EFFECTS OF FEMALE REPRODUCTIVE RATE AND MATING DESIGN ON GENETIC RESPONSE AND INBREEDING IN CLOSED NUCLEUS DAIRY HERDS

Hierarchical (each dam mated to one sire) and factorial (each dam mated to several sires) designs involving juvenile selection in a closed nucleus breeding programme were compared for rates of genetic response and inbreeding using stochastic simulation. Numbers of sires and dams selected and herd sizes varied. Generations were discrete. Sires and dams were selected at 15 months on the basis of an estimated breeding value (EBV) calculated using selection index which considered information on close relatives. Selection was for total merit index. Matings were carried out assuming use of multiple ovulation and embryo transfer (MOET) or in vitro embryo production (IVEP) technologies, over a range of reproductive parameters. Reproductive rates assumed a fixed number of potential offspring per collection with IVEP and MOET. Comparison of mating designs using MOET showed that rates of genetic response and inbreeding with factorial designs were from 88 to 43% and 36 to 111% of those with the hierarchical design. Generally rates of genetic response were increased and inbreeding decreased by increasing the number of mates per dam due to a reduction of correlations among EBV. Reduced rates of genetic response were occasionally observed in factorial designs involving the mating of dams to large numbers of sires. This increased the generation interval due to the time to carry out the matings. Factorial designs were found to be most efficient for increasing genetic response and or decreasing inbreeding, for breeding programmes involving the selection of equal number of sires and dam. For the breeding programmes considered, rates of genetic response with IVEP equalled the best of the MOET designs, achieved at highest MOET rates, at the lowest level of IVEP. But at the equivalent rate of genetic response, rate of inbreeding was increased by 8%. The increased rate of inbreeding was due to the shorter generation interval with IVEP, and the increased probability of selecting related individuals assuming all donors respond to IVEP.