Traditionally, when selling cattle in any industry segment or marketing channel, the more uniform the lot, the higher the price. Also, retailers are increasingly demanding beef cuts that are more consistent in taste and more uniform in size. In response to these market signals, seedstock breeders search for selection practices that will result in herd bulls with less variation in their offspring.

Two types of variation exist in beef cattle production – genetic and phenotypic. Genetic variation in a population (calf crop) relates to the diversity of genotypes for a particular trait. In other words, if all calves have similar genotypes for birth weight, then the calf crop would have very little birth weight genetic variation and vice versa. Phenotypic variation, on the other hand, is the actual (or total) variation that is expressed in the calves. Genetic variation is a part of phenotypic variation, but environmental (management) variation also makes an important contribution.

Increasing uniformity using breeding and selection practices is based on the idea of increasing the percentage of homozygous (identical) gene pairs. If all of the gene pairs that control a trait are homozygous in a particular bull, then he has no genetic variation for that trait. On the other hand, if a bull has a large number of heterozygous gene pairs, he has the potential to pass on many different genetic packages to his offspring. If a breeding program is going to be successful at producing bulls with less genetic variation, then practices should be used that will increase the percentage of homozygosity in those bulls. Currently, the most common practice has been some form of stacking pedigrees.

The term stacking pedigrees has several different meanings depending on whom you talk to. The following are several of those methods, their effectiveness at reducing variation and some potential consequences.

Maximum single-trait selection

This is the practice of mating the very best bull available to the very best cow available for a particular trait. When using Expected Progeny Differences (EPDs), this is the most effective way to make rapid change in the trait being selected.

Effectiveness: Potentially, there may be some improvement in uniformity for the trait being selected, based on the principle that intensive selection for a trait will eventually move a population toward fixation at the alleles that influence that trait. Fixation is simply the elimination of heterozygous alleles, resulting in a higher percentage of homozygous alleles. However, there is no evidence that fixation of any trait has occurred in cattle populations using maximum single-trait selection.

Breeding like to like

Also known as positive assortative mating, this is the practice of breeding a bull and cow with very similar EPDs for each trait. As an example, a producer has a high birth weight cow (Angus birth weight EPD of +8 lbs.). Instead of using a corrective mating sire (Angus birth weight EPD of -2 lbs.), the producer mates the cow to a bull similar to her birth weight EPD (Angus birth weight EPD of 7 lbs.).

Effectiveness: Based on basic genetic principles and verified by research in beef cattle, this system does not significantly reduce trait variability. For most of the traits selected for in beef cattle, mating bulls and cows with similar EPDs does not necessarily result in an increase in the percentage of homozygous alleles in their offspring, and therefore, does not improve the uniformity of future calf crops.

Consequences: If the cows in the herd are being mated to bulls with similar EPDs, then their offspring, on the average, are expected to have the same EPD. Therefore, this herd is not making genetic progress and the progeny produced will likely have no less variation than if corrective or progressive matings were used.

This is the practice of breeding animals that are more closely relate than the average population. Linebreeding is a form of inbreeding where the focus is to concentrate on one particular animal or line of animals in the pedigree.

Effectiveness: Based on genetic principles, this practice should produce bulls that will sire a more uniform calf crop than non-inbred bulls. Genetic variation is reduced when number of possible genotypes is reduced. Theoretically, inbreeding should be an effective means of reducing genetic variability; however, research with beef cattle has shown that this reduction in variation is slight, even when inbreeding was practiced in combination with single trait selection for growth over many generations.

Consequences: Inbreeding can cause several adverse affects, including reduced fertility, reduced survivability, reduced longevity and reduced performance.

Using breeding practices such as those listed above have little to no effect in improving the uniformity of calf crops. Individual bulls show differences in the amount of variation observed in their calf crop, but it is not clear how to make mating and selection decisions that will result in bulls with less variation on a consistent basis. Even though uniformity has an economic impact in the industry, it is difficult to reproduce. At most, understanding breeding practices and the effects of management will provide producers with the knowledge to manage variation.