Graduate Student Literature Review: Role of pain mitigation on the welfare of dairy calves undergoing disbudding*

This review synthesizes research findings on the pain and welfare of dairy calves undergoing disbud-ding procedures. We describe disbudding practices in North America as well as the use and perceptions of pain control for these procedures. Governing bodies across Canada and the United States, including each country’s veterinary medical association and nation-wide initiatives such as proAction and Farmers As-suring Responsible Management (FARM), recommend or require the use of a local anesthetic, a nonsteroidal anti-inflammatory drug (NSAID), and a sedative for disbudding procedures. Although the use of pain relief for disbudding has increased over the past decade or so, some in the dairy industry still do not believe that pain control for disbudding is necessary. As a painful procedure, disbudding has numerous welfare impacts on the calf both during and following the procedure that can be categorized under all 3 principles of animal welfare: natural living, biological functioning, and affective state. The use of pain control for disbudding; namely, a local anesthetic and NSAID, can improve welfare outcomes such as procedure-induced pain behavior, cortisol concentrations, mechanical nociceptive threshold, emotional states, and so on, compared with no pain control for the procedure. Although extensive research exists on pain control practices for disbudding, this review identified further gaps in knowledge and areas for future research. Mechanical nociceptive threshold can be evaluated around the disbudding wounds and is a reliable test in older calves; however, this outcome in very young calves after caustic paste disbudding has been reported to be inconclusive compared with that in older calves. As well, research evaluating xylazine sedation for disbudding has reported both potentially positive and negative results that are difficult to interpret or base suggestions on for the use of this drug. Finally, wounds caused by disbudding take a long time to heal (up to 13 wk) and have increased sensitivity for the entire healing process. Therefore, future research should aim to (1) determine accurate behavioral tests for calves under


INTRODUCTION
Disbudding is a painful husbandry procedure commonly performed on dairy operations; when performed without the use of either anesthesia and analgesia (or both together), it has been identified as a risk factor for poor calf welfare (Calderón-Amor and Gallo, 2020).In the United States, 94% of responding dairy farms reported having dehorned animals (NAHMS, 2018) and in Canada, over 95% of responding producers reported that they disbud their calves (Winder et al., 2018a).Cattle are typically dehorned or disbudded because horned dairy cattle pose a risk of injury to people as well as to other animals (reviewed by Stock et al., 2013).
Disbudding is the removal of the horn-forming tissue before its attachment to the skull, whereas dehorning is the removal of the horn after this occurs, typically at 2 to 3 mo of age (CVMA, 2016).Disbudding is recommended over dehorning, because it is less invasive and less painful (Stafford andMellor, 2005, 2011).
Horned dairy cattle demonstrate a higher proportion of agonistic behaviors without body contact (such as a cow retreating from the horned cow without any body contact necessary) as well as more aggressive behaviors compared with dehorned cattle (Lutz et al., 2019).Additionally, in beef cattle, one of the most prevalent causes of carcass wastage is bruising, and dehorned cattle require less feeding trough space than horned cattle (Stock et al., 2013).Although one of the natural uses of horns on cattle were for protective or defensive purposes, today's modern domesticated dairy cattle do not have the same need for such a tool (Stock et al., 2013).For these reasons, polled or disbudded cattle may be more desirable in production systems and can decrease the amount of aggressive behavior and injury observed among a group of animals.

Disbudding Methods
The 2 most commonly used methods for disbudding are cautery (or hot iron) and caustic paste disbudding, with cautery disbudding being the most common (NAHMS, 2018;Winder et al., 2018a).A 2015 survey of Canadian producers reported that 86% of respondents used cautery disbudding as their primary method and 9% used caustic paste (Winder et al., 2018a).Similarly, in the United States in 2014, 70% of respondents reported disbudding their calves with a cautery iron and 16% with caustic paste (NAHMS, 2018); however, there may be regional differences in the use of disbudding products, as a recent survey performed among Wisconsin producers reported that 61% of respondents use of caustic paste for disbudding (Saraceni et al., 2021).Animals above 3 mo of age typically require surgical amputation dehorning.In Canada and the United States, approximately 11% of producers use surgical amputation (gouge or scoop) to dehorn their calves (NAHMS, 2018;Winder et al., 2018a).A producer's decision to use one method over another may be based on the size of their dairy operation and their labor capacity.Cautery disbudding has been reported as the primary method used on dairy operations under 500 cows [74.2% of small operations (30-99 cows) and 71.7% of medium operations (100-499 cows)], whereas caustic paste is used on a higher percentage of large operations (500+ cows; 28.2%) compared with small operations (9.9%) (NAHMS, 2018).

