Crossbreeding in Dairy Cattle Requires the Use of THREE Breeds
Interest in crossbreeding is at an all-time high among commercial dairy producers globally. The reproductive decline of Holsteins, on both an observed and a genetic basis, has been documented in most places. Also, all dairy breeds in the world have fewer problems than Holsteins for calving difficulty and for stillborn calves.
Post-calving complications of Holsteins have become more pronounced in recent years in most environments; therefore, dairy producers have been forced to place much greater emphasis on management of transitioning cows from pre-calving to post-calving. Holstein cows continue to become larger in size due to genetic selection and are often too large for optimum survival in facilities because of inadequate stall size. The combined effect of these factors is fewer calvings during the lifetimes of pure Holsteins today than in the past.
The global Holstein breed has become more inbred over time. It becomes more and more likely that bulls and cows that are mated to each other are closely related. Most consequences of inbreeding are masked and not readily noticeable. Inbreeding robs dairy producers of income by increasing stillbirths, reducing cow fertility, inhibiting disease resistance, and shortening herd life. Reduced cow fertility should be a major consequence of inbreeding, because inbred embryos are more likely to be non-viable and sloughed. Concerns about inbreeding are eliminated with crossbreeding.
Number of breeds to use in crossbreeding systems
Extent of hybrid vigor realized from crossbreeding systems differs tremendously based on the number of breeds included in the rotation. Table 1 reviews the hybrid vigor for each generation in crossbreeding systems that include 2, 3, or 4 breeds. The average hybrid vigor during the first four generations of 2, 3, and 4 breed crossbreeding systems is 72%, 91%, and 97%, respectively, which means that moving from 2 to 3 breeds increases average hybrid vigor by 19%; however, adding a 4th breed increases average hybrid vigor only 6%.
Once dairy producers become accustomed to hybrid vigor from crossbreeding, they prefer dairy cattle that maintain at least 75% of full hybrid vigor in all generations. For 2-breed crisscrossing systems (the F1 is backcrossed to one of the two original breeds), hybrid vigor dips as low as 50% in the second generation. After eight generations, hybrid vigor plateaus at 67% when only 2 breeds are used for crossbreeding; however, hybrid vigor plateaus at 86% when 3 breeds are used in a fixed rotation. Crossbreeding systems using only 2 breeds will limit the impact of hybrid vigor, and crossbreeding systems using more than 3 breeds will limit impact of single breeds of high merit for specific needs. Therefore, 3 breeds is usually the correct number to use in a fixed rotation for crossbreeding systems.
Some have argued that crossbreeding systems are confusing; however, this is not at all the case. When three breeds are used in a simple 3-breed rotation, a color tagging system eliminates the need for paper or electronic records. Calves sired by breed "A" are tagged with a blue tag, calves sired by breed "B" are tagged with a yellow tag, and calves sired by breed "C" are tagged with an orange tag. Then, whenever employees see a blue tag, they know that semen from breed "B" goes in that heifer or cow, and calves from that heifer or cow are sired by breed "B". Similarly, whenever employees see a yellow tag, they breed the animal with semen from breed "C", and calves from that animal are sired by breed "C." Finally, animals with orange tags are always bred to semen from breed "A", and all calves from those animals are sired by breed "A." This is an extremely simple mating and identification system, and it is much simpler than calculating inbreeding coefficients at the time of mating, which is essential when using only same-breed matings.
Crossbreeding should be regarded as a mating system that complements genetic improvement within breeds. Continuous use of progeny-tested and highly-ranked A.I. bulls is critical to genetic improvement regardless of mating system. Hybrid vigor is a bonus that dairy producers can expect in addition to the positive effects of individual genes obtained by using top A.I. bulls within breed.
Crossbreeding systems should make use of three breeds. The use of just two breeds greatly limits the impact of hybrid vigor, and that is especially true for the initial backcross generation. Backcrossing a first-cross female (F1) to an A.I. sire from one of the original breeds is especially unwise - an A.I. sire from a third breed should be used before returning to one of the original two breeds in the following generation. Dairy producers should carefully choose three breeds for crossbreeding systems that are optimum for conditions unique to their dairy operations.
Published in Dairy Star November 11, 2006