Kuehn L A, Notter D R, Lewis R M
Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg 24061, USA.
J Anim Sci. 2008 Mar;86(3):526-35. doi: 10.2527/jas.2007-0255. Epub 2007 Dec 11.
Flocks participating in sire referencing schemes can achieve greater genetic gains than those achievable by within-flock selection. However, requirements for joining these schemes can be prohibitive to some producers. 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. 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. Founder genetic means for each flock were sampled from a normal distribution with mean 0 and SD equal to the trait's genetic SD. After 10 yr of random mating, flocks had the opportunity to join an SRS and begin selection for the simulated trait. Yearling rams were chosen as reference sires randomly from the top one-sixth of the population ranked on BLUP EBV. Every year, in each flock, 3 reference sires were mated to 10 ewes. 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). 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. Genetic gain was equivalent whether the SRS used AI or natural service matings, although inbreeding was lower with natural service. Across all scenarios, genetic gain and inbreeding were greater when excess ewes were mated exclusively to homebred sires. Genetic gains without SRS were 80 to 82% lower than when the scheme operated for 15 yr, whereas inbreeding was considerably greater. Other scenarios were intermediate in both gain and inbreeding levels. In all SRS scenarios, bias in EBV attributable to differing flock genetic means rapidly decreased in the first 5 yr of sire referencing. 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. Natural service and noncontinuous SRS are viable options to continuous AI SRS in terms of genetic gain, inbreeding, and bias reduction.
参与种公羊参考方案的羊群比通过群体内选择能够实现更大的遗传进展。然而，加入这些方案的要求对一些生产者来说可能过高。本研究的目的是确定限制较少的方案或持续时间较短的方案是否能像连续种公羊参考方案（SRS）一样有效地实现遗传进展并减少近亲繁殖，以及研究这些替代方案是否能减少不同遗传均值导致的偏差。针对每个群体有40至140只母羊的15个羊群，模拟了一个群体内遗传力为0.25的单一性状的系谱和性能数据（50次重复）。每个羊群的奠基者遗传均值从均值为0且标准差等于该性状遗传标准差的正态分布中抽样。经过10年的随机交配后，羊群有机会加入SRS并开始对模拟性状进行选择。一岁公羊从根据最佳线性无偏预测（BLUP）估计育种值（EBV）排名前六分之一的群体中随机选为参考种公羊。每年，在每个羊群中，3只参考种公羊与10只母羊交配。考虑了六种种公羊参考方案，其中所有羊群参与SRS的情况如下：1）15年；2）在停止该方案前5年；3）在停止该方案前10年；4）每3年中有2年参与；5）15年且参考种公羊采用自然交配；6）不参与任何年份（不使用SRS）。未与参考种公羊交配的母羊要么仅与它们自己培育的种公羊交配，要么与品质未知的自家培育和无关购买公羊的混合群体交配。无论SRS使用人工授精（AI）还是自然交配，遗传进展都是相当的，尽管自然交配时近亲繁殖较低。在所有方案中，当多余的母羊仅与自家培育的种公羊交配时，遗传进展和近亲繁殖都更大。不使用SRS时的遗传进展比该方案运行15年时低80%至82%，而近亲繁殖则显著更高。其他方案在遗传进展和近亲繁殖水平上处于中间状态。在所有SRS方案中，由于不同羊群遗传均值导致的EBV偏差在种公羊参考的前5年中迅速下降。当羊群在5年或10年后停止SRS时，偏差水平没有大幅增加，这表明进一步参与SRS可能对管理风险并非必要。就遗传进展、近亲繁殖和偏差减少而言，自然交配和非连续SRS是连续AI SRS的可行选择。
