Management of dairy heifers: can operant conditioning be an effective and feasible tool to decrease stress and ease animals’ close contact and handling?

Beside health monitoring, a regular check of dairy heifers’ growth rate is desirable, but it is rarely done since procedures that require restraint and handling can be associated with substantial stress for both animals and farmers. Inexperienced heifers, especially if they are highly responsive to humans, may find restraint and handling potentially aversive. This study investigated whether training heifers of different age and responsiveness toward humans (RTH), through operant conditioning, could reduce stress in animals, ease close contact and handling, and be feasible in terms of farmer’s effort. We assessed 60 Holstein heifers of 2 age classes (Young, n = 29, 291 ± 39; Old, n = 31, 346 ± 62 d) according to the Avoidance Distance Test (ADT) and classified them as Confident (C, n = 20), Neutral, (N, n = 21) or Non-confident (NC, n = 19). Half of the heifers of each age and RTH class was trained (Tr, n = 29), whereas the other half was not (NTr, n = 31). The Tr heifers were subjected to target training for 8 sessions and positively reinforced with feed to allow being touched on the muzzle, rump and perineum. In case a heifer refused positive reinforcement, the trainer stepped back as negative reinforcement. In the last week of the experiment the effect of training on the reaction to handling was assessed in all heifers. We measured heart rate (HR), root mean square of successive inter-beat interval differences (RMSSD) and fecal cortisol metabolites (FCMs). The presence of behavioral distress signs was recorded as well. The ADT was performed a second time 24 h after the measuring session. All the trained heifers, regardless of RTH class, successfully accomplished the target training task in 6 sessions, spending on average 25.3 s per session, each. All the trained heifers allowed to be touched on the rump and perineum at the end of the 4th session. Training NC heifers required more time compared with the others. Trained heifers showed higher RMSSD (14.2 vs. 16.9), indicating a lower vagal tone, and thus, a slightly lower stress level than NTR. Training did not lead to differences in HR, FCMs or presence of stress behavioral signs. NC heifers had the highest mean baseline FCM values (38.4 vs. 30.3 vs. 29.1 ng/g) compared with N and C. NC heifers showed also the lowest value of FCMs, 12 h after the measuring session (36.7 vs. 44.6 vs. 49.7 ng/g), likely due to a decreased responsiveness of the adrenal gland to a stressor. The average avoidance distance decreased between the beginning and the end of the experiment, especially for N and NC heifers, regardless whether they were trained or not. These results show how using operant conditioning on some heifers, not only decreased their vagal tone, but reduced the responsiveness to humans of all the animals, trained and not trained; in the latter case, through non-associative learning, such as habituation.