ABSTRACT
The initial ovulatory response during synchronization programs is often low in dairy heifers, largely due to follicular dynamics and hormonal dynamics. Specifically, the progesterone (P4) concentration at the time of the first GnRH treatment in a breeding program can influence the LH response, often resulting in a suboptimal ovulatory response. The objective of this study was to determine the effect of the highest label dose 200 µg (100 µg vs. 200 µg) of GnRH (50 µg of gonadorelin hydrochloride per mL; Factrel, Zoetis Inc. Madison, NJ) at the first GnRH of a 6-d CO-Synch plus P4 device program on ovulatory response and pregnancy per AI (P/AI) in first service in Holstein heifers. A total of 1,308 Holstein heifers were randomly allocated at the beginning of a 6-d CO-Synch program at day 0 to receive either i.m. treatment of 100 µg (2CC, n = 655) or 200 µg (4CC, n = 653) of GnRH. Also, at d 0, heifers received an intravaginal insert with 1.38 g of P4 (Eazi-Breed CIDR Cattle Insert, Zoetis Inc.). On day 6, the insert was removed, and i.m. treatment of 25 mg of PGF2α (12.5 mg of dinoprost tromethamine/mL; Lutalyse HighCon Injection, Zoetis Inc.) was administered. On d 7, a second i.m. treatment of 25 mg of PGF2α was given, followed on d 9 by concurrent i.m. treatment of 100 µg of GnRH, and timed AI. A subset of 396 heifers had their ovaries scanned to evaluate ovulatory response, and blood samples were collected to measure the serum concentration of P4 at d 0 and d 6 of the study. The P4 concentrations at d 0 were categorized as low (≤3 ng/mL) or high (>3 ng/mL). The ovulatory response was greater for heifers receiving 4CC than 2CC at d 0 (54.7% vs. 42.8%). The ovulatory response was greater for low P4 than high P4 at d 0 (54.3% vs. 37.8%). However, we did not observe an interaction between treatment and P4 concentrations (low P4 2CC = 48.6% vs. high P4 2CC = 30.0%; low P4 4CC = 60.0% vs. high P4 4CC = 45.5%). The receiver operating characteristic curve analysis indicated that P4 concentrations at d 0 treatment could predict the ovulatory response, although the area under the curve was only 0.6. As expected, heifers that ovulated had increased P/AI (no = 55.6% vs. yes = 67.7%); however, we found no effect of treatment on P/AI (2CC = 63.3% vs. 4CC = 59.6%), and no interactions between treatment and ovulation and treatment and P4 (high vs. low) for pregnancy outcomes. In summary, P4 concentration and increasing the dose of GnRH at d 0 positively affected ovulatory response in Holstein heifers. However, there was no interaction between treatment and P4 on ovulation and no subsequent impact of GnRH dose on P/AI.