Age at Disbudding
Caustic paste is generally used on younger calves compared with cautery disbudding (NAHMS, 2018;Winder et al., 2018a).The majority (73%) of Canadian producers that use caustic paste disbudding disbud their calves before 3 wk of age (Winder et al., 2018a), and the mean age for this same procedure in the United States is 2.3 wk (NAHMS, 2018).In contrast, the mean age for hot-iron disbudding in the United States is 7.1 wk (NAHMS, 2018), and in Canada most (67%) producers using hot-iron disbudding disbud their calves between 3 and 8 wk of age (Winder et al., 2018a).

Disbudding Recommendations and Requirements
Although there is no specific legislation on pain mitigation for disbudding in Canada or the United States, the Canadian Veterinary Medical Association (CVMA) supports the use of disbudding in cattle and recommends the appropriate use of anesthesia and analgesia to help control the pain from the procedure (CVMA, 2016).In the United States, the American Veterinary Medical Association (AVMA) also recommends a combination of sedation, anesthetics, and analgesics as best practice for disbudding dairy calves (AVMA, 2014).In 2017, the Dairy Farmers of Canada implemented a national, industry-led quality assurance program that requires producers to use both a local anesthetic and a nonsteroidal anti-inflammatory drug (NSAID) before disbudding of any method (DFC, 2017).Before this requirement, 66% of responding producers used a local anesthetic, 33% provided sedation, and only 25% of producers reported use of an NSAID for cautery disbudding procedures (Winder et al., 2018a).Similarly, in the United States, the National Milk Producers Federation in partnership with Dairy Management Inc. created the National Dairy Farmers Assuring Responsible Management (FARM) program to work with dairy farmers aiming to provide assurance to customers that the dairy industry is producing milk with integrity.Producers who participate in FARM have new expectations as of 2020 that require disbudding to occur before 8 wk of age, pain control to be used for disbudding, and for a veterinarian to be involved when creating a disbudding protocol (FARM, 2020).Disbudding before 8 wk of age was recently increased in status from a Continuous Improvement Plan to a Mandatory Action Plan through FARM.Although it is not a requirement for producers to participate in FARM, many milk processors do require it; therefore, 99% of the US milk supply is now covered under the FARM program (National Dairy Farm Program, 2020).

Disbudding Changes Over Time
The most common change in disbudding practices over time is the adoption of pain control methods (Winder et al., 2016).Two surveys were performed in 2004 and 2014 in Ontario, Canada, assessing practices for disbudding and dehorning dairy calves by veterinarians and producers (Misch et al., 2007;Winder et al., 2016).Of respondents to the more recent survey, 72% of veterinarians and 63% of producers reported that they had altered their disbudding practices since the first survey (Winder et al., 2016).Both veterinarians and producers reported that the changes that they made involved adding local anesthetics, sedation, NSAIDs, or a combination thereof to their pain control protocol (Winder et al., 2016).It was also reported that 30% of producers who changed their disbudding practice also began providing these medications to calves at younger ages (Winder et al., 2016).When responding veterinarians were asked what influenced their change in pain control provision over time, the most highly cited reason was concern for welfare of the calf, with 75% of responding veterinarians selecting this as a concern (Winder et al., 2016).

