Fertility of Lactating Dairy Cows Treated with Gonadotropin-Releasing Hormone at Estrus, 5 d after AI, or Both, during Summer Heat Stress

Abstract Text: Objective was to evaluate fertility of lactating dairy cows after treatment with GnRH on the day of estrus, 5 d after AI, or both, under heat stress conditions in 2 Kansas dairies. Cows (n = 2,855) detected in estrus based on tail paint removal were assigned to 4 treatments in a 2 x 2 factorial arrangement: (1) control (CON = 722); (2) GnRH treatment at AI (G0 = 739); (3) GnRH treatment 5 d post-AI (G5 = 697); or (4) GnRH treatment at AI and 5 d post-AI (G0G5 = 697). Daily temperature and humidity data were collected during study enrollment and temperature humidity index (THI) was calculated. Blood samples were collected from a subgroup of cows at AI (d 0), d 5 and d 12 to determine progesterone concentrations (CON = 58, G0 = 57, G5 = 58, G0G5 = 65). Pregnancy diagnosis was conducted by transrectal ultrasonography on d 36 and 94. Treatment with GnRH at AI did not affect pregnancy per AI (P/AI) on d 36 (P = 0.89) or d 94 (P = 0.53). Treatment with GnRH 5 d after AI did not affect P/AI on d 36 (P = 0.49) or d 94 (P = 0.36). Furthermore, the interaction between GnRH treatments on d 0 and 5 did not affect P/AI on d 36 (P = 0.90) or d 94 (P = 0.75). In contrast, the interaction between lactation number and treatment with GnRH on d 5 affected P/AI on d 36 (P = 0.01) and d 94 (P = 0.03) because GnRH treatment increased P/AI of ≥3-lactation cows (27.0% vs 19.3%, 23.1% vs 16.1%, respectively). Average THI at AI was 83.8 ± 0.1 and tended (P = 0.08) to be associated with P/AI at d 36, but was not (P = 0.34) associated with P/AI on d 94. Overall, treatment with GnRH on d 0 (P = 0.82), d 5 (P = 0.61), and the interaction between treatments on d 0 and 5 (P = 0.28) did not affect progesterone concentration on d 5 and 12 (1.8 ± 0.1 and 6.7 ± 0.2 ng/mL, respectively). Treating cows under heat stress conditions with GnRH at AI did not increase P/AI, but treatment with GnRH 5 d post-insemination increased fertility of ≥3-lactation cows.

Keywords:

Fertility

Summer heat stress

Dairy cow