Assessing Genetic Merit

Technical / Assessing Genetic Merit

The performance and physical appearance of animals are the result of a combination of factors: the genes they get from their parents; management, such as feeding; and other effects, such as age and rearing rank.

To make genetic improvement, we need to be able to assess how much of an animal’s performance is controlled by the genes alone, as it is only the genetic component of performance that is passed on to the next generation.

Visual assessment of animals – or even assessment on an individual animal’s performance – is a poor guide to the genetic merit of the animal.

Breeding values

Breeding values (BVs) are the best estimate of the animal’s genetic merit for traits. They allow rams to be compared, with all rams included in the same analysis. Mostly, this means all rams from the same flock, but it can be used to compare animals from different flocks, if the analysis has been performed across these flocks.

Breeding values are expressed in the units the traits are measured. For example, weight traits are in kilograms, number of lambs born (NLB) is expressed as lambs per ewe lambing.


An index provides a summary of overall economic breeding merit of animals. The economic value of each trait is summed to give a total economic value of the ram’s genetics. The economic value of a trait depends on the production system; therefore a number of indexes are used to describe different systems. For example, the terminal sire index (TSO) describes a system where all progeny are slaughtered. The dual purpose index (DPO) describes a system where ewe lambs are retained as replacements for breeding.

Sub-indexes are sometimes reported. These refer to the economic value for traits of a particular type. For example, Body Growth vs. Wool vs. Reproduction vs. Meat (carcass quality).

All SIL indexes are expressed as cents per ewe lambing.


BVs and indexes are a function of the population in which they are calculated. In most cases, they are only comparable within a single flock. Although across-flock analysis can allow values to be compared between several flocks, provided there are good genetic connections between the flocks.

It is not possible to compare the genetic merit of rams from different flocks, unless the flocks have conducted an across- flock analysis and there are good connections between the flocks.

For every analysis, SIL sets 1995 as the benchmark year and the average genetic merit of lambs born in 1995 is set to zero. Therefore, the BVs are a measure of the merit of a ram compared to the 1995 average for that flock. If two flocks had different average genetic merit in 1995, the BVs would have to be adjusted for the difference to make them comparable.