Abstract
Genetic trends for 305-d milk yield, genotype-environment interaction and inbreeding levels were evaluated in Holstein (H) and Jersey (J) dairy cattle populations in Costa Rica. Heritability and repeatability for 305-d milk yield were estimated with a statistical model that included the fixed effect of herd-year-season and the random effects of herd-sire interaction, permanent environment and animal. Heritability estimates were 0.19±0.021 (H) and 0.21±0.015 (J) and repeatability was 0.44 (H) and 0.52 (J). Predicted Breeding Values for animals in both breeds were estimated using BLUP techniques. Genetic and environmental trends were analyzed by linear regression. The annual genetic increase was 6.36 kg (H, P<0.01) and 7.95 kg (J, P<0.01) whereas the phenotypic increase was 97.1 kg (H, P<0.01) and 76.4 kg (J, P<0.01), which implies a relatively low rate of genetic improvement. Correlations between PTA estimates for AI sires used in Costa Rica and the USA were 0.62 (H, P<0.01) and 0.78 (J, P<0.01). This indicates a significant level of genetic-environment interaction which was more pronounced for the Holstein breed. Current average inbreeding for these breeds is low in Costa Rica (H=0.75 %, J=1.25 %), but exhibits a positive and significant annual rate of increase in both breeds (H=0.034 %, P<0.01, J=0.051 %, P<0.01).
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