Characterization of novel reproductive phenotypes in dairy cows

• Author / Creator

  Gobikrushanth, Mohanathas

• A reproductive phenotype that has high variability, repeatability, heritability, and association with fertility could become an excellent candidate for genetic selection to augment fertility in dairy cows. Therefore, the general objective of this research was to evaluate the aforementioned characteristics for five novel reproductive phenotypes in dairy cows, namely gonadotropin releasing hormone (GnRH)-induced luteinizing hormone (LH) response, antral follicle count (AFC), anti-Müllerian hormone (AMH), insulin-like growth factor-1 (IGF-1), and ano-genital distance (AGD). The secondary objective was to identify genetic markers (single nucleotide polymorphisms; SNP) associated with three of the above reproductive phenotypes (AMH, IGF-1, and AGD). The first study was designed to determine the variability and repeatability of GnRH-induced LH responses and the associations among plasma LH, follicle stimulating hormone (FSH), estradiol (E2), and progesterone (P4) concentrations. The association between LH response categories and reproductive outcomes was also tested. I hypothesized that cows have variable responses to GnRH treatment, which are repeatable, and cows with a high LH response to GnRH would have increased likelihood of ovulation and consequently improved reproductive outcomes compared to cows with a low LH response. The GnRH-induced LH responses were normally distributed and highly variable but unrepeatable. Plasma concentrations of LH were positively associated with FSH and E2, and negatively associated with P4. Cows with high LH responses had numerically greater ovulation rate and pregnancy to first artificial insemination (P/AI); however, the association with reproductive outcomes needs to be evaluated in a larger population. The second study was designed to test the repeatability of AFC and AMH concentrations at an unknown stage of follicular growth and at an expected day of follicular wave emergence and the association between AFC and AMH at the above two stages in dairy cows. I hypothesized that the repeatability for AMH would be higher than the repeatability for AFC and the association between AFC and AMH would be strong at both stages. The repeatability was high for AMH and moderate for AFC, and a moderate correlation was found between AFC and AMH at both stages. The third study was designed to evaluate the distribution and variation of AMH, identify factors associated with variation in circulating AMH, determine heritability, establish an optimum AMH threshold predictive of P/AI, examine the relationship between AMH and fertility, and identify SNP associated with the phenotypic variation of AMH in dairy cows. Circulating AMH was positively skewed and highly variable, had a quadratic relationship with parity, moderately heritable, and was associated with numerous genetic markers. The relationships between AMH and fertility outcomes, however, were not significant despite numerical indications of better fertility outcomes in cows with high AMH concentration. The fourth study had similar objectives as the third study, with IGF-1 as the hormone of interest. Circulating concentration of IGF-1 was positively skewed and highly variable, influenced by herd, age, parity, pre-calving body condition score (BCS), and season of sampling. It was also positively associated with P/AI, and linked to a few genetic markers related to previously identified candidate genes of fertility in dairy cows.The fifth study was designed to characterize the distribution and variability of AGD, determine the relationships among AGD, age and height, and evaluate the associations between AGD and P/AI and cumulative pregnancy by 250 d in milk (DIM) within parity groups (first, second, and third + parities) in Canadian Holstein cows. I hypothesized that cows with short AGD, presumably exposed to low in-utero testosterone during fetal life, would have greater fertility than cows with long AGD. The AGD was normally distributed and highly variable, weakly-related to cow age and height, and inversely related to P/AI and pregnancy by 250 DIM in first and second parity cows, but not in third + parity cows. The sixth study, with similar objectives as the fifth study, was conducted in Irish Holstein cows. Despite its normal distribution, high variability, moderate heritability, and weak associations with cow age, height, weight and BCS, the inverse relationship between AGD and fertility found in Canadian Holsteins, was not evident in the Irish Holsteins. Overall, this doctoral thesis provided a deeper understanding of the characteristics of five novel reproductive phenotypes and their potential for selection in dairy cows, and opened new avenues for further research.

• Subjects / Keywords

  • Heritability
  • Genomic selection
  • Reproductive phenotypes
  • Dairy cow fertility

• Graduation date

  Spring 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-xfnc-k472

• License

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