The effect of reward duration on demand functions for rest in dairy heifers and lying requirements as measured by demand functions

The present experiment investigated the effect of reward duration on elasticity of the demand function for rest, and assessed for how long dairy heifers are motivated to lie on a 24-h basis. In Part 1, eight heifers housed in tether stalls had free access to lie down for 9 h daily, while they could work for access to lie during 6-h daily test sessions. The price of access to lie down was varied by varying the fixed ratio of pressing a panel per reward (FR) from 10 to 50, and the reward duration was 20, 30, 50 or 80 min. The reward duration affected the elasticity of the demand function. Based on the number of rewards earned, the elasticity of the demand function for 20-min rewards was more elastic (elasticity −0.33) than the demand functions for the longer reward duration periods (elasticity −0.07 to −0.08; P < 0.001). Also based on the lying time during test sessions, the elasticity of demand function for 20-min rewards was more elastic (elasticity −0.32) than demand functions for the longer reward duration periods (elasticity −0.01 to −0.10; P < 0.001). Heifers interrupted lying during the reward periods more often, the longer the reward duration (P < 0.001). With a reward duration of 50 min, heifers earned approx. 4 h of lying in addition to the 9 h where they were free to rest, i.e. they could lie down for a total of 13 h. In Part 2, the same dairy heifers were free to rest for 6 h and could work for access to lie during 12 h daily. Reward duration was always 50 min. Here heifers earned approx. 7 h of lying during the test sessions (elasticity −0.07). During the 6 h where the heifers were free to rest they lay down during on average 5.5 h, i.e. total lying time was on average 12.5 h. This suggests that heifers of this age and stage of pregnancy have an inelastic demand for rest of about 12-13 h per 24 h.