The sucking behaviour and milk intake of one- to three-week-old triplet lambs during natural and competitive suckling situations

Within triplet litters, light-born lambs are at greater risk of dying than heavier lambs in the first week of life. However, the implications of within-litter differences in sucking behaviour for milk intake and growth beyond the first week are unclear. We hypothesized that within litter, triplets born lightest would still be lightest at one to three weeks of age, that these lambs would have lower milk intake than heavier lambs, and that differences among lamb ranks (heaviest, medium, or lightest-born in litter) would be exacerbated in a competitive situation. Triplets in 10 litters were ranked according to birth weight and their sucking behaviour measured in two situations when they were 8-17 days old. In the morning session, undisturbed behaviour was recorded for 100 min, after which the lambs were separated from the ewe for four hours. They were then returned to the ewe and competitive sucking behaviour was recorded for 15 min. Measures of live weight and abdominal girth were taken before and after each observation period to estimate milk intake. At 8-17 days of age, the lightest-born lambs were still lightest but the medium and heavy born lambs no longer differed in live weight (P = 0.002). During the undisturbed period, heavy-born lambs sucked less often (P = 0.026) and for less time overall (P = 0.007) but gained similar live weight to their lighter siblings (P = 0.299), indicating that they were more efficient at extracting milk. In the competitive session, light-born lambs tended to gain less live weight (P = 0.086) and competed with the medium-born lambs for the teat not preferred by the heavy lambs (P = 0.0005). These observations indicate that heavy-born lambs are efficient feeders, that medium-born lambs work harder to achieve the same milk intake, and that light-born lambs achieve lower milk intakes, contributing to their persistently lower weight. Thus, management strategies such as fostering or supplemental feeding should be focussed on the lightest-born triplets.