M. Kaim1, S. Ertracht1, R. Braw-Tal1 and D. Wolfenson2
1Agricultural Research Organization, Bet Dagan, 2The Hebrew University, Rehovot, Israel

Progesterone concentration and number of follicular waves during the estrous cycle preceding first insemination are associated with the success of conception in dairy cows. The objectives of this study were to: 1. establish a hormonal treatment that induces high progesterone levels combined with three follicular waves during the estrous cycle; 2. compare the reproductive performance of dairy cows following hCG treatment with that of untreated control cows.
Experiment 1. Lactating Holstein cows at 66 days postpartum, yielding 40 kg milk/day, which exhibited estrus (n = 83) following synchronization, were treated i.m. with hCG (2500 IU, Pregnyl) or saline on day 6 of the cycle. Follicular and luteal dynamics were monitored by ultrasound, blood samples were taken every 2 to 3 days, and PGF2a was injected on day 21 of the cycle. Most hCG cows (98%) had two corpora lutea (CL) compared with 34% of control cows (p < 0.01). Similarly, most hCG cows (88%) exhibited three-wave cycles, compared with 12% of controls (p < 0.01). Cows treated with hCG, compared with control cows, had pre-ovulatory follicles 7 mm or more later in the cycle (15.7 vs 12.1 days, p < 0.05), had a shorter interval to day of estrus (8 vs 10 days, p < 0.05), and had similar follicle diameters on the day of estrus (16 mm). Mean total CL volume and progesterone concentrations during the cycle were higher in hCG than in control cows (p < 0.05); at mid-luteal phase, progesterone values were 4.8 vs 3.7 ng/ml, respectively (p < 0.05).
Experiment 2. Lactating Holstein cows (153 primiparous and 324 multiparous) on 2 commercial dairy farms, were enrolled into this study over 12 months. In each herd, cows were allocated according to lactation number and days in milk, to one of two groups: hCG - cows were treated im. with hCG (2500 IU, Pregnyl) on day 6 of the estrous cycle preceding 1st AI , or untreated - control cows.
Interval from calving to 1st AI were similar for primiparous (PP) and multiparous (MP) cows in both groups. Conception rates at 1st AI for PP and MP cows, were respectively, 51.3% and 29.9% in the control group vs. 61.0% (n.s.) and 35.0% (n.s.), in the hCG group. Overall conception rates at 1st AI for control and hCG groups were respectively, 36.6% and 43.6% (n.s.).
We conclude, that a single treatment with hCG on day 6 of the cycle increases progesterone concentrations and induces three follicular waves during the cycle; but it did not improve conception rate.