A Comparison of the Health and Production Effects of Local Anaesthetic and Non-Steroidal Anti-Inflammatory Drugs with and Without Xylazine Sedation for Calf Disbudding

Abstract

Use of sedation for disbudding is common practice in a number European countries, with United Kingdom (UK) practices adopting its use. This study assessed the effects of disbudding with and without xylazine sedation on growth rates and calf health on a UK calf rearing unit. Data was collected from 485 dairy crossed with beef breed calves between April and August 2024 from a single calf rearing unit in England. Calves were purchased from multiple farms across the UK and arrived on site at approximately 21 days of age. Calves were disbudded—and, in the case of male calves, surgically castrated—at approximately three weeks after arrival on farm. Pens of calves were assigned to undergo disbudding with (SED, n = 238) or without (CTL = 234) xylazine sedation at a dose of 0.2 mg/kg administered intramuscularly. Calves from both groups were provided with local anaesthetic (procaine hydrochloride) as a cornual nerve block and a non-steroidal anti-inflammatory drug (meloxicam). While other studies have demonstrated some behavioural and physiological indicators of pain to be reduced with sedation, this study found that calves in the SED group had a reduced daily liveweight gain (DLWG) of 0.14 kg/day in the short term (mean 20 days) following disbudding (p < 0.001), but there was no difference in growth rates in the medium-term (mean 43 days) post-disbudding (p = 0.30). Some of this difference could be explained by the slightly higher DLWG pre-disbudding in the CTR group, and it is likely that the physiological impacts of sedation accounted for the rest of this difference. This initial reduction in DLWG following disbudding with sedation should be considered by vets, especially on farms where growth rates may already be compromised. In the sedated calves, 19.3% exhibited either some movement or entry into sternal recumbency. Specifically, a light plane of sedation with calves entering sternal recumbency was associated with a reduction in DLWG of 0.89 kg/day compared to 0.98 kg/day for those that remained in lateral recumbency throughout (p = 0.008). The light plane of sedation may have created additional calf stress, impacted feeding behaviours, and impinged welfare, with further work needed to establish the reasons for insufficient sedation. There was no difference in the number of post-disbudding treatment outcomes between calves disbudded with and without sedation (p = 0.97).

1. Introduction

Disbudding is a routine management procedure in the United Kingdom (UK), conducted to ensure the safety of people and animals on highly stocked cattle farms. It involves the removal of immature horn tissue (buds) prior to their attachment to the skull [1]. As calves grow, their horn buds attach to the frontal sinus, becoming horns which are more difficult to remove [2]. Disbudding generally occurs under the age of two to three months [3], when the horn buds are easily palpable and at a length of around 5–10 mm [4]. Two predominant methods exist for disbudding: thermal cautery and the use of caustic paste. Thermal cautery is often preferred, with a survey reporting that 80% of disbudding in the European Union is carried out by this method [5]. In this technique, a hot iron is applied for approximately 15–30 s to the horn bud. Although techniques vary throughout the literature, the aim is to destroy the epidermal and dermal layers containing the germinal cells around the horn bud [6]. Disbudding in this way is typically performed at around 4–6 weeks of age, with the most common UK farm assurance scheme, Red Tractor (https://redtractorassurance.org.uk/wp-content/uploads/2022/08/Dairy-V5-Standards-Aug-22-FINAL.pdf, accessed on 1 April 2025), stipulating it can be carried out in calves up to 2 months of age when performed by a competent stockperson.

It is well known that disbudding by any method is a painful procedure, with this pain persisting for up to 11 days post-operatively [7]. Wound healing post-disbudding can take around nine weeks, and discomfort can be present for the entire duration of the healing process, exacerbated by the presence of necrotic tissue which inhibits healing. Pain mitigation is therefore a key animal welfare issue, and as such, the European Code of recommendations for welfare of cattle suggests that disbudding is performed under local anaesthetic (LA), usually achieved with a cornual nerve block [8], and systemic non-steroidal anti-inflammatory drugs (NSAIDs). The LA reduces the initial increase in plasma cortisol concentrations, typically in the first three hours after disbudding, whereas NSAIDs work to reduce the secondary cortisol peak, usually 4–6 h after the procedure [1].

