M.L. Santana Jr., A.B. Bignardia, G. Stefani, L. El Faro
objectives of the present study were: 1) to investigate variation in
the genetic component of heat stress for nonreturn rate at 56 days after
first artificial insemination (NR56); 2) to identify and characterize
the genotype by environment interaction (G × E) due to heat stress for
NR56 of Brazilian Holstein cattle. A linear random regression model
(reaction norm model) was applied to 51,748 NR56 records of 28,595
heifers and multiparous cows. The decline in NR56 due to heat stress was
more pronounced in milking cows compared to heifers. The age of females
at first artificial insemination and temperature-humidity index (THI)
exerted an important influence on the genetic parameters of NR56.
Several evidence of G × E on NR56 were found as the high slope/intercept
ratio and frequent intersection of reaction norms. Additionally, the
genetic correlation between NR56 at opposite extremes of the THI scale
reached estimates below zero, indicating that few of the same genes are
responsible for NR56 under conditions of thermoneutrality and heat
stress. The genetic evaluation and selection for NR56 in Holstein cattle
reared under (sub)tropical conditions should therefore take into
consideration the genetic variation on age at insemination and G × E due
to heat stress.
Keywords: Dairy cattle, genotype by envronment interaction. reaction norm.
Fonte: Theriogenology, May, 2017.
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