Reasons for Pain Control Use
Although the use of pain relief for disbudding has increased over the past decade or so (Winder et al., 2016;Robles et al., 2021), there are still some in the dairy industry who do not believe that pain control for disbudding is necessary.Some producers and veterinarians in North America have reported the cost of pain relief as a reason they do not provide pain mitigation for disbudding (Hewson et al., 2007;Robbins et al., 2015;Robles et al., 2021).Calf age at the time of disbudding can also be a factor associated with whether a producer decides to use pain mitigation or not.Caustic paste is more commonly used on younger calves, and it has been reported that producers from Ontario, Canada, have higher odds of providing their calves with pain control (odds ratio = 6.0, 95% CI: >1.0, 35.7) when performing cautery compared with caustic paste disbudding (Winder et al., 2016).Producers in North America have also reported they felt disbudding at a younger age did not require pain relief, as they believed young calves were less sensitive to pain (Robbins et al., 2015); however, there is no evidence to support this belief.

Perception of Pain
Many producers perceive disbudding to be a painful procedure (Hoe and Ruegg, 2006) and are in favor of pain control use (Kling-Eveillard et al., 2015).However, many individuals do not believe that disbudding causes significant or long-term pain for the calf (Robbins et al., 2015).In a survey by Winder et al. (2016), both producers and veterinarians reported that they perceived surgical dehorning at 5 mo of age to be more painful than cautery disbudding at 2 mo of age, and cautery disbudding at 2 mo of age to be more painful than caustic paste disbudding at 2 d of age.Percep-tion of pain caused by gouge dehorning at 5 mo of age had the least amount of variation in score across participants, whereas caustic paste disbudding at 2 d of age showed the greatest variation (Winder et al., 2016).These results suggest greater agreement among producers that gouge dehorning is considered a painful practice for calves, but less agreement on the pain caused by caustic paste disbudding.Although some producers may believe that caustic paste disbudding causes high levels of pain to the animal, others do not view this procedure to cause significant pain and potentially do not see it as a welfare concern compared with other disbudding procedures, such as cautery disbudding or gouge dehorning.

Literature Review Objectives
The welfare of animals can be understood in different ways.For the purposes of this literature review, we assessed the welfare of calves following disbudding procedures using the 3 quality of life concerns proposed by Fraser et al. (1997): natural living, biological functioning, and affective state.The most commonly evaluated ways that the pain caused by disbudding can affect the welfare of the calf can be effectively classified and interpreted under each of these 3 sections.The term "natural living" is determined by both the housing of the animal (i.e., the naturalness of their environment) and the animal's ability to live according to its "nature" or display its natural behaviors (Fraser et al., 1997).The biological functioning approach includes the assessment of disease, injury, nutrition, diet, and other potential interruptions to what would be considered the animal's normal functioning.Last, the affective state of the animal refers to its subjective state and ability to experience affective states such as suffering or pleasure, and it is an important part of the evaluation of their quality of life (Fraser et al., 1997).A recent literature review on welfare and disbudding in dairy calves focused on the effects of combining an NSAID and local anesthetic for only hot-iron disbudding in calves <3 mo of age (Herskin and Nielsen, 2018).However, the aim of this review is to contribute to the conversation on pain and welfare for disbudding and the need for research in these areas to reduce pain and improve the welfare of calves undergoing any form of disbudding with any method of pain control in North America.

Social Behavior
In nature, in the past, calves would have lived in a social environment, but social housing is not com-mon on modern dairy operations (Costa et al., 2016).In Canada, 21% of farms house calves using grouphousing systems and 79% house calves individually (Medrano-Galarza et al., 2017).Calves reared in social isolation (an absence or low frequency of direct peer interaction for an extended period) have been reported to have deficient social skills and cognitive deficits and difficulty coping in novel situations (Costa et al., 2016) compared with their group-housed counterparts.
Even when provided with local anesthetic and an NSAID, calves disbudded using cautery are reported to seek physical and visual isolation in group housing for up to 3 d after disbudding more often than nondisbudded calves in the same group, suggesting that this procedure affects the natural social behavior of calves under current pain control (Gingerich et al., 2020).