Use of sedation for disbudding is common practice, and it is even a legal requirement in some European countries (France, Belgium, Germany), with UK practices adopting its use. Xylazine is an α2-agonist licenced for use in cattle. Sedation with xylazine can eliminate the cortisol stress response for 3 h following disbudding, and it has been suggested that a combination of LA, NSAIDs, and sedation could be optimal for the disbudding procedure [4]. Reedman et al. (2010) [9] assessed the immediate impacts of sedative use in disbudding with LA and NSAID and found that, in the first 24 h following disbudding, calves demonstrated reduced behavioural and physiological indicators of pain when they had been sedated, including a faster resumption of play behaviour. However, they also found that sedated calves had reduced milk drinking speed for 48 h following sedation, although there was no difference in total milk consumed. Similarly, Grøndahl-Nielsen et al. (1999) [10] found that, in the week following disbudding, calves that received sedation grew more than calves that only had LA for disbudding, and Bates et al. (2016) [11] demonstrated that use of sedation and LA at disbudding provided improved growth rates compared with those given NSAIDs as the sole pain mitigation measure.

Given these findings have focused on the immediate period following disbudding, it would be beneficial to know what the longer-term impacts of disbudding with sedation may be on growth rates and calf health. Both LA and NSAID use for disbudding is considered as the gold standard protocol in the UK, and indeed, their use is a contract requirement for many farms. This study therefore aimed to investigate the impact of combining xylazine sedation with LA and NSAID for the disbudding procedure. We hypothesised that calves sedated for disbudding would have improved growth rates in the short and medium term following disbudding, as well as reduced antimicrobial treatment rates, compared to those undergoing the procedure with LA and NSAID only.

2. Materials and Methods

2.1. Farm Overview

This study was granted ethical approval from the Clinical Research Ethical Review Board at The Royal Veterinary College, University of London (URN 2024 2267-3).

A single calf rearing unit in the Southwest of England was enrolled for the study, which took place from April through to August 2024. A new batch of 80 to 160 different dairy cross breed (Holstein, Aberdeen Angus, Belgian Blue, Longhorn, and Hereford predominantly) calves were purchased from multiple farms across the UK, and they arrived on the farm every two weeks at approximately 21 days of age. Calves were weighed on entry using electronic weigh scales, and for the purpose of this trial, they were grouped according to sex and size (bodyweight < 60 kg and ≥60 kg) into pens of ten calves. Following arrival, calves were vaccinated against Parainfluenza 3 virus, Bovine Respiratory Syncytial virus, Mannheimia haemolytica serotype A1, Bovine Viral Diarrhoea Virus, and Bovine Herpes Virus type 1 according to the product descriptions; they also received administration of diclazuril (Dycoxan 2.5 mg/mL Oral Suspension for sheep and cattle, Chanelle Pharma, Galway, Ireland). Calves were fed a commercial milk replacer through a group trough system for each pen, where 3 litres per calf was provided twice a day, totalling 900 g/day of milk replacer per calf per day. The milk feeding strategy was a 42-day on-farm plan, including a 14-day step down process for weaning. Concentrate feed (18% crude protein) and straw was provided for the calves ad lib. The calves were housed in sheds of approximately 120 calves, typically consisting of 12 pens, each with 10 calves. The sheds were naturally ventilated and bedded with straw daily.

Calves were routinely subjected to disbudding—and, in the case of male calves, surgical castration—approximately three weeks after arrival on farm. These procedures were performed by veterinary surgeons with the assistance of trained veterinary technicians from a single veterinary practice in the Southwest of England. Trained farm staff routinely health checked all calves twice daily following a calf respiratory scoring method [12] and administered appropriate treatments where required. This included florfenicol (Fenflor, KRKA UK Ltd, Slough, UK, 40 mg/kg) and meloxicam (Metacam, Ingelheim Animal Health UK Ltd., Bracknell, UK, 20 mg/mL) for respiratory disease. Farm staff were blind to the research treatment group of the calves for these daily clinical assessments, as calves were only impacted by the treatment group at the immediate time of disbudding.

