tissue thickness, mm	0.1173	0.1513	
Distance between optic orbits, cm; thalamic diameter, mm	0.0000	0.9959	
Distance between optic orbits, cm; effective angle	0.1840	0.2161	
Maximum deflection distance, cm; soft tissue thickness, mm	0.0661	0.2879	
Maximum deflection distance, cm; cranial thickness, mm	0.0006	0.9199	
Maximum deflection distance, cm; total tissue thickness, mm	0.0550	0.3340	
Maximum deflection distance, cm; thalamic depth, mm	0.3369	0.0785	
Maximum deflection distance, cm; thalamic diameter, mm	0.1548	0.2607	
Maximum deflection distance, cm; effective angle	0.3593	0.0670	
1BEHIND EAR placement—Bolt entry directly caudal to the pinna of the ear on the same plane as the eyes and targeting the middle of the opposite eye.

Significant (P < 0.05) linear relationships were identified between head weight and FRONTAL cranial thickness (Figure 8) and total tissue thickness (Figure 9). For each kg of head weight, the expected FRONTAL cranial thickness increased by 1.91 ± 0.24 mm (INTERCEPT = 10.53 ± 4.81, R2 = 0.7815, P < 0.0001) and the expected FRONTAL total tissue thickness increased by 1.88 ± 0.22 mm (INTERCEPT = 18.95 ± 4.44, R2 = 0.8028, P < 0.0001). These findings are supported by those of Anderson et al. (2021a, 2024) and Hamilton et al. (2023) who also reported that head weight was a significant predictor of FRONTAL cranial thickness and total tissue thickness for mature swine and market hogs, respectively. Hamilton et al. (2023) also found that head weight was a significant predictor of soft tissue thickness for both physically castrated market barrows and immunocastrated boars, which is in contrast with our findings.

Figure 8. Relationship between FRONTAL cranial thickness measurements (mm) and head weight (kg) of cadaver heads from sows >200 kg (n = 20).

Figure 9. Relationship between FRONTAL total tissue thickness measurements (mm) and head weight (kg) of cadaver heads from sows >200 kg (n = 20).

Distance between optic orbits was a significant linear predictor for BEHIND EAR thalamic depth (Figure 10). For each centimeter of distance between optic orbits, the BEHIND EAR thalamic depth increased by 3.99 ± 0.72 mm (INTERCEPT = 51.68 ± 10.95, R2 = 0.7938, P = 0.0005). To the best of our knowledge, this is the first study to evaluate the relationship between the distance between optic orbits and tissue depth parameters for swine.

Figure 10. Relationship between BEHIND EAR thalamic depth (mm) and distance between optic orbits (cm) cadaver heads from sows >200 kg (n = 10).

A significant (P < 0.05) linear relationship was observed between maximum deflection distance and FRONTAL soft tissue thickness (Figure 11). For each centimeter of maximum deflection distance, the expected FRONTAL soft tissue thickness increased by 0.58 ± 0.20 mm (INTERCEPT = 5.50 ± 0.84, R2 = 0.3183, P = 0.0096). This is in contrast with the findings of Anderson et al. (2021a, 2024) and Hamilton et al. (2023), as a predictive linear relationship between maximum deflection distance and FRONTAL soft tissue thickness was not observed for mature swine or market hogs. In alignment with the current findings, Anderson et al. (2024) did not observe a linear predictive relationship between maximum deflection distance and FRONTAL cranial or total tissue thickness for sows <200 kg BW. Previously, we have observed a predictive linear relationship between maximum deflection distance and FRONTAL cranial thickness and total tissue thickness for sows >200 kg BW (Anderson et al., 2021a), as well as maximum deflection distance and FRONTAL cranial thickness for physically castrated market barrows (Hamilton et al., 2023). Research to further understand the relationship between maximum deflection distance and tissue thickness measurements would contribute to an improved understanding of the relationship between face shape and challenges with the FRONTAL PCB application.

Figure 11. Relationship between FRONTAL soft tissue thickness measurements (mm) and maximum deflection distance (cm) of cadaver heads from sows >200 kg (n = 10).

