SELECTION METHODOLOGY FOR A DAIRY BREED IN A TROPICAL ENVIRONMENT

ABSTRACT Aim of this thesis is to evaluate the genetic and racial structure of a dairy cattle breed reared in Tropics, joint to the investigation of the best selection methodology for productive traits adapted to low/medium input production system. The genetic and racial structure of the Carora Breed, a Venezuelan dairy cattle population, was studied through the analysis of the whole pedigree. The database included 4324 living animals (including culled bulls used for AI), born from 1983 to 2010. The racial composition of Carora breed was studied clustering the current population into 5 racial groups, identified on the basis of the individual racial proportion, in order to detect the breed composition in this synthetic breed. The Carora pedigree resulted quite complete, and the inbreeding level is below the level of 1%, even if the size of this breed is reduced. From this study an increase in the Brown Swiss genes proportion was detected, indicating the possibility of a reduction in adaptability linked to the features of this breed, typically suitable for temperate climates. Genetic parameters of weight at 18 months (W18) and milk yield (MY) of 1781 Venezuelan cows were estimated with a Multiple Trait Animal Model, including the direct breed effects and the epistatic loss. Results showed that direct breeds effects and epistatic loss affected milk trait, while they seemed not important for weight. An high percentage of European genes positively influenced milk production. Four random regression models were used on 95606 test day records to obtain genetic parameters for test day yield and persistency. Akaike’s information criterion (AIC) and minus twice log likelihood (-2logL), were used to compare the models and to choose the best one fitting the data. Estimated heritability for milk yield ranged from 0.21 to 0.30, while repeatability ranged from 0.50 to 0.56. Five persistency measures were calculated taking in account different point of the lactation. In general, heritability values for the five measures of persistency ranged from low to moderate (from 0.18 to 0.26), indicating the possibility of new selection opportunities. The use of persistency indexes accounting for milk yield, may allow the selection of individuals able to express their potential genetic values in tropical environment, without incurring in excessive heat stress losses.