SUBCLINICAL SELENIUM INSUFFICIENCY .2. THE RESPONSE IN REPRODUCTIVE-PERFORMANCE OF GRAZING EWES SUPPLEMENTED WITH SELENIUM

Merino and Border Leicester x Merino ewes grazed improved pastures at 6.3 or 12.5 ewes/ha for 4 years and were supplemented or not supplemented with intra-ruminal selenium (Se) pellets. The Merino and crossbred ewes were mated to Merino and Dorset Horn rams, respectively, and various measures of reproductive performance were recorded. Two cases of muscular dystrophy were observed in 2-3-day-old lambs, but the incidence of oestrus and fertility was not affected by differences in stocking rate, genotype or Se supplementation. Fecundity was higher in crossbreds than Merinos (1.64 v. 1.25, P < 0.001) but was not affected by stocking rate or Se status. Survival of lambs was increased (P < 0.05) by Se supplementation at the high stocking rate (0.74 v. 0.86) but not at the low (0.84 v. 0.83), and a significant (P < 0.01) genotype x litter size x Se interaction reflected greater sensitivity to Se supplementation by Merino singletons (from 0.75 to 0.95) than twins (from 0.75 to 0.69) and by crossbred twins (from 0.76 to 0.87) than singletons (from 0.89 to 0.87). Number of lambs weaned per ewe mated increased following Se supplementation from 0.81 to 1.04 at the high stocking rate, and decreased from 1.08 to 1.00 at the low; the interaction approached significance (P < 0.09). It is suggested that the response to Se supplementation is sensitive to factors in addition to Se status, and such factors may explain some of the variability in responsiveness reported in the literature.