Selection strategies in sire referencing schemes in sheep

Abstract

In sire referencing, genetic links are created among flocks by the mutual use of some rams (reference sires). These connections allow for across-flock genetic evaluations offering a larger pool of candidates for selection. Using stochastic simulation, the effect of three characteristics of the design of such schemes on rates of genetic response and inbreeding were investigated. We considered (i) the selection intensity for reference sires (highest ranking, or from the top sixth or top third of available candidates), (ii) the criteria on which reference sires were chosen (BLUP breeding value or phenotypic performance), and (iii) the extent to which the reference sires were used. For the latter, the number of reference sires used (1, 2 or 3) and the number of ewes mated to each reference sire (a total of 10, 15 or 30 ewes per flock or ∼15, 30 and 45% of the population) was assessed. Fifteen flocks of different sizes (ranging from 40 to 140 ewes) were simulated. Reference sires were picked from a team of six rams of which half were replaced each year. Surplus ewes were mated to rams born within the flock and unrelated rams born outside the scheme. The mating of full and half-sibs was avoided. When selection was most intensive (highest ranking) the rate of genetic progress per annum was 1.51 to 1.73 as great as when it was least intensive (from top sixth or top third). Selection on BLUP breeding values achieved 1.25 to 1.31 times the genetic response of phenotypic selection. When more ewes (30) were mated to reference sires, progress was as much as 1.14 times as large that when fewer ewes were mated (20 and less). The average inbreeding coefficient after 15 years of selection was at most doubled in schemes where genetic improvement was more rapid. Even so, the rate of inbreeding was always lower than 0.3% per annum (less than 1% per generation). By optimising the selection strategy for a sire referencing scheme, genetic progress can be substantially improved with acceptable levels of inbreeding.