Learning, Memory and Stress Evaluation in Lambs Weaned at Different Ages

Weaning is a critical time in a lamb’s life and may be considered its most stressful period. In livestock, it is not possible to stop this process, but it could, at least, be attenuated. The main objective of this study was to evaluate stress, learning and memory in lambs weaned in different ages. Thirteen pregnant ewes were divided in three groups, one with five and two with four ewes. The first group of lambs from these ewes was weaned with four weeks of age, the second, with six weeks and the third, eight weeks. Complete visual, olfactory and auditory separation was applied. Various tests (plasma and faecal cortisol levels, learning and memory tests) were performed on the lambs after the weaning process. Plasma cortisol levels were different in the first group before and after weaning. In the learning and memory tests, animals weaned at six and eight weeks of age showed more stressful behaviour, including high pitched bleating, attempts to escape from the test pen and less success in finding food. The group weaned at a younger age was calmer and, in the learning test, could find the food in 12 out of 15 trials, compared with 1 out of 12 in group 2 and none in the third group. The memory tests were performed in the same way as the learning test, 48 hours and one week after weaning. The results from memory tests were similar to the learning test. Animals weaned at 4 weeks of age showed lower stress behaviour when submitted to tests with isolation and showed better learning and memory capability than animals weaned older (six and eight weeks old).