Crossbreeding- The Lost Art?: Part 1
Livestock Update, February 2001
Scott Greiner, Extension Animal Scientist, Beef, VA Tech
Note: This article is Part 1 in a three part series dealing with crossbreeding.
The economic climate of today's beef business is challenging. Commercial cow-calf producers are faced with optimizing a number of economically important traits, while simultaneously reducing costs of production in order to remain competitive. Traits such as reproduction, growth, maternal ability, and end product merit all influence productivity and profitability of the beef enterprise. Implementation of technologies and systems that both reduce costs and enhance productivity are key. One of the oldest and most fundamental principles that has a positive influence on accomplishing these goals is crossbreeding. This article, in a three part series, will more closely examine crossbreeding and its implications for commercial cow-calf producers.
Crossbreeding beef cattle offers two primary advantages relative to the use of only one breed: 1) crossbred animals exhibit heterosis (hybrid vigor), and 2) crossbred animals combine the strengths of the various breeds used to form the cross. The goal of a well-designed, systematic crossbreeding program is to optimize these advantages of heterosis and breed complementarity simultaneously.
Heterosis or hybrid vigor refers to the superiority in performance of the crossbred animal compared to the average of the straightbred parents. Heterosis may be calculated using the formula:
% Heterosis = [(crossbred average - straightbred average) / straightbred average ] x 100
For example, if the average weaning weight of the straightbred calves was 470 pounds for Breed A and 530 pounds for Breed B, the average of the straightbred parents would be 500 pounds. If Breed A and Breed B were crossed and the resulting calves had an average weaning weight of 520 pounds, heterosis would be calculated as:
This 4% increase, or 20 pounds in this example, is defined as heterosis or hybrid vigor.
The amount of heterosis expressed for a given trait is inversely related to the heritability of the trait. Heritability is the proportion of the measurable difference observed between animals for a given trait that is due to genetics (and can be passed to the next generation). Reproductive traits are generally low in heritability (less than 10%), and therefore respond very slowly to selection pressure since a very small percentage of the difference observed between animals is due to genetic differences (a large proportion is due to environmental factors). The amount of heterosis is largest for these traits that have low heritabilities. This has significance for commercial breeding systems, as crossbreeding can be used to enhance reproductive efficiency. To date, the ability to select for reproduction is limited (ie. there are no EPDs for reproduction). Traits that are moderate in their heritabilities (20 to 30%) such as growth rate are also moderate in the degree of heterosis expressed (around 5%). Highly heritable traits (30 to 50%) such as carcass traits exhibit the lowest levels of heterosis.
Improvements in production from heterosis may be attributed to advantages in having both a crossbred calf and a crossbred cow. The following two tables summarize the effects of individual heterosis (crossbred calf) and maternal heterosis (crossbred cow). These tables include results from numerous crossbreeding studies conducted in the Southeast and Midwest involving several breeds. The advantage to the crossbred calf is two-fold: an increase in calf livability coupled with an increase in growth rate. Perhaps the most important advantage for crossbreeding is realized in the crossbred cow. Maternal heterosis results in improvements in cow fertility, calf livability, calf weaning weight, and cow longevity. Collectively, these improvements result in a significant advantage in pounds of calf weaned per cow exposed, and superior lifetime production for crossbred females.
Advantage of the Crossbred Calf
|Calving rate, %||3.2||4.4|
|Survival to weaning, %||1.4||1.9|
|Birth weight, lb.||1.7||2.4|
|Weaning weight, lb.||16.3||3.9|
|Yearling weight, lb.||29.1||3.8|
Advantage of the Crossbred Cow
|Calving rate, %||3.5||3.7|
|Survival to weaning, %||.8||1.5|
|Birth weight, lb.||1.6||1.8|
|Weaning weight, lb.||18.0||3.9|
|Cow Lifetime Production:|
|Cumulative Wean. Wt., lb.||600||25.3|
The other important advantage to crossbreeding is the ability to take advantage of the strengths of two or more breeds to produce offspring that have optimum levels of performance in several traits. As an example, British breeds generally excel in marbling potential whereas Continental breeds typically are superior for red meat yield (cutability). Combining the breed types results in offspring that have desirable levels of both quality grade (marbling) and retail yield (yield grade). Similarly, milk production and growth rate may be most effectively optimized by crossing two or more breeds. The following table has been created using across-breed EPD adjustments. The table is indicative of the genetic merit of the average bull in each breed listed for the primary growth and maternal traits. The breeds may be directly compared, as the EPDs have been adjusted to a common base. Certainly, considerable differences between breeds exist that may effectively be utilized by crossbreeding.
Breed Average EPDs Adjusted to a Common Base
|Breed||Breed Average EPD|
It is important to realize that the crossbred offspring will not excel both of the parent breeds for all traits. In the example given previously, straightbred calves of Breed B would have had heavier weaning weights (530 pounds) than the Breed A x Breed B crossbreds (520 pounds). However, Breed B females may be larger in mature size and have higher milk production potential resulting in increased nutitional requirements. Limited feed resources in this situation may result in lower reproductive performance. Therefore, the cumulative effect crossbreeding has when several traits are considered is more important than any one particular trait. Effective crossbreeding programs must be designed to optimize performance, not necessarily maximize it.
In short, a well-designed crossbreeding program can improve the productivity of the commercial cow-calf operation. Through the benefits of heterosis, crossbreeding may effectively improve the most economically important traits to the beef enterprise- reproduction and growth. Additionally, by combining the attributes of the various breeds available, crossbreeding programs allow for the matching of genetics with the environment, feed resources, and end product specifications. For this to be accomplished, the crossbreeding system must be manageable and applicable-stay tuned for Part 2.