The use of behavior to assess pain and determine drug effectiveness presents a challenging task. Pain can be classified as either procedural (occurring during the procedure) or postprocedural. Different methods have been used to quantify behavioral and physical measurements (17
). Recording vocalizations (18
), measuring exertion force by the animals against the restraint (19
), and measuring cortisol levels (20
) have all been used to assess pain experienced by animals during painful procedures. Some of these methods are invasive and labor intensive; for example, measuring blood cortisol levels requires multiple samples over time to account for its episodic nature (21
), and such requirements limit the number of methods that can be used to assess pain during experiments. Attempts have been made to quantify the postoperative pain experienced by animals by measuring changes in posture (standing, lying down) and movement (walking, trotting), using continuous (22
) or multiple point in time visual observations (8
). These behavioral observations may lack sensitivity in detecting treatment differences because of large individual variations or because many behaviors are socially facilitated. For example, a calf may stand up or walk because other calves in the herd are standing up or walking. In addition, prey species are generally stoic in response to pain, since there is little biological advantage for them to advertise they are injured or in a weaker state. Thus there are inherent difficulties in assessing and quantifying pain in cattle, which may explain why a variety of measurements have been used by us and others to assess pain.
Although some of the traditional behavioral measurements described above were included in this study, 2 relatively newer techniques of pain detection in calves were included; stride length and pedometer counts. Peham et al (23
) used stride length measurements in horses as a means of assessing the influence of pain following orthopedic surgery. At the onset of our trials, we hypothesized that calves having undergone castration would show some variation in stride length due to pain. Likewise, we believed that pedometers might be useful to detect treatment differences since they had previously been used on cattle to detect estrus (24
), monitor calves’ response to weaning treatments (13
), and determine the distance traveled by cattle on different grazing systems (25
Pedometers have also been used to assess weaning stress in calves (13
), but not as a means of assessing pain in cattle. The results indicate that calves having undergone a painful procedure, like castration, have decreased activity and take fewer steps. The failure to detect differences between treatment subgroups may be attributed to the small sample size in each processing group. We estimated that the sample size required to make the differences that we found significant would be approximately 30 calves per treatment subgroup. There was also wide variation between calves in the number of steps taken per day. For example, one calf recorded a baseline of 2800 steps in 24 h, while another calf in the same treatment subgroup logged 19 600 steps over the same time period. This variation between calves contributed to the nonsignificant difference in the number of steps taken between treatment subgroups.
Calves with a higher baseline average stride length maintained a longer stride length postcastration. Stride length decreased and then increased again after castration, consistent with a quadratic time variable (). The significant differences between treatment groups indicate that stride length can be used as a method to detect post-castration pain in calves. The stride length decreased in all groups after castration, which suggests that castration altered the animal’s locomotion, likely because of pain. Calves in the SURG + EPI + F subgroup showed the least amount of decreased stride length among the 3 treatment subgroups at 4 and 8 h postcastration, which implied that flunixin meglumine is an effective analgesic during this time period. It was anticipated from previous clinical experience that the analgesic effect of flunixin meglumine would last for at least 8 h and our results support this claim. The significantly decreased stride length of calves in the SURG + EPI + F subgroup compared with calves in the SURG and SURG + EPI subgroups at 24 h postcastration suggests that the effect of flunixin meglumine had worn off by this time (). We hypothesize that the inflammation and pain associated with castration may be delayed in calves given a single treatment of flunixin meglumine. If the calves had been observed for 48 h postcastration, a more accurate time line for the effects of flunixin meglumine might have been provided and the level of pain the calves experienced after the drug had worn off determined.
The ability to visually classify the calves as “in pain” was correlated to the length of time after castration. Calves were more likely to be classified as “in pain” sooner rather than later after castration (). This finding was expected, but unfortunately the observer of the calves was aware of this progression in time. Increasing age also increased the odds of being classified as being “in pain” (). This suggests that castration should be done when calves are as young as possible. Or it could suggest that it is easier to assess pain in older calves. We believe that older calves did experience more pain, due to the likelihood of there being more tissue trauma in older (larger) calves and, therefore, more inflammation. Visually scoring the level of pain was a subjective process and perhaps should have been video recorded and viewed in a random playback method to help validate the technique. However, the visual pain assessment results, when taken in conjunction with the data obtained from the stride length measurements, helped to support the conclusion that flunixin meglumine in association with caudal epidural anesthesia provided an analgesic effect for up to 8 h postcastration.
It is interesting to note that the number of calves that vocalized during the castration procedure was small (14%). Previous studies have used vocalization as an indicator of pain (26
), signifying that it can be an effective pain measure. Watts and Stookey (27
) suggested that when the effects of severe stressors are being investigated, vocalization measurements can prove useful. In the present study, the small number of vocalizations may indicate that castration is a less severe stressor than branding, though it is difficult to compare our results with those from studies on branding where different aged animals and different breeds of cattle were used (27
Our results suggest that instantaneous scan sampling at 15-min intervals was too crude a measure to detect postcastration differences between treatment subgroups. However, in the present study, we repeatedly separated calves from their mothers at timed intervals in order to obtain stride length measurements. This likely disrupted their behavior patterns and may have masked changes in behavioral activities detected by our visual observation.
We anticipated that the calves receiving a lidocaine caudal epidural would feel less pain and, therefore, struggle less during castration on the tilt table (as measured via the load cells). The results contradict this hypothesis, suggesting that struggling due to restraint may have masked the pain experiences or that the lidocaine with epinephrine caudal epidurals were ineffective. However, after administration of the epidural, many of the calves were unable to move their tail suggesting that the epidural placement was correct. Anatomical differences may account for some of the differences between calves. There are 3 nerves that act as sensory innervations to the scrotum; the ventral branches of the first 2 lumbar nerves, the genitofemoral nerve, and the pudendal nerve (29
). Differences can be found between animals and between animals of different ages as to the exact location and amount of testicular innervation (30
). In the present study, not all of the nerves may have been affected by the lidocaine, so it is possible that some calves still had feeling in the scrotum or spermatic chord. Another point is that some calves appeared ataxic and were unable to move their tail, but still struggled during the surgical procedure. This suggests that calves could have been struggling against the restraint (tip table) rather than because of the pain. Maximal caudal epidural anesthesia using 2% lidocaine with epinephrine generally lasts for 30 to 150 min (31
). Differences in the processing groups of calves (age, weight, and the time lapse between injection and castration) may be the explanation for some of the associations between subgroups of calves and exertion force outcomes (peaks and standard deviation) obtained. We were unable to standardize and evaluate the precision and accuracy of the MMD machine prior to its use. The validity of the MMD measurements are in question, given that the time between treatment and castration was not associated with the number of calf struggles (number of peaks in the analog signal) and was negatively correlated with the standard deviation of the signal (exertion force of calves).
Stride length and the number of steps taken by calves after castration appear to be useful measures of the postprocedural pain calves may experience. Based on stride length measurements and visual assessments, flunixin meglumine in combination with lidocaine with epinephrine caudal epidural anesthesia can be considered to provide effective pain control for at least 8 to 12 h postcastration.