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1 of 3 This article appeared in Heartland Sheep & Mainland Sheep, the spring sheep specials produced by Country-Wide, in 2008 Genetic Merit – what is it? Georgie Walker & Mark Young, SIL ‘What you see is not always what you get’ very much applies to sheep breeding. When you visually assess the size of a ram, you are looking at his ‘phenotype’, not his genetic merit. Phenotype of an animal is the sum of genotype (genes or DNA) and environment effects (feeding and management). For example, a small, lean looking (phenotype) ram at weaning may have very good growth genes (genotype) but may have been fed less well due to being born late, as a triplet with a hogget mother (environment). So he did not express his genetic potential for growth. Alternatively he may have been fed well (environment) but not have good growth genes (genotype). How do we know which situation applies? We need to use all information available to us to make a “best-bet” estimate of genetic merit as only genetics is passed on to offspring. This concept of actual performance (phenotype) being the sum of environmental effects and genetic effects is fundamental to modern animal breeding. It can be represented as: Performance (Phenotype) Weaning weight = = Genetics (Genotype) Breeding Value (genes influencing weaning weight) + + Environment o Date of birth o Birth litter size o Rearing litter size o Age of dam o Flock o Mob feeding effect Adjustments due to environmental effects Environmental effects are a very important part of the Performance equation. Another very important source of information SIL uses to get “best” estimates of genetic merit is how other family members performed. Two ewes may have similar lambing history, but one has sisters with an average litter size of 1.1 while the other has sisters with an average litter size of 1.6. Which one is the better bet? Other things being equal, it is clear that the ewe from the family with the higher litter size is a better bet to help us increase lambing percentage. SIL ram breeders record performance and environment effect information that allows SIL to produce accurate estimates of genetic merit. Effectively these are predictions of how animals 14:23 29-Sep-08 Genetic Merit 05 WEB.doc sil 2 rate relative to each other, when known environmental effects have been corrected for, and the performance of relatives accounted for. SIL uses the term “breeding value” (BV) for its estimate of genetic merit. BVs are measured in the same units as the trait. For many traits a higher value is better but for some traits a lower value is better (e.g. wool fibre diameter and faecal worm egg count). A ram passes on half his BV to his offspring. It is difficult to assess overall merit across a range of traits particularly when they are very different traits or are measured in different units. SIL addresses this by grouping traits together, where they affect a similar production characteristic, and converting merit to a monetary value. These are called sub-indexes. SIL has sub-indexes for: Growth, Meat (carcass merit), Reproduction, Survival (lamb), Wool and Health traits. These sub-index trait groups can be added together to create an Overall Index of genetic merit. With this system, two animals may be of similar overall merit, but one may be superior for lamb growth while the other is superior for lamb survival and fleece weight. SIL’s genetic merit triangle (see diagram) shows the process of how genetic merit is calculated and represented: Overall Index Sub indexes added up – a farm profit focus Sub-Indexes (Goal trait groups) BVs consolidated - a trait profit focus Breeding Values Field measurements Pedigree information Measurements adjusted for known environmental influences and performance of relatives combined to create BVs What traits are in an overall index depends on the farm production system. SIL’s three main Indexes systems are for Dual Purpose (ewe breed), Terminal Sire (meat breed) and Wool sheep. SIL performs genetic evaluations for many flocks and flock groups. Some sheep are in more than one of these evaluations. BVs and indexes can be different from one evaluation to another since there may be different information available on which to rate these animals against their contemporaries. 14:23 29-Sep-08 Genetic Merit 05 WEB.doc sil 3 SIL regularly performs one large-scale genetic evaluation that is across-flock and acrossbreed. Called SIL-ACE, it uses information from almost 3 million animals in over 300 flocks. Results listing the most highly rated sires are published on the SIL website (www.sil.co.nz). Having a genetic plan that identifies what traits are important to your farm will mean you can make most effective use of SIL figures. Work out what traits are most important for you to improve and make use of your ram breeder’s expertise by asking them to present animals to you that have good figures for these traits. TAKE HOME MESSAGES o o o o o What you see is not always what you get! Animals highly rated for growth may not be the biggest of those presented for sale. Don’t select animals on their own birth rank, or their dam’s lambing history, to improve lambing percentage. Use the BV for number of lambs born (NLB) Use SIL indexes and BVs to get best estimates of genetic merit – this is what the ram will pass on! Ask your breeder to explain SIL information to help you choose rams best suited to your needs. SIL-ACE is a national analysis that compares rams from many different flocks and breeds. Index figures for a ram from SIL-ACE can be compared to those of rams from the same analysis but they cannot be compared to index figures from an analysis done for just one single flock or for another across-flock group Georgie Walker & Mark Young work for Sheep Improvement Ltd (SIL), the national system for genetic evaluation of sheep that is part of Meat & Wool New Zealand. More information about the SIL system for rating sheep on genetic merit is available on the SIL website (www.sil.co.nz). SIL can also answer queries you may have about the SIL system for rating rams on genetic merit – you can telephone 0800-SILHELP (0800-745-435) or send an email to [email protected] 14:23 29-Sep-08 Genetic Merit 05 WEB.doc sil