Fertility of non-puberal estrus, pregnancy rates and progesterone concentrations of beef heifers bred at puberal or third estrus

The objectives of this study were to determine: 1) if pregnancy rates of beef heifers bred at puberal estrus differed from pregnancy rates of heifers bred at their third estrus; 2) if the pattern of progesterone concentrations differed between heifers bred during the puberal or third estrous and relate these changes to pregnancy rates associated with breeding at these times; and 3) if heifers bred at a non-puberal estrus have the ability to become pregnant.Crossbred beef heifers were obtained from Manhattan, MT (L1, N = 102) and Miles City, MT (L2, N = 54), and the experiment was conducted at Miles City.Heifers were assigned randomly within location to one of two treatments: 1) to be bred by a fertile bull on their puberal estrus (El, N = 89) and 2) to be bred by a fertile bull on their third estrus (E3, N = 67).Heifers were fed to gain .56kg • head-1 • d-1 and were observed for estrus twice daily.After exhibiting first estrus (puberty) and breeding, each heifer in El was palpated rectally on d 6, 9 and 12 ± 1d (Estrus = d 0) for the presence of a corpus luteum, and a venous blood sample was collected for assay of progesterone (P4) by radioimmunoassay.Heifers in E3 were palpated and bled on the same schedule as heifers in E1 after first estrus and after being bred to a fertile bull on their third estrus.Pregnancy rates were determined by rectal palpation at a minimum of 38 d post-breeding.Location did not affect (P>.10) weight at puberty or breeding, however, heifers from L1 were younger (P<.05) than heifers from L2 at puberty and breeding.Pregnancy rates were 57 and 78% for heifers in E1 and E3, respectively (P<.05).The probability of a heifer in E1 becoming pregnant increased (P<.05) with increasing age, while age was not a factor (P>.10) for heifers in E3.Progesterone concentrations were higher (PC.05) for heifers in E1 compared to heifers in E3 on d 6, 9 and 12. Progesterone concentrations on d 6, 9 and 12 did not differ (P>.10) between pregnant heifers in E1 and E3.Non-pregnant heifers in E1 had higher (P<.05) concentrations of P4 compared to non-pregnant heifers in E3 on each day.Concentrations of P4 did not differ (P>.10) between non-pregnant heifers in El and heifers in E3 during their puberal cycle.Pregnant heifers in El and E3 had higher (P<.05) concentrations of P4 on each day compared to non-pregnant heifers in their respective treatments.No difference (P>.10) was observed in concentrations of P4 during the puberal cycle of heifers in E3 which were determined to be pregnant or non-pregnant after breeding on their third estrus.There were no interactions (P>.10) between treatment, pregnancy status and day-of-cycle for concentrations of P4.Of. the heifers in E1 (n = 27) that exhibited non-puberal estrus, none became pregnant after breeding by fertile bulls.In conclusion, puberal estrus in beef heifers had a lower fertility compared to breeding at third estrus.Differences in P4 concentrations in pregnant and non-pregnant heifers of El and E3 indicated the possibility that P4 concentrations may play a role in determining fertility of beef heifers bred at puberty.In addition, the potential to become pregnant is apparently absent in heifers which are bred at a non-puberal estrus.