Effects of heat stress on semen characteristics of Holstein bulls estimated on a continuous phenotypic and genetic scale

Semen volume (SV), semen output (SO), semen motility (SM), number of semen doses per ejaculate (NSD) and semen concentration (SC) were analyzed on a phenotypic and quantitative genetic scale in dependency of the continuous environmental descriptor THI (=an index describing combinations of temperature with humidity). Longitudinal semen data included 10,341 observations from 562 Holstein sires kept on an AI station in the northwestern part of Germany. Statistical modeling was based on fixed and random regression methodology (RRM). In this context, the phenotypic and genetic trajectory of traits in dependency of THI was modeled by orthogonal polynomials, i.e. Legendre polynomials of order 3. A general heat stress threshold of THI=60 was identified, and indicating detrimental effects of heat stress beyond this threshold, especially on SV, SO, NSD, and SC. Least square means for semen productivity were generally higher for adult bulls, but compared to young bulls, adult bulls responded with stronger semen production losses for extremely high THI. The optimal environment for semen production was a THI in the range from 50 to 60. Detrimental impact of heat stress was reduced for bulls with longer intervals between consecutive semen collection dates. Heritabilities for semen traits were in a low to moderate range with the following maximal values: 0.18 for SV at THI 30, 0.29 for SM at THI 70, 0.26 for SO at THI 30, 0.26 for NSD at THI 58, and 0.27 for SC at THI 53. Genetic values of sires and additive genetic variances changed with alterations of THI, with highest genetic variances at the extreme ends of the continuous environmental descriptor. Genetic correlations in same traits from THI levels in great distance were lower than 0.80, and indicated genotype by environment interactions. Application of RRM allow to select sires according to THI specific breeding values, which implies an optimization of cow sire and bull sire selection for harsh environments. (C) 2015 Elsevier B.V. All rights reserved.