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Crossbreeding- The Lost Art?: Part 2
Livestock Update, March 2001
Scott Greiner, Extension Animal Scientist, Beef, Virginia Tech
Note: This article is Part 2 in a three part series dealing with crossbreeding.
Part 1 of this series dealt with the fundamental principles of crossbreeding. The primary advantages of crossbreeding beef cattle are heterosis (hybrid vigor) and breed complimentarity. Individual heterosis results in increased calf livability coupled with an increase in growth rate. The crossbred cow exhibits improvements in fertility, weaning weights, and longevity. Since heterosis has its largest impact on reproductive traits which are lowly heritable, a well-designed crossbreeding program offers the opportunity to improve these economically important traits in commercial breeding programs. Breed complimentarity allows for the capture of the strengths of the various breeds, and enables selection of individual animal within those breeds for specific purposes. The ability to combine different breeds to match end product specifications may be justification alone for adopting a crossbreeding plan. The key element to the success of any crossbreeding program lies in its application. This article will focus on the design and management of crossbreeding systems, with specific attention on the application for small herds.
The success of a crossbreeding program will depend on its simplicity and ease of management. There are several factors and challenges that need to be considered when evaluating choice of crossbreeding system, including:
- Number of cows in the herd
- Number of available breeding pastures
- Labor and management
- Amount and quality of feed available
- Production and marketing system
The design of any crossbreeding program should take advantage of both heterosis and breed complimentarity. The goal of a crossbreeding program should be to 1) optimize heterosis in both the calf crop and particularly the cow herd (not necessarily maximize heterosis), 2) utilize breeds and genetics that fit the feed resources, management, and marketing system of the operation, and 3) is easy to apply and manage.
Two-Breed Rotational Cross
The two-breed rotational cross or criss-cross is a relatively simple and popular form of crossbreeding. In this system, two breeds are mated and the resulting female offspring are kept as replacements and mated to one of the breeds. In following generations, females are bred to the opposite breed of their sire. For example, if Angus and Gelbvieh were crossed to make _ Angus x _ Gelbvieh females who were then bred to Angus, the resulting calves would be _ Angus x _ Gelbvieh. These females would then be mated to Gelbvieh bulls. For their entire lives, females would be mated to the bull breed opposite their sire. This system would require a minimum of two breeding pastures (if only natural service is used), one for each breed of sire- and cows need to be identified by breed of sire. A critical component for this system is that the two breeds utilized must be reasonably compatible in biological type. Both breeds must be suitable as both sire and dam breeds. The two breeds utilized in this system should be similar in mature size, and individual bulls selected to avoid large differences in birth weight, milk production, and cow size/nutritional requirements from one generation to the next (see Part 3). An advantage to this system is the use of the crossbred cow, with pounds of calf weaned per cow exposed increased approximately 15% compared to the average of the breeds used in the cross. Over several generations, 67% of the maximum amount of heterosis is realized. Additionally, there are a large number of heifers from which replacements may be selected.
If three breeds are used in the system instead of two, pound of calf weaned per cow exposed increases to approximately 20% and average heterosis over several generations attains 87% of maximum. However, three breeding pastures are necessary and significantly more management is required with the three breed vs. two breed rotational cross. Additionally, finding three breeds compatible in biological type is more challenging. For these reasons, rotational crossbreeding systems beyond a two-breed rotation are not feasible for many producers.
Terminal Sire Systems
The addition of a third breed as a terminal sire to a two breed rotational cross system can further enhance the system. In this rota-terminal system, approximately 50% of the cowherd is mated to the terminal sire breed (different breed than used in the two-breed rotation) with the resulting offspring all marketed (no replacement females retained from resulting mating). The other 50% of the cowherd operates as a two-breed rotation as outlined above. The two-breed rotation functions to produce all replacement females for the herd. Terminal sire breeds should be selected for calving ease, growth rate and carcass merit. Selection emphasis should concentrate on maternal performance, appropriate mature size, and longevity for the two breeds used to produce replacements. These selection criteria may simplify bull selection, and enhance the opportunity to specifically match genetics for their intended purpose. Older (> 4-5 years) and poorer producing cows are the best candidates for mating to the terminal sire. Younger cows should be genetically superior due to selection and should be used to produce the replacement females. The rota-terminal system has been shown to increase pounds of calf weaned per cow exposed by approximately 20%. Maximum heterosis is realized in the calves sired by the terminal breed, and advantages in maternal heterosis are realized as all females are crossbred. The rota-terminal system requires more management in that a minimum of three breeding pastures are required (assuming all natural service). Additionally, less selection may be practiced on potential replacements, as a larger percentage of the eligible heifers must be retained to maintain herd size. The rota-terminal system is difficult to apply herds with less than 100 cows.
Rotating Breeds of Sire
Rotating the breed of sire every three to four years may be a feasible crossbreeding option for producers who have small, single-sire herds. With this type of system two sire breeds are used in rotation by replacing sire breeds every three to four years. A greater number of breeds may be utilized over an extended period of time. In single sire herds, bulls may need to be replaced more frequently to avoid father-daughter matings. This system is relatively simple yet maintains an acceptable level of heterosis. Pounds of calf weaned per cow exposed is increased 10-15%, dependent upon the number of sire breeds used.
A major challenge to making a crossbreeding program work is keeping the system going over time, i.e. keeping the system well-planned and systematic without sacrificing optimum levels of heterosis and breed complimentarity. Purchasing of replacement females and the incorporation of an AI program are two means to assist with these challenges and have particular application for small herds.
Purchasing Replacement Females
The simplest, most manageable crossbreeding system utilizes purchased crossbred females mated to a third terminal sire breed. All calves are marketed in the system. Optimum heterosis can be realized in the cow, as well as the calf crop. There are several advantages to this system, especially for small cow herds. First of all, management becomes simplified as heifers no longer need to be grown, developed, and bred. Bred females may be acquired, which have been confirmed pregnant to highly proven bulls for calving ease and other economically important traits. Secondly, bull selection is simplified since these terminal sires will be not be mated to heifers, and maternal traits are not of interest. Sire selection can focus specifically on acceptable calving ease, and optimum growth and carcass merit. Additionally, only one breed of sire needs to be maintained. Remember that the health program, as well as the genetic package are both acquired from the heifer supplier. Of utmost interest is the economics of raising vs. purchasing replacement heifers. For many producers, purchasing females may be cost effective, especially when the contribution of the heifers to genetic progress of the herd is considered.
Use of Artificial Insemination
The use of artificial insemination may make the application of these described crossbreeding systems more feasible provided the expertise, labor, and facilities are available to make effective use of AI. The use of AI can significantly reduce the number of breeding pastures necessary for rotational cross or rota-terminal systems. Additionally, the use of AI may significantly reduce the number of bulls (and breeds) required for natural service. As an example, in a rota-terminal system the top 50% of the cows could be mated AI for the production of replacement females. Cows that did not conceive AI as well as the other 50% of the cows could be mated naturally to the terminal sire. This would reduce the number of breeding pastures required from three to one or two (depending on cow numbers). Additionally, in any system heifers could be bred AI to calving ease sires. Another major advantage to the use of AI is genetic improvement, as semen from top bulls in any breed could be utilized.
Increased production is realized in commercial herds not only through a crossbreeding program, but also through sire selection. A well-implemented crossbreeding program does not diminish the importance of sire selection. In fact, appropriate sire selection is the key to making the system sustainable. In Part 3, we will examine more closely the importance of sire selection, and describe tools to use to make both across and within-breed selections.
Virginia Cooperative Extension