title: Random regression models to account for the effect of genotype by environment interaction due to heat stress on the milk yield of Holstein cows under tropical conditions.
abstract: The present study had the following objectives: to compare random regression models (RRM) considering the time-dependent (days in milk, DIM) and/or temperature × humidity-dependent (THI) covariate for genetic evaluation; to identify the effect of genotype by environment interaction (G×E) due to heat stress on milk yield; and to quantify the loss of milk yield due to heat stress across lactation of cows under tropical conditions. A total of 937,771 test-day records from 3603 first lactations of Brazilian Holstein cows obtained between 2007 and 2013 were analyzed. An important reduction in milk yield due to heat stress was observed for THI values above 66 (-0.23 kg/day/THI). Three phases of milk yield loss were identified during lactation, the most damaging one at the end of lactation (-0.27 kg/day/THI). Using the most complex RRM, the additive genetic variance could be altered simultaneously as a function of both DIM and THI values. This model could be recommended for the genetic evaluation taking into account the effect of G×E. The response to selection in the comfort zone (THI ≤ 66) is expected to be higher than that obtained in the heat stress zone (THI > 66) of the animals. The genetic correlations between milk yield in the comfort and heat stress zones were less than unity at opposite extremes of the environmental gradient. Thus, the best animals for milk yield in the comfort zone are not necessarily the best in the zone of heat stress and, therefore, G×E due to heat stress should not be neglected in the genetic evaluation.
authors.affiliation: Grupo de Melhoramento Animal de Mato Grosso (GMAT), Instituto de Ciências Agrárias e Tecnológicas, Universidade Federal de Mato Grosso, Campus Universitário de Rondonópolis, MT-270, Km 06, 78735-901, Rondonópolis, MT, Brazil. 10mario@gmail.com.
authors.forename: Mário L
authors.initials: ML
authors.lastname: Santana
authors.affiliation: Grupo de Melhoramento Animal de Mato Grosso (GMAT), Instituto de Ciências Agrárias e Tecnológicas, Universidade Federal de Mato Grosso, Campus Universitário de Rondonópolis, MT-270, Km 06, 78735-901, Rondonópolis, MT, Brazil.
authors.forename: Annaiza Braga
authors.initials: AB
authors.lastname: Bignardi
authors.affiliation: Grupo de Melhoramento Animal de Mato Grosso (GMAT), Instituto de Ciências Agrárias e Tecnológicas, Universidade Federal de Mato Grosso, Campus Universitário de Rondonópolis, MT-270, Km 06, 78735-901, Rondonópolis, MT, Brazil.
authors.forename: Rodrigo Junqueira
authors.initials: RJ
authors.lastname: Pereira
authors.affiliation: Departamento de Genética, Grupo Meragen, Universidad de Córdoba, Rabanales, Cordoba, 14071, Spain.
authors.forename: Alberto
authors.initials: A
authors.lastname: Menéndez-Buxadera
authors.affiliation: Instituto de Zootecnia, Centro de Pesquisas de Bovinos de Corte, Rodovia Carlos Tonanni, Km 94, 14160-900, Sertãozinho, SP, Brazil.
authors.forename: Lenira
authors.initials: L
authors.lastname: El Faro
mesh_terms: D000818:Animals; D001938:Brazil; D002417:Cattle / Q000235:genetics*; D003612:Dairying; D005260:Female; D059647:Gene-Environment Interaction; D006358:Hot Temperature; D006813:Humidity; D007774:Lactation; D008297:Male; D008892:Milk; D008957:Models, Genetic; D012044:Regression Analysis; D013312:Stress, Physiological / Q000235:genetics*; D014329:Tropical Climate
keywords: Dairy cattle; Genetic parameter; Reaction norm; Temperature–humidity index