Genetic variability for heat stress sensitivity in Merino de Grazalema sheep

- Sensitivity to heat stress has been shown in a rustic Mediterranean breed of sheep.
- G × E interactions of production traits through the climatic scale have been observed.
- Reaction norm models are adequate for the genetic evaluation under heat stress.

Different fixed linear models and reaction norm genetic models were used to estimate heat stress effects, which were measured by a combined index of average temperature and relative humidity known as THI, registered on the same test day (TD). A total of 17,602 TD with the results of daily milk yield (MY), daily fat plus protein yield (FP) and daily dry matter yield (DM) were available. These data are from 2622 Merino Grazalema sheep: daughters of 77 bucks and 870 ewes kidding between 2006 and 2011 in a total of 27 dairy units in Andalusia, Spain. During the period of the study, the productive characteristic increased slightly, whereas the climatic variables and the THI index increased substantially. The analyses shows the presence of a comfort zone (THI < 47) where the heat stress had no effect, but at THI ≥ 48 a depressive heat stress thermal zone started to take effect and MY decreased at a rate of −17 g of milk/day for each THI point, which consequently affected the FP and DM daily yield. In genetic terms, it was found that the (co)variance components were heterogeneous throughout the trajectory of the THI scales. Hence, the best animals selected in the comfort zone are not the best in a stress zone. The estimates of heritability (h2) within the comfort zone were h2 = 0.161 for MY, h2 = 0.151 for FP and h2 = 0.157 for DM, but at THI ≥ 48 decreased to h2 = 0.081; h2 = 0.094 and h2 = 0.105 for the same dependent variables. The principal components analysis of the genetic correlations matrices shows that the first eigenvalue accounts for between 71% and 82% of the total genetic variance, whereas the second eigenvalue accounts for around 17%, which indicates the presence of possible changes in the form of response throughout the THI scale. The use of a reaction norm model is highly recommended in the breeding program of this sheep breed, since it shows an important genetic variability in the sensitivity of the animals to heat stress and to identify robust animals with respect to the changes along the trajectory of THI.