2.2. Study Population

A sample size calculation was performed in PS Power and Sample Size (PS Version 3.1.6). Using a 0.11 kg/day difference a day in daily liveweight gain reported by Bates et al. (2016), in calves disbudded with sedation and LA compared to those calves disbudded with NSAID only (p = 0.011), gave an effect size of 0.62. Using a confidence level of 0.95 and a power of 0.8, the sample size for detecting a significant difference of 0.11 kg/day growth was 480 calves.

All treatments were carried out in the pen level groups to reduce stress that may be associated with having a mixture of sedated and non-sedated calves within a pen. The treatment group was systematically assigned to control for sex and bodyweight of calves. All calves were provided with LA using procaine hydrochloride (Adrenacaine Solution for Injection for Cattle) as a cornual nerve block with 3 mL per side, and they were also administered NSAID with meloxicam (Metacam 20 mg/mL solution for injection for Cattle, Pigs and Horses, Boehringer Ingelheim Animal Health UK Ltd., Bracknell, Berkshire, UK), given subcutaneously at a dose of 0.5 mg/kg. A topical chlortetracycline spray (Cyclo Spra, Dechra Veterinary Products, Cheshire, UK) was applied to the disbudding site post-procedure. The treatment groups the calves could be assigned to were either sedation (SED) with xylazine (Chanazine 2%, Chanelle Pharma) at a dose of 0.2 mg/kg administered intramuscularly or a control group (CTL) who just received LA and NSAID. Xylazine sedation was used under an Animal Test Certificate (ATC Type S 51276/0002) obtained from the Veterinary Medicines Directorate; all other medications were used within licenced indication. Immediately prior to disbudding, each calf was weighed using a digital weigh scale (Tru-Test Datamars, Temple, TX, USA), calibrated to measure to the nearest 0.5 kg. All calves in the SED group were then administered xylazine and left for a minimum of five minutes to become recumbent before the administration of LA and NSAID. All male calves were also provided with 10 mL intratesticular LA for castration. All LA was left for approximately 10 min prior to starting the disbudding process, which was then followed by castration as required.

In the SED group, the researchers recorded any response to the disbudding procedure as some movement during procedure, calf entered sternal recumbency, or calf remained in lateral recumbency.

The calves were weighted on entry, at disbudding, and at approximately three and six weeks post-disbudding. At weighing, researchers were blinded to the treatment group of the calves. Weight data was used to calculate average daily liveweight gain (DLWG). The calf treatment data was extracted from farm records. All data were collated and stored in Microsoft Excel^® (Version 2309, Microsoft, Redmond, WA, USA).

2.3. Statistical Analysis

Data was transferred to IBM SPSS Statistics (Version 29.0.0, IBM SPSS Statistics for Windows, Armonk, NY, USA: IBM Corp) for analysis. Calf breed was dichotomised into either continental or native breeds; British Blue X, Simmental X, Salers X, Wagyu X, and Limousin X calves were classed as continental, with the remainder being classed as native (Beef Shorthorn, Devon Red X, Hereford X, Longhorn X).

Following descriptive statistics, the outcome of daily liveweight gain (DLWG) at short-term and medium-term post-disbudding were analysed using a generalised estimating equation (GEE) linear model with an exchangeable working matrix. The subject variable was pen to account for the repeated measures within a pen of calves. Variables included in the model included sex, breed, treatment group, intake (taking into account the housing shed location and date of entry of the calf onto the farm), time period of weights (short or medium term), response to sedation, number of antimicrobial treatments pre- and post-disbudding, weight on entry, pre-disbudding DLWG, days on farm at time of disbudding, days to weigh events, and a pairwise interaction between time period and treatment group. Backwards elimination was performed on the initial model to exclude any variables with a p-value > 0.3 to create the final model. Model fit was checked by assessment of the residuals for a normal distribution.

