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Heat stress and milk quality in Holtein cows: a genomic approach

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Author(s):
Eula Regina Carrara
Total Authors: 1
Document type: Master's Dissertation
Press: Piracicaba.
Institution: Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz
Defense date:
Examining board members:
Gerson Barreto Mourão; Júlio César de Carvalho Balieiro; Luiz Lehmann Coutinho; Leila de Genova Gaya
Advisor: Gerson Barreto Mourão
Abstract

Heat stress causes damage to dairy farming activities. It is possible to select animals for heat tolerance, because there is genetic variation of productive traits when evaluated in different climatic environments. For a correct genetic evaluation according to different environments, the use of models that fit to the data is fundamental. At the same time, genomic tools can aid in the evaluation and genetic selection processes for heat tolerance. In this context, two studies were developed. In the first, the aim was to study the variance functions of milk production and quality traits in relation to a temperature and humidity index (THI), adjusted by Legendre polynomials of orders two to seven, to evaluate which function best fits the data and understand how the variance components and heritability coefficients behave as a function of THI. For this, records of milk yield (MY), somatic cell score (SCS), percentage of fat (FP), protein (PP), lactose (LP), casein (CP) and acid fatty acids (saturated - SAT, unsaturated - UNSAT, monounsaturated - MONO, polyunsaturated - POLY, palmitic acid - C16:0, stearic acid - C18:0; and oleic acid - C18:1) from 5,224 Holstein cows and evaluated using random regression models in 167 different levels of THI were used. With the exception of POLY, there was variation in all variance components along the THI for all traits. Estimates of heritability ranged from 0.07 to 0.43. For MY, FP, LP, SCS, SAT and C16:0, estimates of heritability decreased with increasing THI and for CP, MONO, UNSAT and C18:1 increased. For PP and C18:0, heritability estimates were higher in intermediate THI values and lower at the extremes. In the second study, the objective was to estimate the effect of SNP markers on traits MY, CP, SCS, SAT and UNSAT and to predict the genomic breeding values (GEBV) using these effects, considering two levels of THI: a thermal comfort and an heat stress. The breeding values of the animals for the two environments were used as phenotypes for the genomic wide association studies (GWAS). Genotypes of 1,157 cows to 60,671 SNP markers were used. The GBLUP method (genomic best linear unbiased prediction) was used under a bitrait approach to perform GWAS. With the aim of comparing the environments, the 55 SNP with the greater variance explained for each situation and the 10% animals with the greater GEBV were used. Differences were observed in the variance explained by SNP between the comfort and heat stress environment. There was a re-rankink of animals considering the GEBV for MY (correlations 0.90 and 0.90), CP (correlations 0.88 and 0.86), SAT (correlations 0.88 and 0.87) and UNSAT (correlations 0.97 and 0.97). For SCS, only one animal was reclassified between the environments (correlations 1.00). The first study allowed to evaluate the components of variance along the entire environmental gradient. In turn, the second study allowed to evaluate the individual variance of the molecular markers, complementing the genetic study of the traits. Although small, it was possible to verify genetic differences among the animals among the considered environments. (AU)

FAPESP's process: 16/15066-5 - Heat stress and milk quality in Holstein cows: a genomic approach
Grantee:Eula Regina Carrara
Support type: Scholarships in Brazil - Master