Play Behavior
Certain behaviors can be motivated by both positive and negative affects, whereas behavior such as play in calves is most likely motivated by only one affect (positive; Fraser and Duncan, 1998).Play is a natural behavior for calves that increases in frequency when the basic needs of the animal are met; however, it can be affected by management practices on dairy operations (Jensen et al., 2015).Play behavior reduces when animals are exposed to negative stimuli and it serves as a self-rewarding behavior for calves (Mintline et al., 2013); therefore, it can be considered a potential indicator of positive welfare in animals (Held and Spinka, 2011;Ahloy-Dallaire et al., 2018).However, whether increased play behavior is indicative of optimal welfare or just neutral welfare in animals is controversial (Ahloy-Dallaire et al., 2018).For the purposes of disbudding literature, play is typically considered an indicator of positive welfare.
Providing a calf with pain control for cautery disbudding (a local anesthetic and an NSAID) can result in increased play compared with calves receiving less pain control for the procedure (Mintline et al., 2013).The provision of a xylazine sedative in conjunction with a local anesthetic and NSAID can result in reduced play behavior for 3 h after cautery disbudding but can also increase play behavior 24 h after disbudding (Reedman et al., 2021).These results were only recorded in the short term (3-24 h; Mintline et al., 2013;Reedman et al., 2021), and further research evaluating the longerterm pain effects from disbudding and their effect on the play behavior of calves would be beneficial for fully understanding the impact of this procedure on the welfare of the animal.

Standing and Lying Behavior
Many researchers have used the evaluation or disruption of lying behavior patterns as an indicator of pain in animals.The change in the proportion of time that an animal spends lying down as well as their frequency of moving from standing to lying positions and vice versa (restlessness) are considered good indicators of whether an animal is experiencing pain or discomfort (Molony and Kent, 1997).Stressful events such as disbudding, weaning, or illness have been reported to cause a change in the lying behavior of calves (Molony and Kent, 1997;Black et al., 2017;Sutherland et al., 2018a).
Calves have been reported to be more restless (frequent standing up and lying down) in the first 4 h after disbudding (Morisse et al., 1995) and to spend less time lying down from 180 min (Molony and Kent, 1997) up to 4 h (Sutherland et al., 2019) after a painful procedure.In the 48 h following disbudding procedures, calves have been reported to spend less time lying and have shorter lying bouts compared with before the procedure (Sutherland et al., 2018b).Few studies have focused on examining the lying behavior of calves over a longer period following disbudding.
Provision of NSAIDs and local anesthetics reduces these changes in behavior that occur with disbudding (Theurer et al., 2012;Sutherland et al., 2018bSutherland et al., , 2019)).More specifically, in the short term, the provision of a local anesthetic with or without an NSAID can result in increased lying times in the first hour after disbudding (Sutherland et al., 2018b), and administering an NSAID compared with no pain control can result in increased lying times for up to 4 d after cautery disbudding (Theurer et al., 2012).

Feeding Behavior
Following cautery disbudding procedures, disbudded calves have fewer unrewarded visits to an automated milk feeder compared with calves that were not disbudded; this difference was eliminated with provision of a local anesthetic and an NSAID (Sutherland et al., 2018a).Other researchers have also reported that following disbudding, feeding behavior (time spent eating feed and drinking milk) is reduced in calves (Graf and Senn, 1999), but providing a local anesthetic can eliminate this reaction to the procedure.

