Assessing genetic gain, inbreeding, and bias due to different flock genetic means in alternative sheep sire referencing schemes

¹ Department of Animal and Poultry Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA 24061

^* To whom correspondence should be addressed. E-mail: larry.kuehn{at}ars.usda.gov.

	Abstract

Flocks participating in sire referencing schemes can achieve higher 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 or shorter duration schemes can achieve rates of gain and reduce inbreeding as efficiently as continuous sire referencing schemes (SRS) and investigate if bias from different genetic means can 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 years of random mating, flocks had opportunity to join a 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: i) 15 yr; ii) 5 yr before discontinuing the scheme; iii) 10 yr before discontinuing the scheme; iv) 2 out of every 3 yr; v) 15 yr with reference sire mating by natural service; and vi) no yr (no use of SRS). Ewes not mated to reference sires were mated either to their own homebred 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 higher when excess ewes were mated exclusively to homebred sires. Genetic gains without SRS were 80 to 82% lower than when the scheme operated 15 yr while inbreeding was considerably higher. Other scenarios were intermediate in both gain and inbreeding levels. In all SRS scenarios, bias in EBV due 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 further participation in a 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.

Key Words: simulation, genetic evaluation, bias, sheep