A Poisson loglinear GEE model was constructed to assess the number of post-disbudding antibiotic treatments calves received. The subject variable was pen to account for the repeated measures within a pen of calves. Variables included in the model were sex, dichotomised breed, treatment group, intake, number of pre-disbudding antimicrobial treatments, response to sedation, weight on entry, days on farm, and pre-disbudding DLWG. Backwards elimination was performed on the initial model to exclude any variables with a p-value > 0.3 to create the final model.

3. Results

A total of 485 calves were enrolled in the study, with a descriptive summary given in

Table 1. Descriptive parameters of the calves in the different treatment groups of SED (calves receiving sedation for disbudding) and CTR (calves only receiving local anaesthetic and non-steroidal anti-inflammatory drugs). DLWG is the average daily liveweight gain. Values in brackets are the standard deviation. The number of treatments is split by the number of calves receiving 0 treatments, 1 treatment, 2 treatments, or 3 treatments within the specified time period.

Parameter	Treatment Group
	SED	CTR
Sex (Male/Female)	125:110	107:123
Breed (Cont/Nat)	185:50	187:43
Weight on entry (kg)	65.2 (9.7)	65.9 (12.7)
Days on farm at disbudding	18.1 (3.7)	19.0 (5.2)
DLWG pre-disbudding (kg/day)	0.61 (0.37)	0.65 (0.39)
DLWG short term	0.79 (0.33)	0.86 (0.35)
DLWG medium term	0.96 (0.34)	0.92 (0.30)
Pre-disbudding number of treatments [0:1:2:3]	34:157:43:1	77:121:30:2
Short-term post-disbudding number of treatments (0:1:2:3)	167:60:8:0	175:49:6:0
Medium-term post-disbudding number of treatments (0:1:2:3)	180:46:8:1	188:35:6:0

. Of the enrolled calves, 244 were assigned to the SED group and 241 to the CTL group. Due to missing weight or treatment data, or misidentification of calves at the time of disbudding, 20 calves were excluded, leaving a final study population of 465 calves available for analysis: SED (n = 235) and CTL (n = 230) (Table 1).

After the disbudding procedure, the mean number of days to the first weight was 20 (SD 6.2, range 6–34 days). The mean number of days from disbudding to second weight was 43 days (SD 6.4, range 30–62 days).

Between arriving on the rearing unit and disbudding, 57.3% (n = 278) of calves received one antimicrobial treatment, 15.3% (n = 74) of calves received two, and 0.6% (n = 3) of calves received three. After the disbudding procedure, 16.7% (n = 81) of calves received one antimicrobial treatment, 2.9% (n = 14) received two, and 0.4% (n = 2) received three.

Of the sedated calves, 197 (80.7%) remained in lateral recumbency, with no response noted related to the procedure; 26 (10.7%) presented in sternal recumbency (a very light plane of sedation); and 21 (8.6%) calves exhibited some movement in response to the disbudding procedure (a light plane of sedation) and may have required some extra manual restraint during disbudding.

3.1. Post-Disbudding Daily Liveweight Gain (DLWG)

The overall DLWG of the calves from entry onto the calf rearing unit (approximately 18.5 days prior to being disbudded) until approximately 6 weeks post-disbudding was 0.88 kg/day (SD 0.23). When considering the overall DLWG from the time of disbudding to approximately 6 weeks following disbudding, the DLWG for the CTL group was 0.92 kg/day (SE 0.03), and for the SED group, it was 0.86 kg/day (SE 0.03).