Wound Healing
Cautery disbudding wounds may take anywhere from 3 to 13 wk to re-epithelialize (Huebner et al., 2017; Reedman et al.: LITERATURE REVIEW: PAIN MITIGATION AND DISBUDDING Adcock and Tucker, 2018;Adcock et al., 2019).During this healing process, wounds are painful for at least 3 wk, and the different tissue types present during the healing process are more sensitive than new epithelium (new tissue formed after disbudding wounds have healed) for up to 13 wk (Adcock and Tucker, 2018).Adcock and Tucker (2018) determined that it takes 9 to 13 wk for wounds to re-epithelialize; however, calves in that study were fed a restricted milk volume (<6 L/d; Adcock and Tucker, 2018).Calves given free access to milk will typically drink about 20% of their BW per day or approximately 10 to 12 L/d of milk (Khan et al., 2011).Feeding high levels of milk to young dairy calves has been associated with improved calf health and performance (Todd et al., 2017) as well as reduced healing times after cautery disbudding (Reedman et al., 2022).Adcock et al. (2019) compared the effects of different disbudding irons on wound healing but did not detect any difference, prompting further research to explore other strategies to decrease healing time following this procedure.There are different types and brands of cautery irons and caustic paste as well as highly variable management routines (e.g., housing conditions, nutritional management, health management) among countries, regions, farms, and producers.Further research exploring some of these different attributes may help to inform optimal strategies for reducing healing time after disbudding.

Heart Rate, Respiratory Rate, and Eye Temperature
Disruptions to a calf's normal biological functioning by pain or stress can be quantified using changes to the autonomic nervous system such as heart rate and heart rate variability, respiratory rate, and eye temperature (Stewart et al., 2008).Evaluating these outcomes can help us in determining the effect of pain and disbudding on the calf's bodily functions.Heart rate is increased compared with baseline following cautery disbudding procedures (Stock et al., 2015) and compared with that of sham-disbudded calves (Heinrich et al., 2009).This heart rate increase is lessened with the use of a local anesthetic (Stewart et al., 2008) or an NSAID (Coetzee et al., 2012) or both medications together compared with less or no pain control (Heinrich et al., 2009).The use of a local anesthetic has also been shown to decrease respiratory rate following disbudding (Heinrich et al., 2009), as well as a local anesthetic with an NSAID (Winder et al., 2017).Providing calves with a local anesthetic for cautery disbudding results in a smaller effect on the change in eye temperature compared with no pain control (Stewart et al., 2008).Researchers have also detected no difference in heart rate up to 24 h after disbudding (Stock et al., 2015) or in heart and respiratory rate up to 95.5 h after disbudding (Stock et al., 2016) between groups of disbudded calves provided with an NSAID and a local anesthetic compared with those only provided with a local anesthetic.

