Breeding Better Livestock

by RALPH W. PHILLIPS

THE BREEDER uses three basic tools to bring about the genetic improvement of animals. They are selection, inbreeding, and crossing. The tools have been used to develop existing breeds; they will be used to effect further improvement in breeds, establish new types and breeds, and raise the productivity of commercial livestock. Besides them, the breeder uses a knowledge of the physiology of reproduction to insure maximum fertility and maximum opportunity for selection.

The breeder's ability to select superior animals as parents of the next generation is one of the most important of the factors that determine progress in animal breeding. If the breeder is to select genetically superior animals, he must have yardsticks that measure that superiority. And if he is to utilize effectively in selection the knowledge he obtains through application of those yardsticks, he must know which selection procedures will result in greatest progress. Several recent studies have yielded important information on those points.

A breeder may use one of the three basic methods of selection. These are: First, the "tandem" method, in which he selects for one character at a time until it is improved, then selects for another one, and so on, until all desired traits are improved; second, the "total score" method, in which selection for all desired traits is practiced simultaneously, the total score or index being constructed by adding into one figure the credits and penalties given each animal according to its superiority or inferiority for each trait considered; third, the "independent culling levels" method, in which he sets a certain level of merit for each trait, and discards all individuals below that level, regardless of their rating in other traits.

A careful investigation of the efficiency of the three methods of selection was made by L. N. Hazel and Jay L. Lush, of Iowa State College. From their study of the theories involved, they conclude that selection for a total score or index of net desirability is more efficient than selection on the basis of independent culling levels, and that the tandem method is the least efficient of the three methods.

Although selection on the basis of independent culling levels is generally less efficient than selection for total score, it does permit earlier selection for some traits, without waiting for other traits for which selection can best be made at later ages. The superiority of the independent culling level over the tandem method increases with the number of traits involved and the intensity of culling.

Dr. Hazel also studied the principles of constructing and using selection indexes, or the "total score" method of selection. He points out that the genetic gain that can be made within a group of animals by selecting for several traits at once is the product of the selection differential, or intensity of selection—the superiority of selected animals over the average of the entire group—the multiple correlation (a measure of relationship ) between aggregate breeding value and the selection index, and genetic variability. The first of these, the selection differential, is limited by the rate of reproduction of each species, and it may be small because of the breeder's carelessness in making selections or in emphasizing unimportant points. The third, genetic variability, is relatively beyond man's control. Hence, the greatest opportunity for increasing progress from selection is by insuring that the second, the multiple correlation, is as large as possible.

Hazel gives a multiple correlation method of constructing indexes having maximum accuracy. To use it, one must know the constants:

1. Relative economic values for the different traits.

2. Standard deviations (measures of variation) for each trait.

3. Correlations ( measures of relationship) between each pair of traits.

4. Heritability of each trait (a measure of the extent to which expression of trait is governed by heredity).

5. Genetic correlations between each pair of traits.

The genetic correlations show the extent to which traits are similar be-
cause of genes that affect both traits, and are determined by correlating
one trait in one animal with the other in a relative. Using these principles,
Hazel developed three indexes for swine. The first involved two characters
for which data were available before breeding age. The index (I) was:
I= (0.137 X W) — (0.268 X S)

in which W is the pig's weight at 180 days and S is the pig's market score.

The second index was:

I= (0.136 X W) — (0.232 X S) + (0.164 X P)

in which W and S are the same as in the first formula and P is the productivity of the dam, used as a measure of the pig's productivity, the lapse of one generation being compensated for by a suitable adjustment for the heritability of this trait.

The third index was designed to include information about the average weight (W) and score (S) of the litter in which each pig was born, in addition to the three traits in the second index. These were considered as fourth and fifth variables, using the correlations between the various traits and making allowances for the number of pigs per litter when arriving at the values to insert in the index.