Milk allowance affects play responsiveness in calves.

Play behaviour is increasingly being used as an animal welfare indicator. Calves on standard milk allowances, thought to be experiencing hunger, often play less than those fed more due to lower nutrition, negative affect, or both. However, various aspects of play can be measured, and the most sensitive forms of play to hunger and welfare are unclear. We explored the relationship between milk allowance and various play behaviours and characteristics, including those that have received less attention, such as play responsiveness (i.e. the likelihood of a calf playing in response to their partner's locomotor play). Pair-housed calves (n = 22) on a commercial dairy farm were provided either 6 L milk per day (standard) or 12 L milk per day (enhanced). Each pair included one calf from each diet group. At 16 days old (7 days after imposition of the diets), the calves were habituated to the play arena, before their play behaviours were assessed firstly in pairs and then individually on two consecutive days. Unexpectedly, milk allowances did not affect locomotor play (i.e. running, kicks/bucks) in either test. The expected effects may have been masked by the calves' high motivation to play in the arena, as their home pens greatly limited play opportunities. However, calves fed less milk showed lower play responsiveness, potentially due to them having a higher 'threshold' for play being elicited or having reduced motivation to engage in social play. The group differences in play responsiveness suggest that this variable may be a more sensitive welfare indicator warranting further research.