|Auburn University Digital Library|
|Little Known Asian Animals With a Promising Economic Future|
source ref: b18ase.htm
|Part I : Domesticated Bovine Species|
Yaks and cattle have the same diploid number of chromosomes (60). And in the regions where yaks are found, they are often interbred with cattle, either the humpless cattle (Bos taurus) of Tibet and Mongolia or the zebu (Bos indicus) of South Asia. As with mules, the hybrid offspring of cattle and yak surpass their parents in strength and vigor. Yakows* grow faster than their parents, and they suffer less from high temperatures than yaks. The hybrid cow reaches sexual maturity earlier and yields larger quantities of milk than the yak cow. The males, however, are sterile.
Appearance and Size
The yakow's appearance varies with the type of cattle used in the cross. But because of the phenomenon of hybrid vigor (heterosis), the hybrids are considerably bigger than the mean size of their parents. (For instance, in one test the liveweight was approximately 18 percent higher than the average weight of the parents.) The hybrids also excel in hardiness, working ability, growth rate, and milk production.
Nevertheless, their appearance and performance is closer to that of the mother than of the father. Because of this, the yakows can be "custom designed" for various altitude zones. For example, the farmer at lower altitudes may produce cattle-like yakows by breeding yak bulls to domestic cows, while farmers at higher altitudes would use the reverse cross.
Yakows have shorter hair and a much less downy undercoat than purebred yaks. They are similar in color to yaks. (In Nepal, black is most frequent, but brown shades and even white may occur.)
Yakows are found in parts of northeastern Afghanistan, northern Pakistan, northeast and northwest India, Nepal, Sikkim, Bhutan, Tibet, western China, Mongolia, and southeastern Russia. They are found notably in the Himalayan region of Nepal, where they are bred by people of Tibetan culture and language. Sherpas, for instance, supply hybrid cows and heifers to dairy farmers throughout northeastern Nepal. Hybrids are also much bred in Ladakh (northern Pakistan) and Mongolia.
Whereas farmers in Nepal tend to use zebus to cross with yak, the inhabitants of the Tibetan plateau generally use Bos taurus cattle. People in some areas favor mating yak bulls to cows, while those in other areas prefer the reverse. But even within one village there can be a degree of specialization by different families, depending on wealth and the location of traditional grazing grounds. Generally, however, the crosses between domestic cattle bulls and yak cows are the predominant ones.
The number of yak-cattle hybrids in the world is unknown.
Habitat and Environment
Like the yak, the hybrid can live in cold, barren, upland terrain, but it has the advantage of being able to adapt to lower altitudes. In winter, caravans of hybrids may come down as low as 1,600 m in some parts of Nepal, and hybrid herds (especially those resulting from crossing yak bulls with zebu cows) can spend several weeks grazing at altitudes as low as about 1,700 m in winter.
Female hybrids are fertile when mated with either parent stock. Male hybrids, however, are sterile. Although they have fully developed secondary sexual characters and show libido, their testes do not produce spermatozoa because the seminiferous tubules are poorly developed and the spermatogonia and their nuclei are degenerate.
A few normal or motile spermatozoa may be found in the semen of backcrosses of the F1 hybrids to either parent, but as a rule the spermatozoa are normal and dead.* Backcrossing is reportedly not common, however, as the progeny do not retain hybrid vigor.
Tibetan farmers prefer the hybrids for plowing because of their docile temperament. The yak is said to be more stubborn.
The hybrids are valued as beasts of burden and draft, and are often preferred over yaks. Male hybrids are generally castrated at about 3 years of age so as to increase their strength and size.
Where pastures occur over areas of greatly varying altitude, the use of hybrid livestock is likely to be more efficient, in biological terms, than the use of yaks or cattle. By judiciously selecting cattle, yaks, or hybrids all altitudes from sea level to above 5,000 m can be utilized with best efficiency.
The hybrid's milk is intermediate in composition between that of its parents. However, hybrids yield up to 7 kg of milk per day against the yak's 3 kg. (It seems that the hybrid derived from cattle bull and female yak produces considerably more milk, with a higher fat content than that derived from yak bull x cattle female.) Also the female hybrids produce larger quantities of milk than Nepalese zebu cows on the same hill pastures. They also produce a calf each year, while under normal herd management yaks tend to produce a calf every 2 years.
Hybrids do not tolerate extremes of altitude or cold as well as purebred yaks.
The normal gestation period for yaks is about 1 month shorter than that of cattle, and the gestation period for a hybrid is intermediate between that of the parents - hence there may be problems at calving when a female yak is sired by a bull from a large cattle breed.
Research and Conservation Needs
Research is needed to identify differences in altitude tolerance between various hybrids. This should include measurements of hemoglobin count, respiration rate, and pulse rates at different altitudes.
Studies are also needed on all production characteristics, particularly on the meat and milk production potentials.
Cataloging the breeding strategies of different areas where hybrids occur would be useful, along with a historical perspective.
Research to determine the most productive hybrids by crossing different yak and cattle "breeds" in areas with different environmental conditions could be extremely useful.