Inhibitory effect of selenium supplementation on the reproductive performance in synchronized merino sheep at range conditions in a selenium-deficient area

The effect of selenium (Se) supplementation on the reproductive performance of Merino ewes mated out of the normal breeding season was studied in a 2 (no Se supplementation vs Se supplementation) x 2 (ewes mated at natural oestrus vs synchronized oestrus) factorial design with 50 ewes per treatment combination. Synchronization of oestrus was achieved by intravaginal insertion of 40 mg cronolone sponges for 14 days and administration of 440 IU PMSG at sponge withdrawal. Se supplementation was by the addition to the concentrate with 0.5 mg/kg of Se in the form of selenomethionin, for the 3 months prior to the mating and during gestation. Ewes were kept under range conditions in south-west Spain, and exposed to Merino rams at a female: male ratio of 4 : 1 from 2 days following sponge withdrawal, or the equivalent time, for 21 days. Se supplementation alone did not improve significantly ewe fertility or lamb birth weight, but there was a positive effect of synchronizing ewes mated out of the breeding season as more of these ewes lambed and produced more lambs than those not synchronized. However, a strong interaction was found between synchronization and Se supplementation, causing a deleterious effect on the reproductive performance of ewes. This negative effect, presumably related to high embrionary mortality caused by Se toxicity, should be taken into account for oestrus synchronizing in Se-deficient areas. The supplementation with Se, for the prevention of nutritional myodystrophy degeneration in lambs, should be conducted in a fashion which does not generate high levels of Se in the ewes around the mating period.