The genetic makeup of maternal (sow) lines and terminal sire lines used to produce market hogs has a considerable bearing on how efficiently their progeny convert feed ingredients to lean meat protein.

This article will explain the heritability of growth and feed conversion traits and genetic correlations among the traits, plus provide some guidelines to ensure your genetic suppliers are applying appropriate selection pressure to the economically important traits.

Furthermore, it is important to understand your genetic base as you build diets that will maximize sow performance as well as the pigs' performance potential and their ability to hit carcass targets and maximize packer premiums.

Feed conversion ratio (FCR) reflects the rate at which an animal converts feed to meat. This ratio is calculated by dividing the amount of feed used by the total weight gained. The genetic contribution of both the sire and the dam lines must be considered for all production traits, including FCR.

The sire lines contribute half of the progeny's ability to convert feed ingredients to lean meat protein. Dam lines contribute the other half of the progeny's genetic potential, plus their ability to convert feed ingredients to reproduction and milk production.

Heritability Defined

Heritability, often defined as the resemblance between relatives, ranges from 0 to 1. High heritability indicates a high level of resemblance between parents and progeny. Effectively selecting progeny to become parents based on a particular trait will create a new generation of higher-performing animals.

Genetic correlations are the proportion of variability that two traits share due to genetic causes and, theoretically, may range from -1.0 to +1.0.

Table 1 shows the heritabilities and genetic correlations among production traits, feed intake and feed conversion, plus the traits often used to predict or improve the accuracy of estimating feed conversion.

The yellow portion of Table 1 shows the traits of average daily gain (ADG) in grow-finish, backfat depth (BF) and loin muscle area (LMA). All three traits have a moderate to high heritability and moderate genetic correlations. The sign (direction) of the correlations must be evaluated in addition to the magnitude. The 0.25 correlation between ADG and BF indicates that as ADG increases, backfat will also increase. This is not a favorable result and is indicative of an antagonistic correlation.

In comparison, the 0.36 correlation between ADG and LMA is beneficial and indicates that as ADG increases, so will muscle mass.

Finally, the -0.33 correlation between BF and LMA is also advantageous because a decrease of BF and an increase of LMA are both desirable changes.

Antagonistic correlations, such as the one between ADG and BF, can be controlled by utilizing selection indexes as long as both traits are measured and included in the indexes. Goals for either trait can be adjusted in the formulation of an index.

Heritability and Correlations

A few decades ago, feed efficiency was directly estimated by recording feed consumption of an animal placed in an individual crate. Selecting sires with the best feed efficiency using this method resulted in future generations with better feed efficiency for those animals “if” they were raised in crates. However, FCR progress was often disappointing for various reasons, so that method was dropped.

The research data generated by this method built an excellent case for indirectly selecting feed efficiency via its component traits, namely ADG, BF and LMA. By building the correlations of ADG, BF and LMA with FCR, a new selection index can be developed that will allow improvement in all four traits but only measure three. This is made easier by the advantageous correlations in all three traits with FCR.

Returning to Table 1, the green genetic correlations have the correct sign to allow improvement in FCR and are of a moderate to large magnitude. This selection method has contributed to most of the genetic improvement made in FCR in the 20th century.

Beginning in the 1990s, electronic feeding stations in pens of up to 20 head were used. Feeding stations were an improvement over the individual crates, but they suffered from occasional mechanical breakdown and erroneous data production.

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