RELATIONSHIP BETWEEN CIRCULATING PROGESTERONE AND CONCEPTION AT THE FIRST POSTPARTUM ESTRUS IN YOUNG PRIMIPAROUS BEEF-COWS

The objective of this study was to determine whether patterns of chang e in concentration of progesterone in circulation of primiparous beef cows were related to conception rates at the first postpartum estrus. Data were collected over 2 yr from crossbred primiparous 2-yr-old cows (n = 77). Cows were artificially inseminated when estrus was detected between 3 and 20 (yr 1) or 3 and 23 (yr 2) wk after parturition. Cows were exposed to intact bulls for 32 (yr 1) or 21 d (yr 2) after the p eriods of AI, and estrus detection also occurred during the period whe n natural mating occurred. After parturition, blood samples were colle cted twice weekly to determine time of transient (less than 1 wk) and sustained increases (more than 1 wk) in progesterone of more than 1 ng /mL of serum. Mean times from parturition to transient increase in pro gesterone, first estrus, conception, and sustained increase in progest erone were 91, 100, 108, and 105 d, respectively. Sixty-four percent o f the cows conceived as a result of AI at the first estrus that was de tected after parturition, 32% conceived at a subsequent estrus, and 4% failed to conceive. In 31.1% of the cows, a transient increase in pro gesterone was not detected before the first estrus after calving. Conc eption as a result of AI at the first estrus after calving was less if a transient increase in progesterone did not precede estrus (76 vs 41 %; P <.01). These data indicate that the majority of cows have increas es in progesterone before the first postpartum estrus. If the transien t pre-estrus increase in progesterone occurs in postpartum cows, there seems to be an enhanced conception rate as compared with cows without the increase in progesterone before their first estrus following part urition.