ORIGINAL ARTICLE Genetic parameters for milk fatty acids, milk yield and quality traits of a Holstein cattle population reared under tropical conditions J. Petrini 1 , L.H.S. Iung 1 , M.A.P. Rodriguez 1 , M. Salvian 1 , F. P ertille 1 , G.A. Rovadoscki 1 , L.D. Cassoli 1 , L.L. Coutinho 1 , P.F. Machado 1 , G.R. Wiggans 2 & G.B. Mour~ ao 1 1 Department of Animal Science, University of S~ ao Paulo, Piracicaba, Brazil 2 Animal Genomics and Improvement Laboratory, Agricultural Research Service, USDA, Beltsville, MD, USA Keywords Correlation; dairy cattle; genomics; heritability; single nucleotide polymorphism; somatic cell score. Correspondence G.B. Mour~ ao, Department of Animal Science, University of S~ ao Paulo/ESALQ, 13418-900, Piracicaba, SP, Brazil. Tel: +55 (19) 34294009; Fax: +55 (19) 34294215; E-mail: gbmourao@usp.br Received: 2 July 2015; accepted: 30 January 2016 Summary Information about genetic parameters is essential for selection decisions and genetic evaluation. These estimates are population specific; however, there are few studies with dairy cattle populations reared under tropical and sub-tropical conditions. Thus, the aim was to obtain estimates of heri- tability and genetic correlations for milk yield and quality traits using pedigree and genomic information from a Holstein population maintained in a tropical environment. Phenotypic records (n = 36 457) of 4203 cows as well as the genotypes for 57 368 single nucleotide polymorphisms from 755 of these cows were used. Covariance components were estimated using the restricted maximum likelihood method under a mixed animal model, considering a pedigree-based relationship matrix or a combined pedigree-genomic matrix. High heritabilities (around 0.30) were esti- mated for lactose and protein content in milk whereas moderate values (between 0.19 and 0.26) were obtained for percentages of fat, saturated fatty acids and palmitic acid in milk. Genetic correlations ranging from À0.38 to À0.13 were determined between milk yield and composition traits. The smaller estimates compared to other similar studies can be due to poor environmental conditions, which may reduce genetic variability. These results highlight the importance in using genetic parameters esti- mated in the population under evaluation for selection decisions. Introduction Estimates of heritability and genetic correlations are essential for the design of animal breeding programs and for the prediction of selection response. These val- ues have been reported for several important traits in dairy cattle production, such as milk yield, fat and protein yield, fat and protein percentage, somatic cells score and more recently, fatty acids content. Never- theless, genetic parameters are population specific and studies involving populations under tropical and subtropical conditions are still rare. A tropical envi- ronment is characterized by a long hot season, intense radiant energy and high relative humidity, which can cause several changes in physiology, anatomy and behaviour of lactating cows in an effort to maintain the heat balance (Curtis 1983). Heat stress can reduce feed intake and activity and increase respiration rate, peripheral blood flow and sweating, with a resulting harmful effect on production (West 2003). These challenging conditions also affect the genetic variation and, consequently, the prediction of breed- ing values. In Cienfuegos-Rivas et al. (1999), the vari- ance components were 40% lower in a Mexican population than in US environments, with a signifi- cant rank change of sires. Likewise, Costa et al. (2000) © 2016 Blackwell Verlag GmbH • J. Anim. Breed. Genet. 133 (2016) 384–395 doi:10.1111/jbg.12205 J. Anim. Breed. Genet. ISSN 0931-2668