Effect of different doses of equine chorionic gonadotropin on follicular and luteal dynamics and P/AI of high-producing Holstein cows.

The hypotheses of this study were (1) that the administration of 400IU eCG in a TAI protocol would increase ovarian follicular growth and diameter of the largest follicle (LF), volume of the CL, and produce an earlier rise on serum concentration of progesterone (P4) to ultimately improve P/AI compared to non-treated high-producing Holstein cows; and (2) that 600IU of eCG could enhance any potential effects of a greater gonadotropin treatment upon follicular and luteal size and function, improving P/AI. Cows were subjected to a protocol of synchronization of ovulation for timed artificial insemination (TAI): D0-P4 device insert and estradiol benzoate, D8-P4 device removal and PGF2α; Experiment 1, D10PM - GnRH plus TAI; and Experiment 2, D10AM - GnRH, D10PM - TAI. In Experiment 1, at P4 device removal, cows were assigned to one of the two treatments to receive none (n=232) or 400IU (n=232) of eCG. In Experiment 2, again at P4 device removal, cows were assigned to one of the three treatments to receive no eCG, (n=166) 400 (n=145) or 600IU (n=145) of eCG. Pregnancy was diagnosed 35 days after TAI. Ultrasonographic examination of both ovaries was done in a subset of cows in Experiments 1 [no eCG (n=27) and 400IU eCG (n=14)], and 2 [no eCG (n=15), 400IU eCG (n=14) and 600IU eCG (n=11)]. Exams were conducted at device removal (D8) and TAI (D10) to measure the diameter of the LF; then twice daily from D10 to 13, to determine time to ovulation and the maximum diameter of the LF; and then 3 (D14), 6 (D17), 9 (D20) and 12 (D23) days after presumed ovulation, concurrent with blood sampling, to measure the volume of the CL and serum concentration of P4. In both studies, eCG (400 or 600IU)-treated cows had similar diameter of the LF on D8 and D10, growth rate of the LF from Days 8 to 10, ovulation rate, time to ovulation, volume of the CL, serum concentration of P4 and P/AI as compared to control animals. Thus, adding either 400 or 600IU eCG to TAI protocols was inefficient to alter follicular and luteal dynamics and increase P/AI in high-producing dairy cows under the conditions of these experiments. The lack of positive effects of eCG in the present study might be explained by the small percentage of cows with poor body condition score and lesser incidence of anestrus.