Breed and heterosis effects for milk yield traits at different production levels, lactation number and milking frequencies

ABSTRACTThe 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 (F × J) cows that calved in spring between 2008 and 2012. With an average LN of five, F cows had a higher milk yield (592 kg in OAD and 909 kg in TAD), fat yield (2.1 kg in OAD and 6.3 kg in TAD) and protein yield (11.9 kg in OAD and 19.5 kg 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 system...