The generalised estimating equation (GEE) had the variables sex, pre-disbudding treatment number, and post-disbudding treatment number removed from the final model due to a p > 0.3. The final GEE model fit QIC value was 136.5 indicating a good fit [13]. Overall, the time period was significantly associated with the DLWG such that calves grew faster in the medium term (mean 43 days post-disbudding) than in the short term (mean 20 days post-disbudding) (p = 0.04, 1.07 kg/day compared to 0.81 kg/day, respectively); see Figure 1 and

Table 2. Parameters for the generalised estimating equation model for daily liveweight gain post-disbudding. The treatment groups were SED (calves receiving sedation for disbudding) and CTR (calves only receiving local anaesthetic and non-steroidal anti-inflammatory drugs).

Parameter		Estimate	95% CI	p-Value
Intercept		0.42	0.097–0.75	0.01 *
Breed	Continental	−0.14	−0.21–−0.06	<0.001 **
	Native	-	-	-
Treatment Group	SED	−0.04	−0.12–0.04	0.28
	CTR	-	-	-
Intake	1	0.16	−0.05–0.37	0.14
	2	0.03	−0.20–0.25	0.83
	3	0.15	−0.09–0.39	0.21
	4	−0.08	−0.30–1.3	0.45
	5	0.13	−0.10–0.36	0.27
	6	0.13	−0.09–0.34	0.26
	7	−0.06	−0.27–0.16	0.24
	8	0.19	−0.06–0.44	0.60
	9	0.001	−0.23–0.23	0.13
	10	0.14	−0.12–0.40	0.99
	11	0.008	−0.21–0.22	0.29
	12	-	-	-
	Overall			<0.001 **
Response to sedation	Lateral	0.09	0.03–0.15	0.006 **
	Some Movement	0.1	0.03–0.17	0.005 **
	Sternal	-	-	-
	Overall			0.007 *
Time Period	Short term	−0.09	−0.17–−0.003	0.04 **
	Medium term	-	-	-
Entry Weight	0.008		0.006–0.011	<0.001 **
Days on farm at disbudding	0.005		−0.003–0.013	0.22
Pre-disbudding DLWG	0.069		−0.005–0.142	0.07 #
Days at which weight measured	−0.005		−0.012–0.002	0.15
Interaction Treatment group × Time period	−0.095		−0.20–0.01	0.08 #

# p < 0.10, * p < 0.05, ** p < 0.001.

.

The GEE model demonstrated that although the treatment group alone was not significantly associated with the post-disbudding DLWG (p = 0.28), there was a trend for interaction between treatment group and time period following disbudding (p = 0.08). A post hoc pairwise comparison found that, in the short-term (mean 20 days) post-disbudding period, SED calves had a reduced DLWG by 0.14 kg/day (SE 0.038) compared to CTL calves (p < 0.001, 0.74 kg/day compared to 0.88 kg/day, respectively). In the medium-term period (mean 43 days), there was no significant difference between treatment groups (p = 0.30, 1.09 kg/day compared to 1.05 kg/day, respectively).

Response to sedation was significantly associated with DLWG (p = 0.008) such that calves that remained in lateral had an overall DLWG of 0.98 kg/day, those that demonstrated some movement grew at 0.99 kg/day, and those that entered sternal recumbency grew at 0.89 kg/day (p = 0.007).

The GEE model found that native breeds had significantly higher DLWG than continental breeds (p < 0.001, 1.01 kg/day compared to 0.87 kg/day, respectively). The weight of the calves at entry onto the rearing unit was significantly associated with the post-disbudding DLWG such that, for every 1 kg increase in bodyweight, there was an 8 g/day increase in growth rate (p < 0.001). The intake number onto the farm of the calves was significantly associated with post-disbudding DLWG (p < 0.001), with the mean DLWG ranging from 0.78 kg/day to 1.06 kg/day.

