The occurrence of pregnancy in Merino ewes teased with testosterone- treated wethers before the start of joining

Abstract

Five experiments were conducted to develop simple strategies for using testosterone-treated wethers to increase the reproductive efficiency of ewe flocks to which entire rams were joined before the peak of the breeding season. In experiments 1–4, a number of strategies involving fewer injections, single injections of more testosterone with and without wethers left in the flocks during joining, were compared with the standard technique used for preparing male behaviour in wethers (3 injections of 150 mg testosterone enanthate at 7-day intervals starting 28 days before the introduction of entire rams). In experiment 5, the single dose strategy, either subcutaneous or intramuscular, was investigated. The treated wethers were joined with ewe flocks that had been isolated from rams for at least 28 days, 14 days before the entire rams were joined with the ewes for 6 or 8 weeks. The weeks of conception in all groups were estimated using real-time ultrasound imaging. This allowed patterns of conception in the various treatment groups to be compared. These observations also provided the information for determining pregnancy status and the percentages of fetuses present. In 4 of the experiments, the teased groups had a significantly more concentrated pattern of conception than did the unteased control groups. The results of this study have shown that the reproductive efficiency of ewe flocks is improved by having a more concentrated lambing following the introduction of wethers treated with a subcutaneous injection of 375 mg testosterone enanthate before the entire rams are joined with them, even when the wethers are left in the flock during the joining period. As a result, the length of the joining period can be reduced to 5–6 weeks compared with the 8 weeks usually used for unteased flocks.