The association between lambing status and future reproductive performance of ewes grazing oestrogenic pastures

Eight flocks of ewes grazing improved pastures which contained oestrogenic clover (Trifolium subterraneum cv. Yarloop) were studied. Two data-sets were available for analysis: Data-set 1-four flocks, three with age-structure unknown. Records on the number of ewes lambing per ewe exposed to the ram (EPJ) and the number of lambs born per ewe exposed to the ram (LBJ) were collected during 1972-75. The repeatability of EPJ and LBJ were estimated within flocks by the regression methods. There was considerable variation between flocks in the values obtained. An analysis of variance was conducted including lambing status (0, 1 or 2 lambs at birth) of the ewes in 1972 as a main effect. Class 1 ewes had a better reproductive performance in subsequent years than either class 0 or class 2 ewes. The effects on flock performance of culling barren ewes were simulated, showing that little improvement in subsequent performance would result. Data-set 2-four flocks, age-structure known. EPJ and LBJ were recorded in 1973 and 1974. The repeatabilities of EPJ and LBJ were estimated within age groups of each flock by regression methods. The results were less variable than in data-set 1. An analysis of variance was conducted, including lambing status (0 or 1 lamb at birth) in 1973 as a main effect. Class 1 ewes had a better reproductive performance in 1974 than class 0 ewes. Simulation of the effects of culling ewes which were barren in 1973 showed that only a small improvement in reproductive performance of ewes in 1974 could be obtained. It was concluded that the culling of barren ewes is unlikely to provide a solution to the low reproduction rate of ewes grazing oestrogenic pastures.