The effects of feeding supplements in relation to sward height in the autumn on the reproductive performance of ewes grazing predominantly perennial ryegrass swards were studied in four experiments with three genotypes. Treatments in which either no supplements were given or supplements were offered according to the height of the sward were compared when ewes were grazed on swards which were managed to decline in height from > 5 cm to 2·5 cm either before or after mating. At a sward height of < 3·5 cm, 400 g ground and pelleted barley was offered per head per day; at < 3·0 cm, 700 g was offered; and, at < 2·5 cm, hay was offered in addition to the barley. Responses to these treatments were compared with that of a treatment in which a sward was maintained at > 5 cm. Sward heights were controlled by varying stocking rate. In experiment 1, with 305 Scottish Blackface ewes, supplementation was not necessary until after the start of the mating period. In experiment 2, with 200 Brecknock Cheviot ewes, the supplementation protocol was examined on swards declining in height either before or after a synchronized mating. Experiment 3 repeated experiment 2 with 200 Beulah Speckled-face ewes and experiment 4 repeated only the pre-mating phase of experiment 3 with 195 Beulah Speckled-face ewes.
In experiment 1, ovulation, ova loss and potential lambing rates were determined from the number of corpora lutea and viable embryos at slaughter at 8 to 10 weeks after the start of mating in 103 ewes and lambing rate was determined from the number of lambs at lambing in the remaining 202 ewes. In experiments 2, 3 and 4, ovulation, ova loss and potential lambing rates were determined from the number of corpora lutea and viable embryos at slaughter at return to service or at 4 weeks after a synchronized first mating.
Grazing a sward declining in height below 3·5 cm in the pre-mating period significantly reduced ewe live weight, body condition and reproductive performance, particularly in the relatively prolific Beulah Speckled-face breed, compared with grazing a sward maintained at > 5 cm. Supplementation at the levels offered overcame some of the effects of the lower sward height treatment in the pre-mating period but did not compensate fully. Grazing a sward declining in height below 3·5 cm in the post-mating period, although reducing ewe live weight and body condition, had little effect on reproductive performance, compared with that achieved on a sward maintained above 4 cm. Supplementation at the levels offered at the lower sward height during the post-mating period produced no increase in overall reproductive performance and had a negative effect on some reproductive parameters.