Sources of variation in the number of lambs born in a grazing experiment

Abstract

Considerable variation was observed in the number of lambs born to fine-woolled Merino ewes during a 4-year grazing experiment in which the factors were four pasture management systems x three stocking rates. Attempts at interpretation by standard analysis of variance were ineffectual and accordingly the data were subjected to further examination by using non-orthogonal and contingency models and pattern analysis.

The strongest influence discerned was the tendency of particular ewes to consistently produce 0, 1, or 2 lambs. The majority of births (72%) were singles; twinning was relatively uncommon (<10 %) and was most prevalent in the third year; barrenness was relatively high (18 %) particularly in the first year. Significant associations were found between the number of lambs born and the ewe's liveweight and liveweight change before and after mating. Pattern analysis suggested that part of this variation was associated with differences in the initial pasture composition between plots. There was a highly significant year effect in all the relevant animal variables examined but it was not possible to separate the contributions of weight and age to this effect.

It is suggested that some of the variation in reproduction rate between plots was due to non-uniform allocation of the inherently barren and twinning ewes.