Genetic Evaluation Indexes for Sheep Breeders

The Sheep Flock Improvement Program (SFIP) now has a maternal index for use by sheep breeders. This index combines genetic evaluations for several traits into a single value. It makes it easy to pick animals with superior maternal characteristics.

Currently, genetic evaluations are provided for many traits: birth weight direct, birth weight maternal, 50-day weight direct, 50-day weight maternal, gain from 50 to 100 days of age, number of lambs born, number of lambs weaned, and, for flocks where scanning is done, ultrasonic muscle and fat depth. Trying to improve the flock can be difficult when there are so many traits to consider at once. The top animals are not easy to identify.

Last year the terminal index and growth index were introduced. Those two indexes were designed to select animals that grow faster, have more muscle and less fat. They can be used to increase profitability of slaughter lambs.

The new maternal index complements the terminal and growth indexes. It too works to improve growth and carcass traits. But mostly it concentrates on producing ewes that have larger litters and wean heavier lambs. The maternal index balances the traits so that improvements can be made in more than one area.

The chart below shows the changes that would occur in one generation if only rams with maternal index values in the top 5 to 6% were used as sires. Growth traits improve with the maternal index, though not as much as when the terminal or growth index is used. The maternal index especially increases the number of lambs born and weaned, which is not affected by the other indexes. Changes in litter size appear smaller than changes in growth traits in the chart. One more kilogram in the weight of a marketed lamb is not a huge amount. But one more lamb per litter has an enormous impact. An increase of 0.10 in the number of lambs born is equivalent to one extra lamb for every 10 ewes in the flock.

Expected Genetic Change Using the Maternal Index

Increases in birth weight that naturally go along with greater growth are minimized with the index. The maternal component of birth weight decreases, partially offsetting an increase in birth weight direct. An increase in 50-day weight maternal comes from greater milk production of the ewe and better mothering ability. All this leads to more lambs weaned.

Maternal index values can be negative or positive. An animal that is above average has an index above zero. Higher values are better. If any ram with a value under 3.5 is used as a sire, the genetic change will be inferior to what is shown in the chart.

The maternal index is a new tool for producers wanting to improve the maternal characteristics of their flock. Rams that rate high for the maternal index should be chosen as sires of replacement ewe lambs.