Including ratio traits in multiple trait selection

Background Animal breeding programs aim to improve productivity while reducing input costs and resource use. As sustainability has become a central concern, feed intake and enteric methane emissions now feature more prominently in breeding objectives, supported by increasing availability of individual-animal measurements for those traits. An important question is how to incorporate these traits most effectively in selection indices that aim at improving both resource efficiency and environmental outcomes. A seemingly appealing strategy is to augment existing selection indices with a ratio trait such as production per unit of feed intake or methane emitted per unit of product, as direct indicators of efficiency. This paper will investigate selection indices that include a ratio trait. Selection index theory was used to compare single trait selection for a ratio trait with selection based on a linear index comprising the component traits, as well as with selection including both the ratio trait and one of its components. Genetic parameters for ratio traits were approximated from those of the component traits. When incorporating a ratio trait into the breeding objective, the economic weights of the component traits were correspondingly adjusted. Results When ratio traits are used as a single trait selection criterion, the response to selection is driven largely by the coefficient of genetic variation of each component trait. However, an index including a ratio trait as well as one of the component traits is equivalent to a linear index with the two component traits. A breeding objective including both a ratio trait and a component trait requires an adjustment of economic values of the component traits. Conclusion Ratio traits such as feed efficiency and methane intensity can be used in selection indices as they give equivalent selection responses as indices that include the component traits (e.g. feed intake and methane production) alongside production traits, provided that at least one of the component traits—either the numerator or denominator—is also included in the index and economic weights are adjusted accordingly.