Effect of debonding on stress indicators in cows and calves in a cow-calf contact system

Premature separation, entailing a mixture of stressful events, follows a period of cow-calf contact in dairy production. We hypothesized that timing and length of debonding (i.e., gradually adapting cow and calf to separation) may affect behavioral responses indicating stress and reinstatement of the cow-calf bond. Initially, cows had 24 h/d smart-gate access to their calves before reduction to 12 h/d, 6 h/d, and finally 0 h/d, either commencing at 4 wk after birth over 28 d: long debonding (LDB, n = 16 pairs), or at 6.5 wk, over 10 d: short debonding (SDB, n = 14 pairs). We aimed to compare the vocal response (high- and low-pitched vocalizations) and proximity (<1 m to separation barrier) of LDB versus SDB cows and calves at each reduction of access and finally at 0 h/d contact. We also aimed to assess the impact of calf supplemental milk intake on their vocal behavior. Direct observations (2 h × 2 d) were performed during the following different phases: 24 h/d access (baseline, BL24h), once cow access was reduced (separation, SEP12h), once pairs were habituated to the change (BL12h), after cow access was further reduced (SEP6h), once pairs were habituated (BL6h), and at 0 h/d contact (SEP0h). Mixed effect linear regression models indicate that overall, treatment did not affect cow behaviors nor the main indicator of separation stress, high-pitched vocalizations. Both calves and cows responded with vocal and reinstatement behavior once cows' access was reduced, with great individual variation. The LDB calves spent less time close to separation barrier and emitted more low-pitched vocalizations. Otherwise, the effect of treatment differed with phase: SDB calves emitted more high-pitched vocalizations in response to the initial reductions of cow access than LDB calves. The LDB cows (but not calves) produced more high- and low-pitched vocalizations when access was reduced to 6 h/d and 0 h/d. Calves drinking >1.5 L of supplemental milk/d emitted fewer high-pitched vocalizations. In conclusion, a longer debonding period initiated at a lower age before separation may alleviate the initial behavioral response to separation, especially for calves. The vocal response of calves increases with age at separation but is modulated by intake of supplemental milk. For cows, our results indicate that the studied debonding methods affected the behavioral response to separation only to a limited degree.