Abstract
Simple SummaryInterest in selection for milk yield and thermotolerance in cattle has grown, since heat stress has caused great losses in milk yield. However, few studies on how to carry out concurrent selection are available. Milk yield was analyzed by traditional methods, including heat stress indicators, in genetic evaluation. The results showed that the best sires for milk yield are not the best for heat tolerance, and only a small proportion of individuals have the aptitude for joint selection. Despite a small population fraction allowed for joint selection, sufficient genetic variability for selecting more resilient sires was found, which promoted concomitant genetic gains in milk yield and thermotolerance.Intense selection for milk yield has increased environmental sensitivity in animals, and currently, heat stress is an expensive problem in dairy farming. The objectives were to identify the best model for characterizing environmental sensitivity in Holstein cattle, using the test-day milk yield (TDMY) combined with the temperature–humidity index (THI), and identify sires genetically superior for heat-stress (HS) tolerance and milk yield, through random regression. The data comprised 94,549 TDMYs of 11,294 first-parity Holstein cows in Brazil, collected from 1997 to 2013. The yield data were fitted to Legendre orthogonal polynomials, linear splines and the Wilmink function. The THI (the average of two days before the dairy control) was used as an environmental gradient. An animal model that fitted production using a Legendre polynomials of quartic order for the days in milk and quadratic equations for the THI presented a better quality of fit (Akaike’s information criterion (AIC) and Bayesian information criterion (BIC)). The Spearman correlation coefficient of greatest impact was 0.54, between the top 1% for TDMY and top 1% for HS. Only 9% of the sires showed plasticity and an aptitude for joint selection. Thus, despite the small population fraction allowed for joint selection, sufficient genetic variability for selecting more resilient sires was found, which promoted concomitant genetic gains in milk yield and thermotolerance.
Highlights
Dairy breeding programs have traditionally focused on selection for milk yield
The objective of this study was to identify the best model for characterizing environmental sensitivity in Holstein cattle in Brazil, using the test-day milk yield combined with temperature–humidity index data from public weather stations, and identify sires genetically superior in terms of heat tolerance and milk yield through random regression models
Considering only the temperature–humidity index (THI), approximately 31% of all the test-day milk yield (TDMY) records were obtained under heat-comfort conditions (
Summary
Dairy breeding programs have traditionally focused on selection for milk yield. This intense selection has increased sensitivity to environmental changes in animals. Heat stress is an expensive problem in dairy farming. Garcia et al [2] observed a 21% milk yield loss in a commercial herd of Holstein cows in southern Brazil caused by heat stress. According to Pegorer et al [3], approximately 60% of dairy farms in the world are in heat-stress environments. Heat stress decreases milk yield by 30% to 40% [4], which represents approximately 600 to 900 kg of milk per lactation per cow [5], and can exceed 1300 kg of milk per cow [6]. The impact of heat stress on the dairy cattle industry resulted in an economic loss of USD 900 million in 2012 [4], and the estimated loss in 2014 was USD 1.2 billion for the US dairy sector [7]
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