Breeding for resilience to heat stress effects. A comparison across dairy ruminant species

Abstract Text:

Dairy animals are more susceptible to heat stress (HS) since milk production results in a large metabolic heat strain. As a consequence, selection for increased milk production will tend to decrease animals tolerance to increasing heat loads. A comprehensive approach to characterize HS effects on dairy production and to develop breeding tools to select for heat tolerance (HT) was followed by making use of available milk recording information, climatological data and genomic information on three dairy ruminant populations, Holstein cattle and local breeds Manchega sheep and Florida goats raised in the warm Southern regions of Spain. Heat stress thresholds were around 55/63 (15/18) and 63/65 (19/20) for average daily values of THI (ºC temperature) for fat/protein yields in Holstein and Manchega, respectively. For goats, HS thresholds could not be clearly identified. Sufficient genetic variability was observed in productive response to heat to consider this trait for selection in the three populations. Genetic antagonism between milk production and HT (ability to maintain milk production under high heat loads) was found for the three populations but stronger for cattle and goats. Several selection criteria including eigen-components of the response variability (looking for tolerance criteria independent of production level) were compared and slopes of the genetic response curves in the HS region were recommended. Estimated genetic correlations between production under cold and comfort or heat conditions was different from unity in all species (down to 0.3 in cattle), indicating the different genetic mechanisms involved in heat and cold tolerance. Genome wide association studies using slopes of polynomial curves of response as pseudo-phenotypes have been performed. Beside genomic regions related to fat metabolism (e.g. ACSL3), regions highlighting the effect of HS on the regulatory activity of transcription factors (TBL1XR1, DCHAU1), a number of genes involved in basic processes related to proteins transport and intracellular signal transduction (CNIH3 and CNIH4 ubiquitines and chaperones such as NVL from the chaperone-like AAA-ATPase family) showed signinficant signals. Although the use of milk recording and weather data has been proven useful to identify HT animals, the use of finer phenotypes together with genomic information are deemed important to succeed in selecting HT animals while maintaining productivity.

Keywords: Heat tolerance, selection, dairy ruminants