2 Response to Treatment with Human Chorionic Gonadotropin on Pregnancy Rate of Heat-Stressed Lactating Cows: Interactions with Parity and Genotype

Abstract

Reported effects of treatment of lactating dairy cattle with hCG after AI have been variable. The current objective was to evaluate whether fertility response to hCG in lactating Holsteins exposed to heat stress depends on parity or genotype. The experiment was conducted at a commercial dairy in northern Florida during the summer (June to September). Ovulation was synchronized using the Double Ovsynch protocol for 282 primiparous and 490 multiparous first-service lactating cows. Cows were randomly assigned to receive either 3,300 IU of hCG (Chorulon, MSD, Millsboro, DE, USA) or diluent, IM, on Day 5 after AI. Cows were genotyped by PCR-based KASP assay (LGC Genomics, Middlesex, United Kingdom) for 4 single nucleotide polymorphisms (SNP) previously associated with fertility, embryonic development, or stress tolerance. The SNP were as follows: heat shock protein A1L (HSPA1L, HSP70C895D), prostate androgen-regulated mucin-like protein 1 (PARM1, rs111027720), coenzyme Q9 (COQ9, rs109301586), and progesterone receptor (PGR, rs109506766). Pregnancy diagnosis was performed at Day 60 after AI by ultrasonography. Data were analysed using the GLIMMIX procedure of SAS (SAS Institute Inc., Cary, NC, USA). When genotype was not considered in the model, there was a tendency (P = 0.08) for a treatment × parity interaction, with hCG increasing pregnancy rate in primiparous cows (32.1% ± 0.04 v. 42.0% ± 0.04) but not multiparous cows (27.7% ± 0.02 v. 27.0% ± 0.03). When genotype for COQ9 was included in the model, the parity × treatment interaction was significant (P = 0.036). Moreover, the response to treatment was affected by COQ9 genotype (P = 0.023) and the 3-way interaction (P = 0.060). In cows treated with vehicle, pregnancy rate was greatest for the AA allele (40.2% ± 0.05 for AA, 21.5% ± 0.03 for AG, and 33.3% ± 0.05 for GG). However, for cows treated with hCG, pregnancy rate was lowest for AA (23.5% ± 0.05 for AA, 32.9% ± 0.04 for AG, and 34.0% ± 0.03 for GG). The 3-way interaction occurred because the negative effect of hCG on fertility in AA animals only occurred in multiparous cows. Pregnancy rate was also affected by genotype for HSPA1L (25.0% ± 0.02 for the major allele CC, 38.7% ± 0.04 for CD, and 27.5 ± 0.04 for the minor allele DD; P = 0.003) but there were no interactions with treatment. There was a tendency for pregnancy rate to be affected by genotype for PGR (P = 0.076) but there was no interaction with treatment. The PARM1 genotype was not associated with pregnancy rate. In conclusion, treatment with hCG 5 days after AI improved pregnancy rate in primiparous lactating cows under heat stress but had no effect on pregnancy rate of multiparous cows. Moreover, actions of hCG to improve fertility were associated with a SNP in COQ9. Thus, genotype can affect response of cows to a fertility-promoting drug.