Introduction Directory UMM :Data Elmu:jurnal:L:Livestock Production Science:Vol67.Issue1-2.Dec2000:

Livestock Production Science 67 2000 129–141 www.elsevier.com locate livprodsci Selection strategies in sire referencing schemes in sheep R.M. Lewis , G. Simm Animal Biology Division , SAC, King’s Buildings, Edinburgh EH9 3JG, UK Received 23 August 1999; received in revised form 4 February 2000; accepted 9 February 2000 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.  2000 Elsevier Science B.V. All rights reserved. Keywords : Sire referencing schemes; Sheep; Genetic gain; Inbreeding; Simulation

1. Introduction

In many countries, the average size of pedigree sheep flocks is small with little scope for intensive Corresponding author, Animal Biology Division, SAC, Sir within-flock selection. Historically it has not been Stephen Watson Building, Bush Estate, Penicuik, Midlothian simple to select animals from outside flocks to EH26 OPH, Scotland, UK. Tel.: 144-131-535-3227; fax: 144- overcome the constraint of small flock size. Since 131-535-3121. E-mail address : r.lewised.sac.ac.uk R.M. Lewis. flocks differ in their husbandry, the performance 0301-6226 00 – see front matter  2000 Elsevier Science B.V. All rights reserved. P I I : S 0 3 0 1 - 6 2 2 6 0 0 0 0 1 8 2 - 2 130 R .M. Lewis, G. Simm Livestock Production Science 67 2000 129 –141 records of animals in separate flocks and born in 100 females linked by the same reference sires. The different years could not be fairly compared. intensity and criteria used for selecting reference With the introduction of best linear unbiased sires and the extent of their use within the scheme prediction BLUP methodologies to sheep breeding was not explicitly considered. Nimbkar and Wray programmes, across-flock genetic evaluations are 1991 considered some of these selection strategies possible. But for BLUP to disentangle genetic from but their combined effect on response in sire refer- environmental sources of variation in performance, encing schemes has yet to be tested. genetic relationships or links among animals are Our objective was to investigate the effects of needed across flocks Garrick, 1991; Kennedy and specific operational characteristics of sire referencing Trus, 1993. If the degree of linkage is adequate, the schemes in flocks of small size on rates of genetic size of an individual flock is effectively enlarged, progress and inbreeding for a trait of moderate since the genetic merit of animals in separate flocks heritability 0.25. This was undertaken using sto- and born in different years can be directly and chastic simulation. We consider i the selection accurately compared Kinghorn and Shepherd, intensity for reference sires, ii the criteria by which 1990. This allows more intense selection and there- reference sires are chosen and iii the extent to fore quicker rates of genetic progress. which reference sires are used. For the latter, differ- Sire referencing schemes are used to establish ent combinations of the number of reference sires genetic links across flocks. In these co-operative used and the number of ewes mated to reference breeding programmes a team of rams or reference sires were assessed. sires is selected, typically from among member flocks. Each member of the scheme then uses some, although not necessarily the same, rams from this

2. Methods