Abstract

The objective of this study was to evaluate the effects of production level (PL) and lactation number (LN) on the expression of breed, and heterosis for yield traits in cows milked once (OAD) or twice daily (TAD) in New Zealand. Records were used from 35,192 Friesian (F), 31,118 Jersey (J) and 88,606 crossbred (FxJ) cows that calved in spring between 2008 and 2012. With an average LN of five, F cows had a higher milk yield (592kg in OAD and 909kg in TAD), fat yield (2.1kg in OAD and 6.3kg in TAD) and protein yield (11.9kg in OAD and 19.5kg in TAD) compared with J cows. Expressed as a percentage, heterosis effects for milk yield traits ranged from 4.1%-7.6% for production traits, and were similar across milking frequencies. In absolute values, however, heterosis was different for fat and protein yield only in first-lactation cows, and no differences were found in milk yield. In a second analysis, breed effects, defined as F-J, increased as PL of the herd increased in both OAD and TAD systems. The highest expressions of heterosis were found at medium (5.9%-6.4%) and low (5.2%-6.8%) PL in cows milked OAD and TAD, respectively. However, in absolute values, the greatest heterosis was observed at high PL in the population milked TAD. The estimates of breed and heterosis obtained in this study can be used for simulation studies to evaluate the profitability of crossbreeding systems under OAD and TAD milking systems in New Zealand dairy farms, at different PL.