3.2. Number of Antimicrobial Treatments Post-Disbudding

The number of antimicrobial treatments given to calves ranged from 0 to 3, and this was almost exclusively due to detection of respiratory disease through the daily calf scoring protocol. The GEE for the number of treatments given had the variables DLWG, and the number of days between disbudding and final weight removed from the final model due to a p > 0.30. The final model found no significant association with the treatment group on the number of post-disbudding treatments a calf received (p = 0.97), with 71.1–82.6% of calves not requiring any antimicrobial treatments (Table 1). The model demonstrated that number of post-disbudding treatments was significantly associated with breed such that continental breeds received more treatments than native breeds (mean 0.45 compared to 0.38 treatments, respectively, p = 0.023); see

Table 3. Parameters for the GEE with a Poisson loglinear distribution for the number of post-disbudding antimicrobial treatments given to calves based on disease detection by clinical health scoring. The treatment groups were SED (calves receiving sedation for disbudding) and CTR (calves only receiving local anaesthetic and non-steroidal anti-inflammatory drugs).

Parameter		Estimate	95% CI	p-Value
Intercept		0.15	−0.31–0.60	0.53
Sex	Female	−0.09	−0.19–0.01	0.065 #
	Male	-	-	-
Breed	Continental	0.17	0.02–0.31	0.023 *
	Native	-	-	-
Treatment Group	SED	0.003	−0.14–0.15	0.97
	CTR	-	--
Intake	1	−0.1	−0.36–0.17	0.48
	2	0.01	−0.23–0.25	0.92
	3	−0.04	−0.25–0.18	0.72
	4	0.03	−0.23–0.29	0.84
	5	0.12	−0.18–0.42	0.44
	6	0.07	−0.24–0.39	0.64
	7	0.37	0.07–0.67	0.02*
	8	0.21	−0.22–0.64	0.34
	9	0.26	−0.07–0.58	0.12
	10	0.15	−0.11–0.41	0.26
	11	0.26	−0.21–0.73	0.27
	12	-	-	-
	Overall			0.002 *
Response to sedation	Lateral	0.09	−0.06–0.24	0.24
	Some Movement	0.04	−0.15–0.22	0.71
	Sternal	-	-	-
	Overall			0.04 *
Pre-disbudding treatment number	0	−1.64	−1.95–−1.34	<0.001 **
	1	−0.69	−0.82–−0.56	<0.001 **
	2	−0.18	−0.30–- 0.07	0.002 *
	3	-	-	-
	Overall			<0.001 **
Entry weight		−0.004	−0.008–0	0.077 #
Days on Farm at disbudding		−0.04	−0.06–−0.02	<0.001 **
Pre-disbudding DLWG		−0.16	−0.31–−0.01	0.037 *

# p < 0.10; * p < 0.05; ** p < 0.001.

. The number of days that the calf had been on the farm at the time of disbudding was significant to the extent that calves that had been on the farm for longer (p < 0.001) had reduced numbers of treatments. The number of pre-disbudding treatments that a calf had received was significantly positively associated with the number of post-disbudding treatments (p < 0.001) such that as pre-disbudding treatments increased, so did the post-disbudding treatments. Calves with no pre-disbudding treatments had a mean of 0.15 post-disbudding treatments, calves with one pre-disbudding treatment had a mean of 0.39 post-disbudding treatments, calves with two pre-disbudding treatments had a mean of 0.65 post-disbudding treatments, and calves with three pre-disbudding treatments had a mean of 0.78 post-disbudding treatments.

4. Discussion

This study compared the production impacts of using xylazine sedation with local anaesthetic and non-steroidal anti-inflammatory drugs (NSAID) for disbudding calves. Overall, the DLWG of the calves continued to increase from entry onto the farm until approximately 6 weeks following disbudding (and in the case of male calves also being castrated), which suggests that there were minimal negative production consequences of these painful management procedures, despite only one dose of NSAIDs being provided to each calf (Figure 1). This is of interest given that disbudding wounds can be painful for 11 days following the procedure [7] but do not appear to negatively impact production parameters, even if they do alter behaviour.

