Herd conversion strategies for A2 beta-casein were investigated within an inter-temporal simulation model with annual time-steps comprising three sub-herds based on allele status (A1A1, A1A2, and A2A2). Cows that are homozygous for the A2 allele of the beta-casein gene are known as A2 cows (A2A2), and produce milk known commercially as ‘A2 milk’. The simplest herd conversion process is to use semen from A2A2 bulls. However, the conversionfunction is curvilinear and asymptotic and this strategy by itself cannot achieve herd purity. Conversion rates can be increased markedly through genotyping of calves and/or cows. This is necessary for completion of the conversion process. Mating of yearlings with A2 semen can have considerable impact, particularly in association with increased replacement rates. Combining these strategies can lead to pure A2A2 herds within 5 to 8 years, depending on initial herd structure. Use of sex-selected semen can further reduce the conversion period. There is potential for reduced cow herd gain in breeding worth as a result of non-use of elite bulls that are not A2A2. For example, a loss of 20 breeding worth units in the bull team would result in ten years being required to achieve breeding worth gains otherwise achievable in nine years.
Proceedings of the New Zealand Society of Animal Production, Volume 73, Hamilton, 149-154, 2013
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