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Progesterone Concentrations in Beef Heifers Bred at Puberty or Third Estrus^1,2,

Montana State University, Bozeman 59717 and U.S. Department of Agriculture, Miles City, MT 59301

Abstract

Peripheral serum progesterone concentrations were evaluated in beef heifers following breeding at puberal (E1, n=63) or third (E3, n=45) estrus. A blood sample was collected on d 6 ± 1, 9 ± 1 and 12 ± 1 (estrus = d 0) after the puberal estrus of all heifers and after the third estrus of E3 heifers. Progesterone concentrations were higher (P<.05) for heifers in E1 compared with heifers in E3 on d 6, 9 and 12 after breeding to a fertile bull. Progesterone concentrations on d 6, 9 and 12 did not differ (P>.10) between pregnant heifers in E1 and E3; however, non-pregnant heifers in E1 had higher (P<.05) concentrations of progesterone compared with non-pregnant heifers in E3 on each day. Concentrations of progesterone did not differ (P>.10) between non-pregnant heifers in E1 and heifers of E3 during their puberal cycle. Pregnant heifers in E1 and E3 had higher (P<.05) concentrations of progesterone on each day than non-pregnant heifers in their respective treatments. There were no interactions (P>.10) between treatment, pregnancy status and day-of-estrous cycle for concentrations of progesterone. Results of this study indicated that luteal function differed between heifers that failed to conceive at their puberal estrus and heifers that failed to conceive at third estrus. However, concentrations of progesterone did not differ between heifers that conceived at puberal or third estrus. The relationship of changes in luteal function from the puberal through the third estrous cycle and pregnancy is not clear.

¹ Contribution no. J-1947 from Montana Agric. Exp. Sta. The authors express their appreciation to Ann Darling, Dave Phelps and the Physiology Crew at Miles City for their technical assistance.

² This study was a contribution to the Western Regional Project, W-112; Reproductive Performance in Domestic Ruminants.

³ Dept. of Anim. and Range Sci., Montana State Univ. Address reprint requests to James G. Berardinelli.

⁴ Present address: Anim. and Dairy Sci. Dept., Univ. of Georgia, Athens 30602.

⁵ Fort Keogh Livest. and Range Res. Lab., USDA-ARS, Miles City, MT.