Progesterone concentrations in beef heifers bred at puberty or third estrus.

Peripheral serum progesterone concentrations were evaluated in beef heifers following breeding collected on d 6 +/- 1, 9 +/- 1 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 less than .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 greater than .10) between pregnant heifers in E1 and E3; however, non-pregnant heifers in E1 had higher (P less than .05) concentrations of progesterone compared with non-pregnant heifers in E3 on each day. Concentrations of progesterone did not differ (P greater than .10) between non-pregnant heifers in E1 and heifers of E3 during their puberal cycle. Pregnant heifers in E1 and E3 had higher (P less than .05) concentrations of progesterone on each day than non-pregnant heifers in their respective treatments. There were no interactions (P greater than .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.