Of note was the fact that despite male calves also being castrated at the same time as disbudding, sex had no effect on DLWG, although male calves did have a trend towards receiving more treatments post-disbudding than female calves (p = 0.07). Others have found that male calves have higher growth rates than female calves [14,15], so it may be that males calves were impacted by the dual procedures, but without a control group, it is not possible draw further conclusions. However, the lack of difference in DLWG would suggest that applying disbudding and castrating procedures together is generally well tolerated by healthy male calves, and such an approach can be used to reduce the number of stressful events that a calf is exposed to.

Anecdotally, many farmers and vets prefer to carry out disbudding under sedation due to the easier handling and lower labour requirement, but understanding the implications of sedation for the calf itself is important. Regarding the overall DLWG from the time of disbudding to approximately 6 weeks following disbudding, the DLWG for the CTL group was 0.92 kg/day (SE 0.03), and for the SED group, it was 0.86 kg/day (SE 0.03). More specifically, when the interaction between time period and disbudding treatment group was considered based on DLWG, the CTR calves that were disbudded consciously without xylazine had a 0.14 kg/day increase in growth rate in the short term (mean 20 days) following disbudding compared to the SED group (p < 0.001). However, by the medium-term (mean 43 days) post-disbudding, there was no difference between the SED and CTR groups’ DLWG. This highlights that disbudding under sedation does result in a reduction in DLWG in the few weeks immediately following disbudding. The Bates et al. (2016) study assessed growth rates over an extended period where calves receiving sedation grew faster in the first thirty days post-disbudding, but this was in comparison to calves that did not receive any local anaesthetic in the control group [11]. Given that local anaesthetic eliminates escape behaviour during disbudding and reduces the plasma cortisol response [4], this would explain the differences between the study findings. The paper by Grøndahl-Nielsen et al. (1999) assessed growth in the two weeks following disbudding and found no difference between any of the treatment groups, but no information was given on the feeding protocols used for these calves, nor the actual method for determining their weight [10].

When considering the reason for the difference in DLWG, this study found 80.7% of sedated calves remained in lateral recumbency throughout the procedure (an appropriate plane of sedation), but with 19.3% exhibiting some movement or entering sternal recumbency. It should be noted that subjective classification by the researchers disbudding for the level of calf responsiveness was used in this study, and this could be improved upon in future work. However, calves under a light plane of sedation that entered sternal recumbency had an overall DLWG of 0.89 kg/day compared to 0.98 kg/day for those that remained in lateral throughout (p = 0.008). This suggests that the level of sedation had a large impact on the growth outcomes in these calves. Xylazine provides conscious sedation through muscle relaxation that prevents movement or reactions to stimuli [16]. The behavioural impacts of incomplete sedation are unknown, but the onset of xylazine sedation is known to cause mild/moderate distress to calves [17], which may be amplified if the sedation is incomplete and calves are able to physically move. Other studies have reported an increase in plasma cortisol concentrations in xylazine-sedated calves for disbudding [16,17], which may be amplified when there is a light plane of anaesthesia and calves are partially able to react to stressful events. This may have resulted in the overall reduction in DLWG of the whole SED group. It has been anecdotally reported that some breeds are more sensitive than others to the sedative impact of xylazine and that the response of an individual can be variable, particularly if the animal is over excited [18]. Further work aimed at understanding the reasons behind incomplete sedation could help improve operating protocols and minimise any negative welfare outcomes from poor sedation outcomes.

Other causes of reduced DLWG in calves given xylazine may be related to feeding behaviours following sedation, with slower milk drinking speeds observed in the first 48 h post-disbudding [9]. Feed intakes were not measured in this study, but reduced intakes would contribute to the lower DLWG seen, and given the studied calves were fed in a trough system, slower drinking speeds could have a greater impact on milk intakes compared to individual teat feeding methods. In addition, disbudding leads to anhedonia (pain-induced deficits in appreciation of rewards) for days following disbudding [19], which may also impact feeding behaviours in calves, especially those that may have had raised stress levels due to a light plane of sedation. Xylazine also decreases body temperature, with calves that experience a greater body temperature decrease shown to drink less milk post-disbudding [20]. Alpha-2 agonists also decrease gastrointestinal motility, including a marked drop in intraduodenal flow rate [21], secondary to inhibiting the release of acetylcholine, which can contribute to ruminal tympany [22]. Again, this could be a possible mechanism by which xylazine influences feeding behaviour following disbudding. Although this reduced short-term DLWG is relatively small, many producers do struggle to achieve adequate growth rates in their youngstock, so the impacts that sedation might have should be considered when making disbudding restraint decisions. This finding also calls into question the claims around sedation use being positive for calf welfare.

