137. Investigating the Relationship Between Service Sire Effects from Heifer Pregnancy Evaluations and Semen Quality Traits in Angus Bulls.

Abstract

Heifer pregnancy (HP) outcomes are typically analyzed as binary response of the female. However, pregnancy success may also be influenced by the service sire through fertilization ability and early embryo viability. This contribution is usually modelled as a service sire effect (SSE) in HP genetic evaluations. When modelled as independent random effects, SSE capture variation among sires in pregnancy success but do not explicitly partition variation into genetic and non-genetic components, and thus can be interpreted as a pseudo-phenotype. Quantifying the genetic relationship between SSE, derived from HP outcomes, and semen collection (SEM) traits provides insight into potential selection strategies for improving male fertility. Our objective in this study was to estimate the genetic relationships between SSE and SEM traits in American Angus bulls. A two-stage analytical procedure was implemented using BLUPF90 programs. In Stage 1, a total of 128,431 HP records representing 4,450 service sires were provided by the American Angus Association and modelled as binary phenotypes. Using a threshold model (as implemented in GIBBSF90+) we obtained best linear unbiased predictions (BLUPs) for SSE, and subsequently de-regressed them to serve as pseudo-phenotypes in the second stage. In Stage 2, weighted bivariate repeatability models were fitted using BLUPF90+ to jointly analyse SSE and SEM traits recorded at Artificial Insemination (AI) facilities. The dataset comprised 69,387 SEM records from 5,304 bulls, of which 446 overlapped with SSE from Stage 1. The traits analyzed for SEM were scrotal circumference (SC), volume (VOL), concentration (CONC), initial motility (IMOT), post-thaw motility (PTMOT), percentage of primary abnormalities (PAB), percentage of secondary abnormalities (SAB), and percentage of normal spermatozoa (NORM). Heritability estimates were 0.29 (SC), 0.05 (VOL), 0.05 (CONC), 0.10 (IMOT), 0.10 (PTMOT), 0.05 (PAB), 0.10 (SAB) and 0.05 (NORM). Genetic correlations between SEM and SSE were -0.05 (SC), -0.02 (VOL), 0.12 (CONC), 0.27 (IMOT), 0.20 (PTMOT), -0.82 (PAB), 0.63 (SAB), -0.12 (NORM). The generally low heritability estimates, and genetic correlations may reflect prior selection, as bulls must pass semen evaluation before entry into AI programs, reducing variation in SEM traits. Overall, the results indicate that SEM traits contain genetic variation and have potential for improvement through selection, although their ability to indirectly improve breeding values for male fertility expressed as SSE appears limited.

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