Comparison of GnRH vs. estradiol benzoate plus GnRH to initiate a progesterone-based timed-artificial insemination resynchronization protocol in lactating dairy cows.

The present study compared 2 strategies to initiate a progesterone (P4)-based timed-artificial insemination (TAI) protocol for lactating dairy cows: only GnRH or estradiol benzoate (EB) plus GnRH. Lactating Holstein cows (n = 487; 184 primiparous and 303 multiparous) from 2 commercial dairy herds were used for their second or greater services postpartum. Weekly nonpregnant cows at pregnancy diagnosis 32 d after a previous AI were randomly assigned to 1 of 2 experimental groups that differed only in the strategy to initiate (d 0) the TAI protocol. On d 0, every cow received a 2.0 g P4 implant, and in Group EB+GnRH, cows were treated with 2.0 mg i.m. of EB and 16.8 µg i.m. of buserelin acetate (GnRH), while in Group GnRH, cows received only 16.8 µg i.m of GnRH. Seven d later (d 7), 0.530 mg i.m. of cloprostenol sodium (PGF) was administered in all cows, followed by a second dose on d 8, concomitant with 1.0 mg i.m. of estradiol cypionate (EC) and P4 implant withdrawal. The TAI was performed on d 10 (48 h after P4 device withdrawal) in both experimental groups. Only conventional Holstein semen was used throughout the study. The percentage of cows with corpus luteum (CL) on d 0 (73%) and overall ovulation rate after d 0 (54%) did not differ between groups. The CL regression between d 0 and the first PGF treatment was greater in EB+GnRH than GnRH group (42 vs. 31%). Consequently, the proportion of cows with CL at PGF was greater when only GnRH was used on d 0 compared with EB plus GnRH (86 vs. 82%), and the mean number of CL at PGF was greater (1.23 vs. 1.11). The expression of estrus near TAI was greater in GnRH group (84 vs. 77%), and cows showing estrus had greater (44 vs. 10%) pregnancy per AI (P/AI) on d 32 for both treatments. There was no effect of the presence of CL on d 0 or at PGF, nor of ovulation after d 0 or CL regression between d 0 and d 7 on fertility. However, fertility was critically impaired when cows did not have CL at both times, d 0 and at PGF treatment. There was no interaction between treatment and other variables, and the P/AI was similar in cows receiving EB plus GnRH or only GnRH on d 0 (37.8 vs. 36.6%). In summary, although there was no detectable difference in P/AI between treatments, this study demonstrated potential negative physiological outcomes caused by EB treatment on d 0 (greater incidence of luteolysis after d 0 and fewer cows with CL at PGF treatment). In conclusion, there was no benefit of adding EB at the initiation of a P4-based TAI protocol on fertility compared with using GnRH alone, despite differences in ovarian dynamics and expression of estrus.