Effect of Pre-oestrus Body Weight on Lambing Success in Primiparous Sheep ( Ovis aries ).

We investigated the relationship between reproductive performance in Norwegian domestic sheep at the age of 12 months and autumn body weights (i.e., pre-oestrus condition), among primiparous females in two different populations, Buskerud and Nordland. We hypothesized the existence of a threshold weight below which lambing either does not occur, or the probability of lambing is very low. We also expected heavier females to have higher probabilities of successful lambing. Using mixed-effects logistic regression analyses, we observed spatio-temporal variability in lambing success of primiparous sheep. The proportion of primiparous females giving birth between 1993 and 1999 varied from 87.4 to 92.75% (mean=91%) and from 82.02 to 90.53% (mean=87%) in Buskerud and Nordland, respectively. As expected, higher autumn weight increased the probability of lambing in the first year of life, even after accounting for the random variability between year and farm. We were unable to find a threshold weight necessary for successful lambing, probably as a result of varying management practices. We conclude that juvenile weight before oestrus is, among others, a good predictor of the probability of successful lambing in primiparous sheep. [ABSTRACT FROM AUTHOR]