Effect of heat stress on estimates of genetic parameters for tick count in Hereford × Nelore cattle in Brazil.

The objective of this study was to evaluate the impact of heat stress (HS) on genetic parameter estimates for tick count (TC) in Hereford × Nelore cattle. A total of 15,981 TC records of Rhipicephalus (Boophilus) microplus were analyzed, collected from 4,306 cattle with different breed compositions (½ Hereford ½ Nelore, 5/8 Hereford 3/8 Nelore, ¾ Hereford ¼ Nelore, and pure Hereford), born between 2008 and 2016. Tick count data were provided by a breeding program conducted by Conexão Delta G (Delta G Connection), encompassing information from 15 farms located in the Pampa Sul region of the state of Rio Grande do Sul, Brazil. Climatological data were obtained through the POWER tool. Tick counts were log-transformed (log10 + 1), and the effect of HS on TC was estimated using the temperature-humidity index (THI) function. The general model comprised the contemporary groups (management group, farm, and month of counting) as a fixed effect and the linear effect of animal age as a covariate. The heritability coefficient and estimated breeding values (EBV) for TC, without considering the THI, were estimated using a univariate model with the AIREMLF90 software. To estimate the variances and covariances of TC considering the effect of HS, a reaction norm model was used, with THI as the environmental descriptor. Spearman’s correlation between the EBV of the top 1% most outstanding bulls was estimated to assess changes in the ranking of animals under different levels of the THI function. The mean of the untransformed TC was 38.27 ± 44.60. An increase in the estimate of additive genetic variance was observed with increasing HS. Heritability estimates ranged from 0.014 ± 0.003 (THI 75) to 0.048 ± 0.004 (THI 70), both higher than the estimate obtained without considering the effect of HS using the univariate model (0.01 ± 0.008). The genetic correlation between the intercept and the slope was high and negative (-0.84). The Spearman correlation for the extreme THI values (71 and 80) was greater than 0.80, indicating the absence of genotype × environment interaction (GEI). Therefore, due to the limited evidence of GEI and the low heritability, both with and without the effect of HS, it is recommended that breeders consider both the selection environment and the rearing environment in the short term. In the long term, selection for this trait may lead to genetic gains, although the response to direct selection is expected to be slow.