![]() Levels of bias did not substantially increase when flocks discontinued SRS after 5 or 10 yr, suggesting that further participation in an SRS may not be necessary to manage risk. In all SRS scenarios, bias in EBV attributable to differing flock genetic means rapidly decreased in the first 5 yr of sire referencing. Other scenarios were intermediate in both gain and inbreeding levels. Genetic gains without SRS were 80 to 82% lower than when the scheme operated for 15 yr, whereas inbreeding was considerably greater. Across all scenarios, genetic gain and inbreeding were greater when excess ewes were mated exclusively to homebred sires. Genetic gain was equivalent whether the SRS used AI or natural service matings, although inbreeding was lower with natural service. Ewes not mated to reference sires were mated either to their own home-bred sires exclusively or to a mixture of homebred and unrelated purchased rams of unknown merit. Six sire referencing scenarios were considered, in which all flocks participated in a SRS for 1) 15 yr 2) 5 yr before discontinuing the scheme 3) 10 yr before discontinuing the scheme 4) 2 out of every 3 yr 5) 15 yr with reference sire mating by natural service and 6) no years (no use of SRS). Every year, in each flock, 3 reference sires were mated to 10 ewes. Yearling rams were chosen as reference sires randomly from the top one-sixth of the population ranked on BLUP EBV. After 10 yr of random mating, flocks had the opportunity to join an SRS and begin selection for the simulated trait. Founder genetic means for each flock were sampled from a normal distribution with mean 0 and SD equal to the trait's genetic SD. Pedigree and performance data for a single trait with a within-flock heritability of 0.25 were simulated (50 replications) for 15 flocks with 40 to 140 ewes per flock. The objectives of this study were to determine whether less restrictive schemes or schemes of shorter duration could achieve rates of gain and reduce inbreeding as efficiently as continuous sire referencing schemes (SRS) and to investigate whether bias from different genetic means could be reduced by these alternative schemes. However, requirements for joining these schemes can be prohibitive to some producers. Flocks participating in sire referencing schemes can achieve greater genetic gains than those achievable by within-flock selection.
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