The entry weight of the calf (p < 0.001) was significantly associated with the post-disbudding DLWG such that for every 1 kg extra bodyweight, the post-disbudding DLWG increased by 8 g/day. This suggests that larger calves tend to grow better in these multi-origin rearing systems. The pre-disbudding DLWG demonstrated a positive trend (p = 0.07), with the calves who grew faster pre-disbudding also continuing to grow faster following disbudding. It should also be noted that the pre-disbudding DLWG for the CTR group was 0.04 kg/day higher than the SED group (Table 1), which may partly explain some of the difference in post-disbudding DLWG. Breed was found to be significantly associated with DLWG and treatment number post-disbudding. Surprisingly, the continental breeds grew slower than the native breeds (p < 0.001, 0.87 kg/day compared to 1.01 kg/day, respectively) and received more treatments than native breeds (mean 0.45 compared to 0.38 treatments, respectively, p = 0.023). It should be noted that there were multiple different breeds within the bichromatised classification used in this analysis, with nearly four times more continental breed calves than native breeds overall. Traditionally continental breeds are reported to have higher birth weights and growth rates, but given the fixed milk feeding volumes used in this system, it may be possible that restricted feeding may have limited the continental breeds ability to optimise their growth potential.

Overall, the intake number variable which took into account the shed where the pens were located and the date of entry of the calf onto the farm (numbered 1–12) was found to be significantly associated with DLWG (p < 0.001). This likely represented the multiple farms of origin; the different environmental conditions experienced by the calves depending on when they entered the study, due to the weather and distances travelled from their farms of origin; and potential differences in the shed environments in which the pens were located.

When considering the post-disbudding treatments, we found no association with the use of sedation (p = 0.97), with 71.1–82.6% of calves not requiring any post-disbudding antimicrobial treatments (Table 1). However, the number of treatments pre-disbudding was relatively high, with 63.5–86.0% of calves having between 1 and 3 antimicrobial treatments, predominantly for respiratory disease. Transportation and comingling of calves from different source farms is known to be a risk factor for respiratory disease [23], with this likely accounting for the relatively high treatment rate following arrival on the farm. Unsurprisingly, the number of treatments prior to disbudding was positively associated with the number of treatments post-disbudding (p < 0.001). The high detection rates for disease pre-disbudding by farm staff suggests that post-disbudding clinical disease was unlikely to have been missed, but further assessment by thoracic ultrasound would have enabled more rigorous assessments of lung health and should be considered in future studies.

5. Conclusions

While other studies have demonstrated some behavioural and physiological indicators of pain to be reduced with sedation use, this study found a 0.14 kg/day decrease in DLWG during the short term following disbudding for calves disbudded using xylazine sedation calves, but the growth rates in the groups became equivalent by the medium-term post-disbudding. Some of this difference could be explained by the slightly higher DLWG pre-disbudding in the CTR group, and it is likely that the physiological impacts of sedation accounted for the rest of this impact. In calves disbudded with sedation, 19.3% exhibited at least some movement or entered sternal recumbency, with these latter calves having the greatest DLWG reductions. Understanding sedation protocol efficacy, especially with respect to breed sensitivities and the impact of calf excitement prior to sedation is therefore necessary, with a light plane of sedation likely to increase stress and possibly changing feeding behaviours to a greater extent. There was also no difference in the number of post-disbudding treatment outcomes between calves disbudded with and without sedation.