|dc.description.abstract||A2 milk is cow's milk containing beta-casein that is exclusively of the A2 type. A2™ milk comes from certified herds in which all cows carry two copies of the A2 allele of the beta- casein gene. There are a number of strategies that farmers can use to create a pure A2 herd. These include the use of semen from certified homozygous A2 bulls, genetic testing of cows and calves to determine the beta-casein allele status, and the use of A2 sexed semen. The rate of herd conversion is also mediated by breeding decisions such as replacement rate, culling rate or artificial insemination of yearlings with A2 semen. Other relevant herd parameters include the incidence of the allele at the start of the conversion process and the level of involuntary culling in the herd. Given these complexities, a time-dependent simulation model was developed to investigate the impact of various decision variables and herd parameters on herd structure and genetic gain. Four alternative conversion strategies were explored based on the assumption of using only A2 semen: (1) non-testing of cows and calves, (2) genetic testing of cows, (3) genetic testing of calves, and (4) genetic testing of both cows and calves. For each generic strategy, a range of additional decision variables were explored, including herd replacement rate, culling rate, artificial insemination of yearlings with A2 semen, and A2 sex-selected semen. Results reported here refer to three herd baseline compositions, with initial A1:A2 allele ratios of 1:1, 2:1 and 1:2. In a non- testing situation, the proportion of homozygous A2 cows increases with a function that is in the early years almost linear, but subsequently becomes curvilinear and asymptotic. Hence, regardless of the initial A1:A2 allele ratio or decision variables, a pure A2 herd will never be achieved. In testing situations, A2 achievement of herd purity typically takes between four
and fifteen years depending on specific strategies and herd parameters. In most situations, genetic testing of calves is more efficient than genetic testing of cows. However, maximising the rate of conversion requires genetic testing of both cows and calves, together with artificial insemination of yearlings with A2 semen and with high cow replacement rates. The use of sex-selected semen can further speed up the process. High replacement rates are only effective if undertaken in conjunction with other specific strategies and in some situations can be counter-productive.
There is potential for reduction in the rate of herd genetic gain if the breeding worth (BW) of the bull team declines as a consequence of the elimination of non-A2 bulls. However, artificial insemination of yearlings with A2 semen and high replacement rates can act to reduce any such reduction in the rate of genetic gain by reducing the generation interval.||en