An Animals Own Performance

Animals were originally selected on the basis of their own phenotype. That is, cows that give the most milk, sows that have the largest litters, hens that lay the most eggs, and horses that run the fastest are examples of this type of selection. Although selection on phenotypes is very easy to apply, the accuracy of phenotypes as an estimate of genetic merit is equal to the square root of heritability. Heritability is the proportion of the variability in a trait that is attributable to genetics (Table 2). For most economically important traits, heritability ranges from 0.05 to 0.50. Consequently, response to selection on an animal's own performance may not be very high. Accuracy can be improved slightly for some traits by averaging several observations taken on the same animal.

Table 1 Minimum possible and typical generation intervals for common livestock speciesa

Sexual

Gestation

Minimum

Typical

Typical

Offspring

Species

maturity

length

possible

males

females

per gestation

Cattle (Bos taurus)

12 mo

280 d

33 mo

6 9 yr

3 4 yr

1

Horses

15 24 mo

340 d

41 59 mo

8 12 yr

8 12 yr

1

Swine

120 250 d

112 d

19 20 mo

1.5 2 yr

1.5 2 yr

10 12

Sheep

185 d

150 d

17 mo

2 3 yr

2 4 yr

12

Goats

165 d

150 d

15 16 mo

2 yr

2 yr

12

Rabbits

125 d

31 d

11 mo

1 yr

1 yr

8 12

Chickens

140 d

In egg

11 mo

1 1.5 yr

1 1.5 yr

250

Turkey

224 d

In egg

15 16 mo

1 1.5 yr

1 1.5 yr

100

Rainbow trout

2 3 yr

In egg

3 yr

3 yr

3 4 yr

1,000

aValues may vary with breed within species.

aValues may vary with breed within species.

Progeny Performance

For sex-limited traits, the average performance of an animal's progeny can be used. Males can generally have many dozens or hundreds of progeny and the accuracy of evaluating the genetic merit of that male can approach 100%. A disadvantage is that one has to wait until the progeny are born, grow, and make their own performance records, thereby increasing the generation interval and lowering response to selection. An optimal balance between accuracy of evaluation and generation interval has to be achieved.

Relatives' Performance

Animals' genetic merits could be evaluated using information on parents, full-sibs, or half-sibs. Accuracy of evaluation is limited by the amount of information on relatives and depends on the particular combination of relatives. Animals could be evaluated before they are old enough to be used for breeding purposes. The generation interval can be shortened using relatives' records, but accuracy of evaluation may suffer.

Combining All Sources of Information

The best method of evaluating genetic merit is through the use of an animal model.[3] An animal model is a statistical method for combining information from the animal's own performance, all other relatives, and all progeny, and at the same time, account for any nongenetic factors that might influence performance of animals, such as contemporary groups, ages, years, and seasons. Animal models make efficient use of the data and generally provide the highest probability of correctly ranking animals for genetic merit. Animal models have been used in dairy, beef, and swine since 1989 in various countries around the world. The methodology generally applied to the animal model is called best linear unbiased prediction, or BLUP. Methodology and genetic models, however, are continually being improved.

EVALUATION OF MORE THAN ONE TRAIT

Livestock are often evaluated for many traits, such as production, reproduction, conformation, and health. Selection can be applied separately to each trait (independent culling levels), or traits may be combined into an economic index. The first step is to define the breeding goal, i.e., all of the traits that the breeder hopes to change. The next step is to identify the traits that will be in the economic index. For example, there could be five traits in the breeding goal, and 12 traits in the economic index. Traits in the breeding goal may not be measurable directly on animals, and so two or three other correlated traits are used in the economic index as indicators of the trait in the breeding goal. The genetic variability of each trait, in the breeding goal and in the index, must be known as well as the genetic correlations between all traits. A genetic correlation between two traits is an estimate of the proportion of genes that influence both traits. Relative economic weights can be computed

Table 2 Heritabilities of different kinds of traits

Low (0 0.15)

Medium (0.15 0.40)

High (0.40 0.70)

Litter size

Milk production

Carcass traits

Fertility

Growth traits

Meat tenderness

Disease

Feed efficiency

Meat yields

susceptibility

Locomotion

Body lengths

Milk components

Conception rate

Racing speed

Gaits

Longevity/

Egg production

Fleece weights

survival

from figures on costs and returns using the estimated genetic correlations and genetic variabilities of traits.

Selection on an economic index leads to optimum change in economic value over all traits. Often, one trait has greater economic importance than the other traits, and so relative economic values are often debated. Economic indexes may be derived in many different ways. For example, the weights for the index may be based on the desired responses that the user seeks for each trait. Or, one or more traits, perhaps, should not change either for the better or worse, and weights can be derived to accomplish this goal. Some traits are related to each other, or to economics in a nonlinear manner. For example, legs on animals can be either too straight or too curved, but both lead to economic losses compared to an animal with desirable legs between too straight and too curved. Thus, economic indexes can be nonlinear functions.

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