The objective of this study was to estimate genetic parameters for lactation yields of milk (MY), fat (FY) and protein (PY) for cows milked either once- (OAD) or twice-a-day (TAD) at different production levels (PL), calculated as average milk solids yield (FY + PY) for dairy cattle in New Zealand. Lactation records of 124,620 and 194,631 cows from 298 and 350 herds milked OAD and TAD, respectively were used. Heritiabilities (h2) were lower in low PL herds milked OAD compared to low PL herds milked TAD (0.11-0.18 vs. 0.22-0.37). Across PL, h2 for MY at medium and high PL were greater than 0.30 at both MF, while for FY and PY, those values were less than 0.30. Genetic correlations (rG) between MY and PY were greater (>0.70) compared to rG between FY and any of the other traits (<0.65). The lowest rG was observed between MY and FY at low PL (0.09±0.087). In conclusion, lower h2 at low PL from cows milked OAD reveal unequal genetic expression of milk traits genes compared to higher PL. Ignoring the heterogeneity of variance in the genetic evaluation could result in systematic inaccuracies and biases in the estimation of breeding values for cows milked OAD.
Proceedings of the New Zealand Society of Animal Production, Volume 76, Adelaide, 49-53, 2016
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