New Zealand dairy production per cow has increased by over 33% in the past fifteen years. Consequently, animal breeders are interested in understanding how expression of heterosis for dairy production traits is affected in higher-production systems. To investigate this question a natural-experiment approach was taken, utilising herd-testing records in the 2010-11 dairy production season in herds that contained two breeds, Holstein-Friesian cows ('control' breed) and Holstein-Friesian x Jersey cows ('treatment'). Herds were divided into two production system classes. The system classes were national-average herd production per cow ('control' system) and high per cow production ('treatment'). Per cow yields were over 45% higher in the treatment herds than in national-average herds. Herd test records were analysed for 29,329 two-year-old heifers, and for 20,753 five-or six-year-old mature cows. Dependent variables for the statistical models were deviations from contemporary group means for 270-day yields for production traits. Explanatory variables included Breed, System, and individual cow Production Values as estimated in the national genetic-evaluation system (which include genetic, permanent environment and heterosis effects on yields). In the high-performance production systems the crossbred cows had higher yields than the Holstein-Friesian cows (compared with Production Value predictions) by 5.7 kg milksolids (heifers) and 1.9 kg milksolids (mature cows). This study indicates that heterosis for dairy production traits is more fully expressed in production systems that favour higher per cow yields than in the more restrictive production systems common in the 1990s.
Proceedings of the New Zealand Society of Animal Production, Volume 74, Napier, 41-45, 2014
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