Heter-what? Understanding the benefits of crossbreeding

Still, one of the most tried-and-true principles to maximize production traits while simultaneously reducing costs in commercial beef systems is crossbreeding.

Kashly Schweer

There are two primary benefits to crossbreeding: heterosis and breed complementarity. Understanding these advantages and how they can be applied to individual systems is key.

Q. What is heterosis?

A. Heterosis is the advantage in performance that crossbred individuals have over the average of their purebred parents. Whether the crossbred animal is composed of two, or three or more breeds, that little boost is often noticed in economically relevant traits, which in turn contribute to the overall bottom line.

The level of heterosis varies depending on the trait and breeds used to compose the cross. Generally, animals of unrelated breeds will exhibit higher levels of heterosis than those more closely related. For example, a cross between Angus and Brahman will exhibit higher levels of heterosis than when Angus and Hereford are mated.

Q. How is heterosis calculated?

A. Percent heterosis is the mathematical difference between the average performance of the crossbred animals and the average performance of the straightbred parents on a percentage basis. This can be expressed as:

Percent heterosis = [(crossbred average – straightbred average) ÷ straightbred average] x 100

For example, if it is a two-breed cross between Breeds A and B, and Breed A has an average weaning weight of 480 pounds while Breed B has an average weaning weight of 550 pounds, the average weaning weight of the straightbred parents is 515 pounds.

When the two breeds are crossed, the resulting crossbred calves have an average weaning weight of 530 pounds. The percent heterosis would be calculated as:

[(530-515) ÷ 515] x 100 = 3 percent

In this example, the 3 percent or 15-pound increase in weaning weight is defined as heterosis.

Q. Does crossbreeding only benefit growth traits?

A. Actually, crossbreeding exhibits the highest levels of heterosis in reproductive traits, not those associated with growth. An inverse relationship exists between heritability and heterosis. Heritability is the proportion of variation that is under genetic control.

When traits have low heritability, they respond slower to selection pressure and are difficult to make genetic progress for. Conversely, these traits that are lowly heritable have the largest response to crossbreeding. Reproductive traits tend to have heritability estimates in the 10 percent range. Crossbreeding can be used to enhance reproductive efficiency in commercial systems.

Growth traits are moderately heritable (20 to 30 percent) and are also moderate in their level of heterosis. Highly heritable traits, such as carcass traits, express the lowest levels of heterosis. Table 1 summarizes common traits, their heritability and level of heterosis.

Q. Are there different forms of heterosis?

A. It is important to understand that there are different forms of heterosis: individual, maternal and paternal. You can think of individual heterosis as the extra boost in calf livability and performance expressed in the crossbred calf.

Maternal heterosis is the advantage a crossbred dam has in her maternal capabilities such as the uterine environment, milk supply, longevity or post-natal care.

The majority of the economic benefit of crossbreeding systems is due to having crossbred cows. Building production systems that maintain heterosis in the cow herd is beneficial for improved fertility and longevity and the overall economics of the beef system.

In order to maintain maternal heterosis, crossbred females need to be used. This can be accomplished by either retaining the F1 heifers or purchasing individuals off-farm. Paternal heterosis is the effect of using a crossbred sire on the performance of its offspring, but less is known about its contribution to the beef system.

Q. How do composite bulls contribute to crossbreeding systems?

A. Composite bulls are growing in popularity. Using composite sires in a crossbreeding system is a simple way to inject heterosis into high-percentage purebred herds.

Composite breeds result from a planned mix of purebreds that is maintained at consistent breed percentages. Using composite bulls eliminates the need of multiple breed rotations.

Q. How do I choose what breeds to use?

A. Online dating sites claim to find soulmates based on compatibility. Crossbreeding can be much like playing matchmaker. It allows the producer to take advantage of the strengths of multiple breeds to have offspring with superior performance in a variety of traits.

It is important to remember that crossbred calves may not outperform their purebred parents across all traits. Crossing breeds with different strengths is designed to optimize performance for a given environment and breeding objective.

Keeping the end target in mind while selecting breeds can help direct producers to breeds that will benefit their operations.

Q. How do I compare bulls of different breeds?

A. Typically, more than one sire breed is used in crossbreeding systems. Since sire selection is of high importance for genetic improvement, it is critical the best bulls for the specific systems are being chosen both within and across breeds.

Comparing bulls across breeds is much like comparing apples to oranges; different types of fruit with unique characters. To compare apples to apples, the expected progeny differences (EPDs) need to be put on a similar basis. This is accomplished using adjustment factors.

The across-breed EPD adjustments are updated annually from the U.S. Meat Animal Research Center.

Q. Will my cows continue to be uniform in size?

A. It is important to keep uniformity in the cow herd for a variety of reasons. When the cow herd boasts a multitude of sizes, it also has different nutrient requirements. This can lead to feed waste if overcompensating for the larger cows in the herd or, on the opposite end of the spectrum, depriving the bigger ladies of their nutrients if underfed.

One way to help maintain cow herd uniformity is to use bulls of similar frame score. With a strong relationship between frame score and mature size, minimizing differences in frame scores of bulls used to produce replacement females will help maintain cow herd uniformity.

In order to have a successful crossbreeding system, it must align with the resources available and breeding objective. Capturing the benefits of heterosis allows producers to optimize genetic progress in economically important traits, especially those associated with reproductive efficiency and maternal performance.