Effect of birthweight on survival in triplet-born lambs

Abstract

Lambing percentage is the main contributor to higher profit on New Zealand sheep farms and has increased from 98% in 1960 to 124% in 2006. As ewe litter size increases so does the proportion of triplet-born lambs and consequently preweaning mortality rate. The purpose of this study was to investigate the effect of birthweight (BWT) on survival in triplet-born lambs within and among litters. Data from a total of 594 triplet crossbreed lambs born to mixed-age ewes was analysed. For the statistical analysis, the lambs were allocated according to their BWT, and within the litter were then allocated to three BWT groups (light, medium and heavy). The following parameters were calculated: total litter BWT (TBWT), percentage of TBWT for each lamb (PBWT = BWT/TBWT), coefficient of variation for BWT within litter and lamb average daily gain from birth to weaning. The mortality rate for the light, medium and heavy lambs was 56, 40 and 28%, respectively, and consequently the lightest lamb in a litter was 3.2 times more likely to die than the heaviest lamb. In the light lamb group, as BWT increased mortality decreased (P = 0.006), for medium lambs BWT did not influence mortality (P = 0.88) and in the heavy group there was a trend of increased BWT associated with increased mortality (P = 0.10). The negative relationship between lamb mortality and PBWT was identical in the three BWT groups (P < 0.0001). The effect of BWT on lamb mortality became non-significant when PBWT was fitted in the same model. This means that, in triplet-born lambs, mortality is not influenced by the BWT of the lamb itself, but by the BWT of its littermates. It can be calculated that a 4-kg lamb has a 24% chance of surviving in a litter of 16 kg, a 60% chance in a litter of 12 kg and an 87% chance of surviving in a litter of 8 kg. It is concluded that reducing the variation in BWT within a litter, or specifically managing the lightest lamb at birth, will increase the competitive ability for survival of each lamb and, therefore, decrease overall mortality and further improve farm profitability.