Genetics of tolerance to heat stress in milk yield of dairy buffaloes assessed by a random regression model

• Autor(a) principal: Stefani, Gabriela [UNESP]
• Data de Publicação: 2020
• Tipo de documento: Tese
• Idioma: eng
• Título da fonte: Repositório Institucional da UNESP
• Texto Completo: http://hdl.handle.net/11449/202400
• topic: Interação genótipo-ambiente; Búfalos; Estresse calórico; Regressão aleatória; Índice de temperatura-umidade; Genotype by environment interaction; Buffaloes; Heat stress; Random regression; Temperature-humidity index

Buffaloes are recognizably rustic and well adapted to adverse tropical climates. However, there are reports in the literature that these animals show signs of suffering when exposed to high temperatures and solar radiation. Despite being an important issue, the effect of heat stress on milk yield in buffaloes has never been studied in Brazil. The objectives of this study were to assess the effects of heat stress on the milk yield and investigate the presence of genotype x environment interaction (G×E) in Brazilian dairy buffaloes reared under tropical conditions. With this, 53,113 test-day (TD) records for milk yield from 3,179 first-lactations of dairy buffaloes, collected between 1987 and 2018 were evaluated. A mixed model considering days in milk (DIM) and temperature-humidity index (THI) was applied to quantify milk yield losses due to heat stress. The most detrimental effect of THI on TD milk yield was observed in the intermediate stages of lactation, after lactation peak, in DIM 105-154 and 155-204 days (-0.002 and -0.014kg/day per THI, respectively). The least squares means of TD milk yield was used to identify a heat stress threshold by a piecewise linear regression model. A substantial reduction in TD milk yield due to heat stress was observed for THI values above 76.8 (-0.26 kg/day per increase of 1 THI unit). An analysis using a single-trait random regression animal model was carried out to estimate variance components and genetic parameters for TD milk yield over THI and DIM values. The additive genetic variance and heritability estimates were higher for lower THI values and earlier DIM. Lower genetic correlations between TD records were observed between opposite extremes of THI scale (THI=60 vs THI=80), reaching zero value. The genetic trends observed for the regression coefficients related to general yield level (0.02) and specific ability to respond to heat stress (-0.006) indicated that selection to increase milk yield did not affect the specific ability to respond to heat stress until the present moment. These trends reflect the low genetic correlation between these components (0.085±0.157), and are indicative of GxE with reordering of the estimated breeding values across the environment classes. Thus, the best animals for milk yield in the comfort zone are not necessarily the best in the zone of heat stress. Therefore, actions for monitoring trends of genetic components related to response to heat stress are recommended.