Cortisol
In calves, cortisol has been used as an indicator of pain and stress following cautery disbudding (Black et al., 2017).After disbudding, cortisol concentrations rise rapidly in the first 15 to 30 min (Winder et al., 2018b); these concentrations also increase with stress from being handled and sham disbudded (Heinrich et al., 2009).Cortisol has anti-inflammatory effects (Hannibal and Bishop, 2014) and, following an increase in cortisol concentrations due to pain from disbudding when no pain control is provided, cortisol concentrations decrease to baseline values after 3 to 7 h, likely due to these anti-inflammatory effects (Winder et al., 2018b;Reedman et al., 2020).
Amputation dehorning has been reported to elicit a larger cortisol response than disbudding (Petrie et al., 1996), and it may take more than 8 h for these concentrations to return to baseline (McMeekan et al., 1998).Providing calves with a local anesthetic for disbudding procedures eliminates the acute cortisol peak normally observed in the first 30 min after the procedure (Graf and Senn, 1999;Morisse et al., 1995;Reedman et al., 2020); however, local anesthetics are only effective for 1 to 3 h after administration.Once the local anesthetic wears off, a delayed cortisol peak is observed, similar to that observed immediately after the procedure without a local anesthetic (Stilwell et al., 2009(Stilwell et al., , 2012)).Therefore, although a local anesthetic is effective at reducing the acute pain from the disbudding procedure for 1 to 3 h (Stafford et al., 2003), it does not eliminate all pain when used alone.Providing a calf with an NSAID alone does not eliminate the initial cortisol peak in the first 30 min after disbudding, but it does aid in decreasing cortisol concentrations sooner than if no pain control is given (Glynn et al., 2013;Stock et al., 2015;Reedman et al., 2020).When both a local anesthetic and NSAID are provided to the calf for disbudding, both the acute and delayed cortisol responses are eliminated (Stilwell et al., 2009;Winder et al., 2018b;Reedman et al., 2020).Although the local anesthetic is effective at eliminating acute pain (the first cortisol peak) for 1 to 3 h after disbudding, the NSAID works to prevent the delayed peak, which usually occurs 4 h after disbudding, through anti-inflammatory effects.In some cases, the cortisol response of calves provided with both medications does not differ from those of sham-disbudded control calves (Winder et al., 2018b).Assessing cortisol concentrations is helpful for understanding the benefit of pain mitigation in improving the welfare of the calf.
Results in the literature on cortisol concentrations evaluating the efficacy of xylazine sedation for disbudding have been inconclusive.Xylazine can provide effective restraint for the disbudding procedure.Some researchers have reported that xylazine (especially when paired with a local anesthetic) can reduce (but not eliminate) the initial cortisol response after disbudding for 45 min (Caray et al., 2015) up to about 3 h (Stafford et al., 2003), whereas others have reported plasma cortisol concentrations to be greater in xylazine-sedated calves compared with calves that did not receive this medication (Stilwell et al., 2010).Some of these differences could be attributed to how cortisol was collected and evaluated (through salivary or plasma concentrations) or to a difference in study designs.However, there is very little research on the effects of xylazine sedation for disbudding and the literature suggests that cortisol may be an unreliable indicator of pain in xylazine-sedated calves (Stilwell et al., 2010), potentially due to the effects of xylazine on the calf, which include decreased arterial pressure and reduced tissue oxygenation (Campbell et al., 1979;Hodgson et al., 2002).Xylazine also does not have an anesthetic effect but provides conscious sedation by causing muscle relaxation and limits the ability of the animal to move or react to stimuli (Stilwell et al., 2010); thus, it has been reported that sedation alone does not control the pain of disbudding (Grøndahl-Nielsen et al., 1999;Stilwell et al., 2010).Further research examining the effects of xylazine sedation for disbudding would be beneficial in understanding the effect of providing this medication to calves.

Haptoglobin
Inflammation caused by procedures such as disbudding can result in increased haptoglobin concentrations over time (Allen et al., 2013;Glynn et al., 2013).The provision of a local anesthetic and an NSAID results in lowered haptoglobin concentrations (for up to 2-3 d) compared with calves provided with less or no pain control for disbudding procedures (Ballou et al., 2013;Erdogan et al., 2019;Reedman et al., 2020).

Growth
Researchers have reported improved ADG in calves that received analgesics for scoop dehorning procedures compared with controls (Glynn et al., 2013).Pain relief for disbudding has been reported by multiple researchers as a factor that contributes to improved growth rate and weight gain after disbudding (Glynn et al., 2013;Bates et al., 2015Bates et al., , 2016)).Specifically, calves provided with an NSAID for disbudding had higher growth rates than calves not provided an NSAID (Bates et al., 2015), and calves that received a sedative and a local anesthetic for disbudding had higher growth rates than calves that received no pain control or an NSAID alone (Bates et al., 2016).
Although feeding behavior and milk intake have been reported to change in response to pain from disbudding, there are conflicting results in the literature on whether this pain (and the changes that it causes) affects the growth and weight gain of calves.Although some researchers have reported no detectable differences between cautery-disbudded and non-disbudded calves (Grøndahl-Nielsen et al., 1999), others have detected a decrease in these parameters after painful procedures such as cautery disbudding (Black et al., 2017).It would be beneficial to conduct a meta-analysis on this topic to help discern whether there is a true difference in growth and weight gain in calves after disbudding compared with non-disbudded calves and depending on pain control methods used.

