Figures are a fantastic tool when the data behind them is robust and the limitations and validity of the figures are well understood by the people using them. In the Corriedale world, indexes have been a contentious topic for years.

While not the only breed grappling with some of the Sheep Improvement Ltd (SIL) complexities and politics, Corriedales and other mid-micron sheep pose a slightly more complicated picture than some other breeds. The most sensible wisdom is that estimated breeding values (EBVs), indices and structural traits need to be balanced. Terminal breeds are a relatively simple breeding proposition compared with a maternal. The NZ terminal worth (NZTW) and NZ maternal worth (NZMW) indices reflect the main important traits of each type of sheep. Most maternal breeds are strong woolled and provide an umbrella for breeders largely focused on growth and fertility ahead of wool traits. SIL can handle wool traits, and as a result Corriedales and other breeds are able to use a tailored index that reflects the wool value of the sheep and weights it accordingly to other traits.

When compared with strong wool breeds using the NZMW index in isolation, Corriedales generally struggle to stack up on figures because their wool value, a key trait and focus for Corriedale breeders, is not adequately reflected. As a result, mid-micron breeders can use a mid-micron index. This becomes more relevant to them and their clients than the NZMW.

Suggesting NZMW is a uniform index, which all maternal breeds can be adequately compared with, is misleading. Some breeders adopt their own indices using the range of traits they rate as most important – this is different again.

There is a conflict between using raw data and EBVs. Many clients still revert to looking at a ram’s weaning weight or fleece weight in kilograms. However, both of these are influenced by feed and environmental conditions. EBVs are more relevant because they put that sheep’s value for a particular trait in perspective relative to the flock, and account for other influences such as birth rank. The closest breeders come to consensus on the use of indices and EBVs is when they discuss balance.

Figures are a tool to use but they do not pick up all the faults that a good eye does. Conversely, EBVs and indices complete part of a big picture concerning an animal’s merit, which no one can see simply by looking at an animal. Breeders and clients who consider a range of tools available, and balance the traits on offer in a way that aligns with their own business goals, tend to make the best decisions.


  • Know the index a ram breeder is presenting – check if they use a standard index or tweak it in any way, for example, adding a trait index on to the standard index.
  • Check if the index is from a within-flock or across-flock analysis.
  • Take time to understand the traits included in the index and the weighting of each. If the weighting is understood then a client might choose a top performing ram based on key EBVs rather than index alone.
  • Check if the breeder records all the included traits or whether some rely on default values.
  • Are there traits that are important that the index does not include? Looking at EBVs as well as the index is a good way to find the sheep that best suit the client’s goals. For example, dag score, longevity or wool curvature might be valued traits that the index does not account for.