Physical castration is a common procedure performed on young piglets usually at 2–3 days of age and is a standard practice in many countries, including the United States. Castration is a procedure that causes prolonged pain and is detrimental to the welfare of piglets (1, 2). Therefore, some countries (such as the EU and Israel) are incorporating analgesia during castration and other mutilations (3), or are considering the use of long-acting analgesics or anesthetics in all castrated pigs (4, 5).

Pigs are castrated early in life to prevent boar taint in the meat of sexually mature males and to reduce aggressive behavior. During castration one incision can be made or two incisions, one on each side of the scrotum (1). The testicles are externalized, grasped, and pulled out, tearing the spermatic cord, or the spermatic cord is cut with the scalpel blade (6). Literature is limited on the benefits of one incision versus two incisions, but there is speculation that one incision would be less painful than two, considering that the skin of the scrotum and tissues have sensory and motor innervations (1). Additionally, there is anecdotal evidence to suggest wounds heal quicker when only one incision is made because the amount of tissue damage is less, so the pain would be less. However, there is no scientific evidence that suggests which method may be preferable from an animal welfare perspective.

Castration is usually performed without any type of analgesia (7–9) and therefore, it is a painful (8, 10, 11) and stressful procedure in young pigs. Various studies have shown that physical castration leads to changes in behavior, which may be indicative of pain (12–16). Additionally, physical castration can lead to acute changes in physiology including the activation of the hypothalamic-pituitary-adrenal axis (HPA) and the activation of the sympathetic nervous system (SNS) (6, 17, 18). Physical castration in piglets also induces strong vocal responses (17–20). Screams (classified as having a higher frequency than squeal) have been reported as being more predominant during castration without anesthesia (19). These high-frequency calls are indicative of pain, mainly associated with the pulling and severing of the spermatic cord. Behavioral observations over a period of 5 days showed that physically castrated piglets spent significantly less time at the udder, had decreased activity while awake during the first 2.5 h, and overall tended to walk more throughout the 5 days post castration (6). Behaviors such as stiffness, prostration, and trembling are common for the first few hours after physical castration but in the days following, piglets show other pain-like behaviors like scratching their rump and tail wagging (6).

The use of general anesthetics and local anesthetics may be limited by the regulations and economics of the swine industry. Currently, the most used local anesthetic is lidocaine (21, 22). However, lidocaine injected into the scrotum and testicles can also cause additional pain and distress to piglets. Furthermore, lidocaine concentration is highest 3 min after injection, and thus, piglets would have to be castrated within that period, which may be impractical at farms because of the increased time and stress caused by repeatedly handling pigs to administer the injection and then castrate them (18). Ethyl chloride sprays are topical anesthetics with immediate skin anesthesia by rapid evaporation of volatile liquid, which produces a drop in skin temperature, resulting in temporary and swift interruption of skin sensation (23). Ethyl chloride has previously been used for breast needle biopsies, during intravenous cannulation (24, 25), and before immunizations (26–28) in humans. In livestock, they have been used to reduce the pain of ear tagging and ear notching in unweaned calves (29). Although the use of ethyl chloride in humans is widely reported to reduce pain prior to painful procedures, its use in farm/livestock animals is vaguely reported.

Meloxicam is a non-steroidal anti-inflammatory drug (NSAID) that has analgesic effects due to its peripheral anti-inflammatory actions (30). Meloxicam has been researched extensively for its postoperative analgesic effects in multiple species, including humans and pigs. Meloxicam has marketing authorization for use in non-infectious locomotor disorders to reduce symptoms of lameness and inflammation, but more recently for relief of postoperative pain associated with castration in piglets (meloxicam, 5 mg ml⁻¹ solution for injection (3, 30). In a study, von Borell et al. (2) reported that 2 mg/kg BW meloxicam given intramuscularly 15 min before castration prevented a rise in cortisol concentration after castration, suggesting that it was effective in reducing pain. Therefore, one incision castration with the use of ethyl chloride in combination with meloxicam (0.4 mg/kg BW, according to the manufacturer's label) could reduce some of the pain associated with castration compared to not using anything at all. Therefore, this study was divided into two parts, Study 1 and Study 2. The objectives of study 1 were to evaluate the effect of one incision compared to two incisions for physical castration and the use of ethyl chloride applied before castration, and the objective for study 2 was to evaluate the effect of the best method found in objective 1 and the use of meloxicam before castration using measures of behavior, performance, and physiology.

All animal procedures were approved by the Texas Tech University (TTU) Animal Care and Use Committee (protocol number: 17079-09). The experiment was conducted partially at the Texas Tech University New Deal Swine Facility and a commercial site.

Piglets were housed in farrowing crates with their dam. The farrowing crates were slatted, and piglets were on a milk-liquid diet. Heat lamps were provided in every crate. The temperature in the barns was kept at an average of 75°F.