Mechanical Nociceptive Threshold
A pressure force algometer or von Frey monofilaments can be used to assess the mechanical nociceptive threshold (MNT) of the wound area following cautery or caustic paste disbudding (Adcock and Tucker, 2018;Reedman et al., 2020).After disbudding procedures, MNT in disbudded calves is much lower than baseline values or MNT in sham-disbudded calves (Allen et al., 2013;Adcock and Tucker, 2018).Using this test, it has been reported that during the healing process (which can take up to 13 wk), every tissue type present during this time was more sensitive compared with newly epithelialized disbudding wounds (Adcock and Tucker, 2018).Researchers have reported that disbudding wounds are more sensitive after the procedure anywhere from 4 to 9 h (Frahm et al., 2020), 24 h (Stock et al., 2015;Frahm et al., 2020), 48 h (Stock et al., 2016), 3 wk (Adcock and Tucker 2018), and up to 105 d (Casoni et al., 2019) compared with their baseline values before the procedure or compared with values in sham-disbudded calves.Therefore, an increased response to the MNT test can persist for long periods following disbudding.
The provision of a locally applied anesthetic before disbudding can reduce short-term pain and sensitivity, as evidenced by higher MNT values from 1 to 3 h after the procedure (Glynn et al., 2013;Stock et al., 2015;Reedman et al., 2020).Pain mitigation such as NSAIDs (which provide longer-term relief than local anesthetics) given in addition to a local anesthetic can result in higher MNT values for up to 4 to 6 h after the procedure (Stock et al., 2016;Winder et al., 2018b).However, not all studies comparing a local anesthetic alone to a local anesthetic plus an NSAID have detected a difference in MNT values (Winder et al., 2018b).Although NSAIDs serve a similar purpose, many different types of NSAID are used to control pain following disbudding procedures (e.g., meloxicam, carprofen, ketoprofen, firocoxib).A study by Stock et al. (2015) examined MNT in calves following disbudding given both a local anesthetic and firocoxib compared with a local anesthetic alone and did not detect any differences between the treatment groups.In contrast, a study by Stock et al. (2016) detected a tendency for calves treated with both a local anesthetic and carprofen to be less sensitive than a local anesthetic alone.Winder et al. (2018b) compared MNT values in calves provided both an NSAID and local anesthetic with calves provided a local anesthetic alone in a meta-analysis and found calves to be less sensitive 4 to 6 h after disbudding when provided with both medications.
Most research evaluating MNT around the horn buds of calves after disbudding have been conducted on calves older than 1 wk of age, and there is limited research evaluating MNT in calves younger than this.However, MNT results in 1-to 9-d-old calves after caustic paste disbudding differed from those in calves older than this (also disbudded using caustic paste), as no difference was detected in MNT values between 1-and 9-d-old calves disbudded with no pain control and sham-disbudded calves from 15 min to 300 h after disbudding (Karlen et al., 2019;Reedman et al., 2020).This illustrates a gap in the knowledge for evaluating pain from disbudding in young calves under 1 wk of age.The MNT tests in these young calves are inconclusive and show different results compared with calves over 1 wk of age.Further research in this novel area would be beneficial for understanding the overall pain experience of these very young calves, as it is common to disbud calves before they are 1 wk of age.

Behavioral Indicators of Pain
Calves exhibit certain behavior patterns more frequently after disbudding with less pain control; these are considered behavioral indicators of pain and are indicative of a negative affective state.Examples include tail flicking, head shaking, ear flicking, and vocalizing.Calves show more head shaking, ear flicking, and head rubbing after cautery disbudding compared with shamdisbudded calves (Faulkner and Weary, 2000).
The provision of a local anesthetic for disbudding results in reduced tail flicking, head shaking, and ear flicking compared with calves with no anesthetic (Winder et al., 2018b).Following the duration of effect of a local anesthetic (approximately 1 to 3 h following administration), calves that received a local anesthetic displayed behavioral indicators of pain that were not different from those of calves that did not receive an anesthetic (Graf and Senn, 1999;Stilwell et al., 2009).These indicators are decreased to the level of a sham control calf for up to 24 h (study end) when both a local anesthetic and an NSAID are used (Winder et al., 2018b).However, recent work by Thomsen et al. (2021) reported that 42% of calves disbudded with a hot iron and provided with a local anesthetic showed at least 1 of 3 behaviors examined (getting up, kicking, or lifting head), suggesting inadequate local anesthesia.Differences in these results could be attributed to study design or local anesthetic type and application method.
Although there is evidence in the literature of an NSAID (in combination with a local anesthetic) reducing behavioral indicators of pain for up to 24 h, few studies have examined longer-term pain.One study evaluated the behavioral indicators of pain of calves 11 d after the cautery disbudding procedure and the effect of additional medication at this time (Adcock et al., 2020).Calves that received an additional injection of a local anesthetic 11 d later had reduced head shaking and ear flicking compared with calves that did not; this difference was no longer detectable after 1 h when the local anesthetic wore off (Adcock et al., 2020).These results aid in understanding the length of time that calves experience pain after the disbudding procedure.Further research evaluating the length of time over which calves display behavioral indicators of pain would be beneficial in understanding the potential benefits of additional pain medication and the full impact of the procedure on the long-term welfare of the calf.

