Absence of variation between sheep in motivation to feed

Abstract

Behavioural demand methodologies allow us to quantify the behavioural needs of farm animals. Traditionally, few animals are used due to the difficulties of training, and individuals are cycled through all experimental treatments. In order to conduct a behavioural demand experiment whereby animals are randomly allocated to treatments, it is necessary to know the inherent differences in motivation between animals. Therefore, the aim of this experiment was to investigate individual differences in the motivation of sheep to work for a food reward at a fixed level of feeding. Twenty-four two-year-old Merino ewes were trained to press a lever for an 8.5g reward of lupin seed. Each animal was tested in 20-minute sessions, carried out over 5 days, with an ascending sequence of fixed ratio values (FR; 5, 10, 20, 30, 50), changing daily. Total daily rations were 1.2 times maintenance requirements, half of which was fed as lupins 20 minutes prior to testing. The balance, minus reward consumption, was fed as lucerne pellets 2 hours after test completion. Data were log transformed before performing an Analysis of Variance. The number of rewards obtained declined as the FR increased (P < 0.001) with mean rewards obtained being 45, 36, 24, 18 and 11 for FR-5 to 50 respectively. The difference between animals in the number of rewards obtained was significant (P < 0.001), and there was no animal X FR interaction (P = 0.20). If motivation is defined as the rate of change in rewards obtained as workload increases, then the individual differences in this study imply a difference in work intensity not motivation, which would be reflected in a significant animal X FR interaction. These results suggest that it would be appropriate to design a behavioural demand experiment for feeding motivation, in which sheep are randomly assigned to treatment groups.