PREGNANCY RATE IS GREATER WHEN THE CORPUS-LUTEUM IS PRESENT DURING THE PERIOD OF PROGESTIN TREATMENT TO SYNCHRONIZE TIME OF ESTRUS IN COWS AND HEIFERS

Our hypothesis was that conception in bovine females would be enhanced if the corpus luteum was present during the period of progestin treatment to synchronize estrus. In this study, 67 heifers (one replicate) and 124 cows (two replicates) were randomly assigned to one of two treatment groups. Seven days after estrus (Day 0), all animals were implanted with norgestomet and the implant remained in place for 10 days. All implants were removed on Day 17. Cows and heifers in one group received prostaglandin F2alpha (PGF2alpha) on Day 7 of the estrous cycle (PG 7; norgestomet without corpus luteum), and animals in the second group received PGF2alpha on Day 17 (day of implant removal; PG 17; norgestomet with corpus luteum). All heifers and cows exhibiting behavioral estrus were artificially inseminated 12 h after estrus was detected during a 7-day period following removal of norgestomet. Blood samples were collected from cows of replicate 1 to determine serum concentrations of progesterone and 17beta-estradiol. Percentage of females that had calves as a result of artificial insemination was greater (p < 0.01) in the PG 17 group (87% and 78% cows [two replicates) and 58% heifers) compared to the PG 7 group (31% and 44% cows [two replicates] and 41% heifers). Concentrations of 17beta-estradiol in plasma were not different (p > 0.05; pooled SEM = 0.67) at Day 7 (beginning of the norgestomet treatment; 2.34 and 3.09 pg/ml, respectively); but they were different (p < 0.01) at Days 10 (9.53 and 1.65 pg/ml), 13 (12.22 and 1.31 pg/ml), and 16 (11.76 and 2.82 pg/ml for cows in the PG 7 and PG 17 groups, respectively). As we hypothesized, incidence of conception in bovine females receiving norgestomet with the corpus luteum present (PG 17) was greater than in cows treated with norgestomet in the absence of the corpus luteum. Greater concentrations of 17beta-estradiol during the treatment period may contribute to the decreased incidence of conception of cows treated with the synthetic progestin in the absence of the corpus luteum.