Genetic variation for tolerance to heat stress in dairy small ruminants: Results obtained in Spain

Research has been carried out to study the genetic response to heat stress of Payoya and Mur ciano-Granadina goats and Merino de Grazalema sheep. Test day records of milk, fat and protein yield and pedigree records and values of an index (THI) combining average temperature and relative humidity, registered on test day in weather stations located at less than 20 km of the farms, were analysed with norm of reaction models. Heritability of dairy traits at and genetic correlations between different points of the trajectory of THI values were estimated, showing a heterogeneity of the genetic (co)variance components through this trajectory. Breeding values were also estimated (EBV) for all animals for different THI values. Two types of animals were identified: Those showing a large variation of their EBV through the THI trajectory (sensitive) and those showing an almost constant EBV (tolerant). This allows for the selection of animals showing a higher tolerance to heat stress with practically no further expense added to the present cost of the breeding programs.