Castration is a stressful and painful procedure for pigs (18, 34, 35). However, finding a method to reduce the pain of castration has proven to be challenging, as it can be time-consuming to apply and expensive. The main goal of this study was to identify if modification of the castration technique such as one incision versus two incisions would improve pig welfare, based on measures of performance, behavior, and physiology. An additional goal was to determine if analgesics, such as a topical ethyl chloride or an injectable analgesic, such as meloxicam, would mitigate the pain during or after castration or both.

Based on performance measures, pigs castrated with one incision (C1) and one incision plus ethyl chloride (C1V) had approximately 50 gm/day higher average daily gain (ADG) compared to the pigs castrated with two incisions with or without ethyl chloride (C2 or C2V). The 50 gm/day of higher average daily gain in one incision castration with or without ethyl chloride can be considered to have significant biological relevance compared to other two 2-incision castration groups pigs when a long production timeline is considered. The effect of C1 and C1V on performance measures indicated that these methods of physical castration were no different from non-castrated pigs. Though McGlone et al. (13) reported increased weight at weaning in piglets castrated at 14 days of age. Hay et al. (6) and Carroll et al. (34) found no significant effect of castration on the body weight of pigs than non-castrated pigs.

The behavioral data showed no differences among treatments in the hour post-castration nor during the 24 h period the pigs were observed except for vocalization behavior. This finding contrasts with other studies that have shown that there are behavioral changes after castration (6, 13, 18). All the pigs were handled multiple times after castration for weighing and blood sampling, which can be stressful and may have affected their behavior by masking some of the effects of castration among treatment groups. Vocalization data did show a difference among treatment groups. Pigs in the C1 and C1V treatment groups had louder scream but unlike C1, C1V did not differ from the C2 and C2V treatment groups. Pig scream, associated with the stress and pain induced during castration, should have a high sound power (dB) (36) and is different than other vocalization sound (19, 37). Von Borell et al. (37) reported 15 dB higher relative sound energy of vocalization compared to grunting and squealing sound. It was observed that it is more difficult to grasp the second testicle out of one incision versus two incisions. During castration when the incision was made on the skin and the first testicle was removed, there is a connective tissue called perineal raphe that needs to be incised in order to access the second testicle (38). When cutting through the perineal raphe, if the researcher cut too deep, the incision went all the way through the testicle. It was observed that incising through the testicle caused exaggerated pain to the pig, as a researcher recorded that the screams seemed louder during the training phase. The use of ethyl chloride seemed to mitigate some of the pain but not all as can be seen in the data where the use of the ethyl chloride during one-incision castration did not differ from two-incision castration and two-incision castration plus ethyl chloride. One-incision castration is not commonly used in the swine industry. Thus, if adopting this technique, the workers must be trained until proficient or the acute pain of cutting through a testicle is worse than the pain of 2-incision castration. Due to the shortage of labor in the swine industry, it will be difficult to adopt a technology that is more labor-intensive if the benefits are not economically significant.

Blood chemistry measures such as total protein (TP), blood urea nitrogen (BUN), and glucose (GLU) was found to be different between treatment groups. Total protein was higher in the C1 treatment group, which may indicate they were possibly not nursing as much, or they experienced more stress and protein catabolism. Total protein and albumin concentrations are markers for protein homeostasis, which increase with dehydration (39). However, reduced nursing behavior was not supported by the behavior data collected, as it showed no difference in nursing behavior among treatment groups. Additionally, the stress of handling and bleeding may have affected these values. This can be seen in the difference in TP values for NO and SH pigs. SH pigs were exposed to handling and weighing plus sham castration (which may have added more stress), possibly leading to the TP difference between NO and SH pigs. BUN values were also higher for the C1 treatment compared to the other treatment groups. Blood urea nitrogen is a metabolic waste product in the blood generated from the breakdown and probably due to muscle and protein breakdown caused by muscle exertion, and has been reported during transport (39). GLU was significantly different among treatment groups. GLU was the highest for NO and C2 pigs, but both groups were not significantly different from SH pigs. C1, C1V, and C2V were not significantly different than each other. There was no time effect. This may indicate that pigs continued to nurse during the 24 h post-castration. In this regard, E. Pérez-Pedraza et al. (38) found no significant differences in glucose concentration among treatment groups in a study on the effect of the number of castration incisions and the use of local anesthesia on physiological parameters. A difference in cortisol values among treatment groups was not identified. The cortisol findings support the use of meloxicam in reducing stress associated with castration at T30 and T180 post-castration. At T15, the stress of castration was high regardless of the use of meloxicam. This may indicate that the administration of the product was not effective at the time given and may need to be administered more than 15 min prior to castration.

