Heterosis effects on 305-day milk yield in a Girolando dairy cattle population in different lactation orders

The aim of this study was to evaluate the heterosis effects for 305-day milk yield (305MY) in different lactation orders of Girolando cows, by fitting test-day milk yield records to Wood's, Mixed Log, and Wilmink's non-linear models. A total of 400,328 test-day milk yield records of 39,077 Holstein (H), Gyr (G), and Girolando (1/2 H, 1/4 H, 3/4 H, 3/8 H, 5/8 H, and 7/8 H genetic groups) cows from 822 herds were collected in the state of Minas Gerais, Brazil, from 1998 to 2014. The pedigree file comprised 36,640 animals, including 3677 sires and 24,472 cows. Genetic parameters were obtained by single-trait analysis using the AIREMLF90 software. Heritability estimates of 305MY and of 305MY fitted to the different non-linear models were higher in the first lactation and ranged from 0.10 to 0.29. We observed a greater heterosis effect for 305MY (and 305MY fitted) verified in cows of genetic group 1/2H, which presented average production superior to the average of their parents (of pure breeds) in the first (1034.33 kg) and second (1303.94 kg) lactation, respectively. Both Wood's and Wilmink's models stood out in quality of fit, presenting the best estimates for 305MY adjusted for different lactation orders. Wood's model is the most suitable to estimate 305MY of cows with a higher proportion of H genes. However, Wilmink's model allows estimating heterosis effects for 305MY and components of the lactation curve more similar to those observed in 305MY in the different genetic groups. The results indicate that local breeders can promote an improvement in milk production through exploitation of the heterosis effect, making choices of genetic combinations (breeders) more suitable for mating to obtain greater economic and productive efficiency of their herds during lactations.