Conclusions related directly to the efficacy of either the FRONTAL or BEHIND EAR PCB placement, or the INLINE or PISTOL PCB device types, cannot be made from this study because cadaver heads were used (Anderson et al., 2021a, 2024). Our findings suggest that the FRONTAL placement may be favorable to the BEHIND EAR placement for sows >200 kg, due to less tissue for the bolt to travel through to reach the brain, and the thalamus within the brain. However, to fully understand the efficacy of these PCB placement-treatment combinations for sows >200 kg BW, studies utilizing live animals should be conducted, expanding upon this work and the work done by Kramer et al. (2021). All cadaver heads in this study had the expansive sinus cavities that are characteristic of mature sows (EFSA, 2004, 2020; Woods et al., 2010; HSA, 2016; NPB and AASV, 2016; Anderson et al., 2021a, 2024).

It is important to acknowledge that additional limitations from the use of cadaver heads exist. Because cadaver heads were sourced from animals which were completely exsanguinated, there was no blood supply to the brain and it was not possible to assess hemorrhage as described by Millar and Mills (2000), Woods (2012), and Kramer et al. (2021, 2022). Additionally, TBI scoring systems, including those used by Millar and Mills (2000), Woods (2012), and Kramer et al. (2021, 2022), could not be used because whole, intact brains were not available for this type of brain damage assessment. The application of all PCB shots in a highly controlled laboratory environment likely resulted in a level of accuracy and precision with placement that could not be achieved on-farm or in slaughter establishments. As a result, the lesser total tissue thickness and thalamic depth, in addition to the greater effective angle, associated with the FRONTAL PCB placement may be advantageous.

Implications

Of the 2 PCB placements evaluated in this study, the FRONTAL placement appeared to be more reliable in reaching the brain than the BEHIND EAR placement. This was demonstrated through less total tissue thickness and thalamic depth at the FRONTAL placement, along with prevalent brain damage that resulted from both the INLINE and PISTOL PCB devices. The FRONTAL placement also allows for a greater degree of error with the angle of bolt travel from the site of the PCB application. Of the 2 PCB devices evaluated in this study, the INLINE device may be more appropriate for use with mature sows than the PISTOL device. This was indicated by the combination of greater manufacturer-indicated bolt travel distance for the INLINE PCB and less total tissue thickness in the FRONTAL placement, which may allow for the bolt and the bone fragment produced from the bolt to reach the level of the thalamus with greater consistency than the PISTOL PCB device. Although it was not the focus of this study, the different bolt velocities, and associated kinetic energies, of the PCB devices used in this study may impact the efficacy of these placement-device combinations for rendering sows >200 kg BW immediately insensible and causing subsequent death. The greater amount of soft tissue at the BEHIND EAR placement may absorb more kinetic energy than the FRONTAL placement, thus leading to a lesser concussive impact. Ultimately, the FRONTAL placement with an INLINE PCB may present the least risk of failure for the PCB euthanasia of mature sows >200 kg BW.

Acknowledgments

We would like to thank the commercial slaughter establishment that provided the cadaver heads for this research. We greatly appreciated the data collection assistance of O. Barber, M. Cowell, B. Dittrich, L. Geist, R. Heap, E. Hamilton, E. Hensman, A. Kinnaird, K. Kirk, A. Matzek, H. Olsen, B. Plazcz, M. Redmann, M. Seehusen, S. Sigl, L. Streich, A. Tomandl, and G. Wellnitz, (all affiliated with the University of Wisconsin—River Falls at the time of this study). We also appreciated assistance from A. Rendahl with upper reference limit calculations. The provision of captive bolt stunning equipment by Jarvis Corporation and technical support by Bunzl Processor Division was greatly appreciated. We appreciated the space and accommodations provided by the University of Wisconsin—River Falls Meat Lab and manager R. Rehnelt. We would also like to thank the commercial meat processing company that provided the bandsaw used for this research. This work was supported by the Agriculture and Food Research Initiative competitive award no. 2022-67016-36344 from the USDA National Institute of Food and Agriculture.

Abbreviations

ARAS ascending reticular activating system

BW body weight

CI confidence interval

HCW hot carcass weight

PCB penetrating captive bolt

TBI traumatic brain injury

URL upper reference limit (97.5% reference interval)

URL 90% CI the 90% confidence interval of the upper reference limit

Conflict of interest statement

The authors declare no real or perceived conflicts of interest.
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