Wound scores were also not different among treatment groups. This contrasts with Sutherland et al. (18) who used a topical anesthetic (lidocaine) to reduce the pain of castration and observed differences in wound healing at 9 and 14 days.

Based on measures of performance, behavior, and physiology it was determined that one incision castration would be used versus C1V because the differences were not great between the two treatments, and the data were not sufficiently clear to conclude that C1V was better than C1. C1V did not have a significant effect on reducing the pain of the incisions made during castration. In fact, when the castration incisions were made, pigs were not observed to squeal but mainly grunt. When the testicles were gripped and the spermatic cord was cut, pigs squeal at high frequencies and intensities. This is in agreement with other studies that have suggested that cutting the spermatic cord is more painful (17, 40, 41) significantly more than the incision (16). Thus, C1 was chosen to be part of Study 2.

Study 2 consisted of evaluating if one incision castration and the use of an anti-inflammatory 15 min prior to castration would reduce the pain of castration based on behavior, performance, and physiology. Performance measures did not differ among treatment groups. The behavioral data showed no difference among treatment groups in the hour post-castration nor during the rest of the 24 h period, the pigs were observed.

During surgical castration, the incision induces localized stinging pain of cutaneous origin, but the pain and vocalization are greater while extracting and severing the spermatic cord than sham handling or initial cutaneous incision (19). NSAIDs produce analgesia principally by suppressing the synthesis of prostaglandin and other inflammatory mediators during post-castration (42). NSAIDs may act to mitigate surgical stress and pain (42, 43) but the acute pain associated with scrotal incision and severing of the spermatic cord is not mitigated by NSAIDs like meloxicam (44). In a study, von Borell et al. (2) reported that 2 mg/kg BW meloxicam given intramuscularly 15 min before castration prevented a rise in cortisol concentration after castration, suggesting that it was effective in reducing pain. However, this study showed that the use of meloxicam was not effective at reducing pain, as vocalization data did not differ among treatment groups in this study. Consistent with this finding, Keita et al. (30) reported no significant effect of meloxicam in vocalizations for piglets surgically castrated without the administration of pre-emptive meloxicam. Similarly, Kluivers-Poodt et al. (40) using temporal, waveform, and spectral parameters to measure the vocal response during castration found no differences in vocalization in meloxicam-treated piglets compared to piglets surgically castrated without meloxicam.

Physiological measures included blood hematology and chemistry. Blood measures were not significantly different among treatment groups, except for red blood cells (P = 0.0304; Figure 10). C1 pigs had higher values than C1M pigs and No pigs. This may be due to a disruption in RBC synthesis, and blood clotting that may be impacted negatively by meloxicam. Cortisol values were different among treatment groups. The cortisol findings support the use of meloxicam in reducing stress associated with castration at T30 and T180 post-castration. At T15, the stress of castration was high regardless of the use of meloxicam. This may indicate that the administration of the product was not effective at the time given and may need to be administered more than 15 min prior to castration. In this regard, Keita et al. (30) found an increase in cortisol in blood for short time followed by a significant reduction post-castration in pigs that were administered meloxicam preoperatively only. Similarly, Zöls et al. (45) in a study on the effects of analgesics on the castration of piglets found significantly high cortisol concentrations in 1 and 4-h post-castration in piglets castrated without preoperative meloxicam whereas no significant rise in cortisol was noted in piglets administered with meloxicam preoperatively. Likewise, the wound score was not significantly different among the treatment groups.

This study had some limitations. First, a standard stainless-steel weight of 2 kg was used to calibrate the weighing scale and was used throughout the study to measure the weights of pigs. This could be a limitation as the linearity of the measurements was not established for weight greater than 2 kg.

In addition, it is not sure if meloxicam is an effective analgesic for acute pain of castration. Previous literature suggested it has some benefit in a castration model (46, 47). This is a general problem for farm animal analgesia products; few have been extensively tested in different species in different potentially painful situations. The marginal efficacy of meloxicam in this study suggests that meloxicam is not expected to modify the acute surgical pain but more to change postoperative pain associated with inflammation. In addition, we do not have a good indication of dose and timing for use of Meloxicam (and most analgesics) in farm animals experiencing different types of pain.

Likewise, Ethyl chloride was intended to address the acute pain associated with the procedure of castration. As with Meloxicam, ethyl chloride analgesic properties were not previously validated as effective in each painful experience.

In conclusion, the data collected gives sufficient insight into the benefits and limitations of one-incision castration compared to two-incision castration. The use of vapocoolant; ethyl chloride has no effect on reducing the pain associated with castration incisions. Additionally, meloxicam may have removed stress due to inflammatory pain, but no other effects were seen. Further research could potentially identify further positive and negative effects of meloxicam, and the correct timing and dose for administration to truly identify if it could mitigate the pain associated with one incision castration.