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Effect of Feed Treatment and Exogenous Estrogen and Progestogen on Puberty and Lambing Rates in Ewe Lambs^1,2,

Montana State University, Bozeman 59717

Abstract

Four hundred seventy-four ewe lambs (5 to 6 mo of age) were assigned within breed to two postweaning feed treatments; 1 = alfalfa pellets ad libitum and 2 = alfalfa pellets plus 20% barley or wheat ad libitum. Ten days prior to the start of breeding, approximately one-half of the ewe lambs within each feed treatment were treated with a single injection of 2.5 mg of estradiol valerate and 1.5 mg of norgestomet and implanted with 3 mg of norgestamet for 8 d. At the start of breeding, fertile Suffolk rams were fitted with marking harnesses and penned with ewe lambs; evidence of estrus and breeding was determined on a weekly basis by Visual examination for breeding marks. In the second year of the study, 7 to 13 d after recording estrous activity, all marked ewe lambs were bled; and blood was assayed for progesterone. In 1983, 60% of the ewe lambs showed estrus and 11% lambed compared with 48% exhibiting estrus and 30% lambing in 1984 (P.01). More (P.01) ewe lambs on feed treatment 1 displayed estrus, but more (P.05) lambed in the feed treatment 2 group. Steroid treatment resulted in more (P.01) ewe lambs showing estrus but fewer lambing (P.01). Examination of progesterone concentrations for evidence of ovulation and corpus luteum development indicated that treatment with steroids caused a large percentage of ewe lambs to exhibit estrus. However, many of these failed to develop a corpus luteum and become pregnant. Furthermore, pregnancy rate was reduced (P.05) in steroid-treated ewe lambs that developed a corpus luteum.

¹ Published with the approval of Director of the Montana Agr. Exp. Sta. as Journal Series Paper No. J1805. The authors express their appreciation for the excellent assistance of R. Adair, K. C. Curry, D. Byerley and K. Hanford throughout the course of this study.

² This study was supported in part by the United States Agency for International Development Title XII Small Ruminant Collaborative Research Support Program, Grant No. DAN-1328-G-SS-4093-00.