The proportion of ewes lambing of those exposed to the ram, litter size and weight at lambing, and at weaning, pre-weaning lamb mortality and changes in ewe weight during suckling, were studied in 326 ewes (931 lambings) representing two pure breeds, Oxford and Suffolk, and four crossbreds, Oxford♂ × Suffolk♀, Suffolk♂ × Oxford♀, North Country Cheviot♂ × Oxford♀ and Cheviot♂ × Suffolk♀, mated to purebred and crossbred rams during 9 years. The proportional fertility in the Oxford and Suffolk ewes (0·82 and 0·85 respectively) was lower than that of Suffolk × Oxford (0·93), Oxford × Suffolk (0·94), Cheviot × Oxford (0·88) and Cheviot × Suffolk (0·88) ewes. Litter sizes of Suffolk ewes at birth and at weaning were 1·6 and 1·3 lambs, 0·3 (0·22) and 0·3 (0·27) lambs largeir than those of the Oxford ewes, respectively. The corresponding figures for litter weight were 1·29 kg (0·22) and 11·8 kg (0·36). Oxford ewes lost 0·10 of their weight during nursing compared with 0·14 for the Suffolk and the two reciprocal crosses. The estimates of heterosis in litter size and litter weight at birth were 0·085 and 0·082, and at weaning were 0·13 and 0·18 respectively.
The results showed that Oxford crosses were better in performance than the pure Oxford, whereas the Suffolk crosses were similar in performance to the pure Suffolk, except in the proportion of ewes lambing of those exposed and pre-weaning mortality, in which the crosses were 0·07 and 0·20 superior to the pure Suffolk, respectively.
Breed of sire had a significant effect on litter weight at birth and at weaning, whereas the interaction breed of sire × breed of dam was a significant source of variation on litter size and litter weight at birth and at weaning. Litter performance at birth was significantly lower in 2-year-old ewes and in ewes sired by 3-year-old or older rams than in other age groups