Other Indicators of Affective State
Researchers can assess the emotional states or responses in calves by evaluating their cognitive changes through the assessment of cognitive bias (Neave et al., 2013;Lecorps et al., 2020) and potential signs of anhedonia (Lecorps et al., 2020).Neave et al. (2013) reported that calves are more likely to judge ambiguous cues as negative after disbudding than before the procedure, indicating that the calves were more pessimistic after the painful procedure.In further work, researchers began more thoroughly examining the emotional state of calves after disbudding through different methods such as examining potential signs of anhedonia following disbudding (Lecorps et al., 2020) and calf perception of pain after disbudding (Adcock and Tucker, 2020).Calves have been reported to be more pessimistic after disbudding and to display signs that may be interpreted as anhedonia, either consuming less of a highly desirable sweet solution (Lecorps et al., 2020) or perceiving the value of a reward that they were conditioned to expect as lower compared with before the procedure (Lecorps et al., 2019).Alternatively, using risk aversion methods, Adcock and Tucker (2021) reported disbudded calves to accept more risk compared with control calves to suckle (for up to 3 wk after disbudding) possibly as a strategy to mitigate pain, suggesting that disbudding influences motivational states for weeks after the procedure.These changes in the emotional state of calves after disbudding were previously reported to continue anywhere from 5 (Lecorps et al., 2020) to 20 d (Adcock and Tucker, 2020) after the procedure.
Another way that animal welfare researchers have attempted to understand the experience of calves after disbudding is through conditioned place avoidance.These experimental paradigms can provide a strong basis for examining the affective component of pain through learned responses from the animal rather than relying on physiological and behavioral indicators.Ede et al. (2019) examined this paradigm in calves in response to post-procedural pain from hot-iron disbudding and following intramuscular, intranasal, and subcutaneous injections (Ede et al., 2018).These researchers reported that an NSAID (meloxicam) treatment (in conjunction with a local anesthetic) made hot-iron disbudding less aversive to calves in the 6 h after their procedure (Ede et al., 2019).It was also reported that intramuscular injections were found to be the most aversive, with calves injected intramuscularly exhibiting a longer latency to drink milk compared with the other treatment groups (Ede et al., 2018).These novel methods of exploring the affective state of calves after disbudding show great promise and should be explored further and used more frequently in research.

CONCLUSIONS
Disbudding is a common procedure in the dairy industry and has been well documented to be painful and a significant welfare concern.Pain mitigation can alleviate some of the negative pain-related outcomes associated with the procedure; however, some people still do not believe that pain control is necessary when disbudding calves.Although research on the welfare of calves after disbudding has mainly focused on evaluating outcomes relating to the health and biological functioning of the calf, researchers have begun to shift focus to better understand the affective state of the calves during and following this procedure.Newer research has identified long-term effects of disbudding on the affective and motivational states of calves, and on their biological functioning, including very long healing times (up to 13 wk).Future research should aim to (1) determine accurate behavioral tests to assess pain for calves less than a week of age undergoing disbudding, (2) further understand the effects of xylazine sedation for disbudding, and (3) determine more ways to reduce the healing time after disbudding procedures.
Reedman et al.: LITERATURE REVIEW: PAIN MITIGATION AND DISBUDDING