It’s easy to look at your lambs and pick out those that are bigger than the rest. But is that the best way to identify the lambs with the greatest genetic potential for growth? Unfortunately, selection for growth traits is not that simple because your biggest lambs are likely to be those that are single born or twin ram lambs out of a ewe in her prime. Lambs like that have had the easiest path to fast growth, but may not carry the genetics for this important trait.  Here are some things to consider when selecting for growth:

Adjust your lamb’s wean weights:  Clearly a single born lamb is going to weigh more than a triplet.  Lambs raised by mature ewes have an advantage over those raised by yearlings or old ewes.  Males generally grow faster than females.  Katahdin-specific adjustment factors have been calculated to allow producers to compare lambs of different sexes, birth types and ages of dam (see chart below).

Accurate comparisons:  In addition to adjusting weights for sex, birth type and ewe age, all lambs in a contemporary group must be weighed on the same day and their weights adjusted for days of age.  A good contemporary group includes lambs that are about the same age and who were raised under the same conditions (same pasture, same feed).

Genetics:  Estimated Breeding Values (EBVs) for weight traits evaluate an animal’s merit for growth.  EBVs are based not only on the individual animal’s weight gain, but their relatives’ genetic merit for growth too:  parents, siblings, half siblings, grandparents, etc. If your flock is not in NSIP, you can still get a pretty good idea of genetic merit by choosing animals that had the best adjusted weight out of sires with the best genetic growth potential. If you used more than one ram, average the weights of all lambs out of each sire to determine if there are significant differences between sires based on the average weaning weight of their offspring.

Remember that selecting for growth traits can have disadvantages too.  Fast growing lambs often have larger frames that require more feed to reach optimal market condition.  Growth traits are correlated, so animals with high weight EBVs will likely produce lambs with higher birthweights that could lead to lambing difficulties, and heavier mature body weights that could affect stocking density.  

By: EAPK Communications Committee