STRATEGIES FOR CONTROLLING RATES OF INBREEDING IN MOET NUCLEUS SCHEMES FOR BEEF-CATTLE

A closed MOET (multiple ovulation and embryo transfer) nucleus scheme, with overlapping generations, was modelled for beef cattle by stochastic simulation. Selection was carried out for 25 years on a trait measurable in both sexes and with a heritability of 0.35. Different strategies to control the fate of inbreeding were investigated: 1) decreasing female selection intensity whilst keeping the number of donors constant; 2) culling selected animals after having been used for a period of time; 3) using more donors; 4) using factorial mating designs; and 5) selecting on modified indexes. Comparisons among different schemes were made on the basis of equal number of transfers per year. Strategies 1, 2, and 3 reduced inbreeding but also reduced response. When the schemes were compared at the same level of inbreeding, culling of animals gave higher rates of genetic progress than decreasing selection intensity. Factorial designs decreased the rate of inbreeding by up to 19% in comparison with nested designs, with no effect on response. The most successful strategies were those that reduced the emphasis on family information in the selection criterion and especially selection on estimated breeding values obtained by BLUP (best linear unbiased prediction) using a deliberately increased heritability. With this method, it was possible to reduce inbreeding by up to 30% without affecting genetic progress. The reduction in inbreeding with different raised heritabilities averaged 42% and ranged from 26 to 61%. Under all the strategies studied to control inbreeding, proportional reductions in rates of inbreeding were always higher than those in genetic response.