Traits, BVs & Indices
Traits, BVs & Indices
What is a trait?
A trait is something you can record for your livestock that measures their biological and economic performance. The best traits are simple and repeatable to measure. Liveweight is an example of an excellent trait to record that reflects growth performance. On Deer Select there are 17 liveweight traits including birthweight and monthly weights up to 12 months of age.
Overall there are 200 different traits on Deer Select at present. However nobody needs to record anywhere near that many! The traits chosen by each breeder reflects his, her or its individual breeding goals. For example, breeders for venison production focus on liveweight and carcass traits, but seldom record antler traits.
Generally, more than one trait will contribute to calculation of one BV. For example, Conception Date BV requires data on estimated foetal age, date of stag joining and the mating group.
The most overlooked piece of information is ‘Mob’. Mob or ‘management group’ is a crucial piece of information that must be recorded with each trait measurement. Mob information is essential to enable the Genetic Engine to account for within-farm environmental effects.
Breeding values (BVs) (otherwise known as ‘estimated breeding values' or 'eBVs') are a prediction of the genetic merit or worth of an individual for a particular goal trait after accounting for environmental variation. They represent the genetic potential of an individual based on
- its own performance records (within its contemporary group)
- the performance records of all offspring (‘progeny’) and other related individuals
Units for breeding values
Deer Select BVs are expressed in the units that the traits are recorded in. For example, liveweight traits are expressed in kilograms. The BVs can be positive or negative, where a zero value is the average animal recorded on Deer Select in 1995. Individuals with no performance records on Deer Select for the trait in question would be assigned a BV of zero.
Accuracy of BVs
All BVs reported on DEERSelect are accompanied by an estimate of their accuracy. The BV’s accuracy is determined by the records used to estimate the BV, and includes information on performance of the individual, performance of its progeny and performance of other relatives recorded on Deer Select. Well-used sire stags have the highest accuracies as they have large numbers of progeny records (which most accurately reflect their genetic merit). It is difficult for a hind to generate high accuracies, as they are only able to produce a limited number of progeny in their lifetime. As a guide, accuracies for liveweight traits for sires should be at least 75% for reliance to be placed on the BV.
When using BVs to evaluate genetic contributions of individuals to a breeding programme, it is important to remember that, on average, as a parent only half of the BV is contributed to the progeny.
Example 1: Expected progeny BV
In a venison production system, the effect of buying a stag with a high (+8.6 kg) carcass weight BV (CWeBV) to lift the average carcass weight of your terminal progeny, when your replacement hind herd average CWeBV is + 2.8kg is +5.7kg. This is determined using the following simple equation:
½ x 8.6 (= 4.3) + ½ x 2.8 (= 1.4) = Calf average +5.7kg
BVs are useful for looking at a single trait. But breeding programmes generally involve balancing the advantages of multiple traits, as the breeder wishes to combine more than one goal. However, different traits do not contribute at the same weight to the overall economic value of an individual and its offspring. In order to simplify this, particularly for stag purchases, Deer Select provides Economic Indices.
Currently economic indices are available to red deer breeders with a venison production focus. These indices are made up from multiple goal trait BVs. Each of these BVs has an adjustment factor (or ‘weighting’) designed to account for the input costs required and value realised to achieve the final combined goal. As the input costs and value realised are both measured in dollars, these indices are also expressed in dollars. The more complex the goal the more BVs are taken into account in the index. The indices are for: replacement hind and early kill venison production systems and terminal venison production systems, as shown in the example below.
Example 2: Terminal Index v Replacement Early Kill Index
The Terminal Index is relatively straightforward, think of it as offspring that get big and meaty, fast and efficiently. It is all about growth, carcass and meat yield (particularly of high value cuts). There is no consideration of reproductive traits.
However, while the Replacement Early Kill Index is more complex it is still about meaty, fast and efficient, but balances that with early conception and efficient calf production. So while also placing value on high growth, carcass and meat yield, it also considers reproductive performance of replacement hinds and their maintenance as a mature female.
More detailed information on the indexes is in the Archer and Amer paper below in show me the science.
As with BVs, economic indices are a prediction of the genetic merit of an individual. Therefore, when considering the value that a potential sire may contribute to your herd through his progeny, it is important to halve his dollar value expressed on the economic index you are interested in, in the same way as any amount for a single BV must also be halved (as in example 1 above).
The economic indexes are currently being re-estimated to update them as they were originally estimated in 2008/09.
Show me the science
Archer, J.A. and Amer P.R. (2009) Breeding objectives for seasonal production systems: an example from the New Zealand deer industry.Proceedings of the Association for the Advancement of Animal Breeding and Genetics 18: 207-211.
Archer, J.A., Ward J.F., Newman S-A.N., Cruickshank G.J and Pearse A.J. (2005) Implementing genetic evaluation in the New Zealand deer industry. Proceedings of the Association for the Advancement of AnimalBreeding and Genetics 16: 4-7.
Archer, J.A., (2007) Genetic technologies for deer breeding. Proceedings of the New Zealand Society of Animal Production 67: 91-94.