Hybrid Yak Fertility
By Greg Dike
About six months ago I saw an advertisement for a white hybrid yak bull calf. He was beautiful. I I was interested in obtaining a white yak hybrid to start the process to backcross up to a scientifically based white yak – one with eight or less cattle alleles under the GeneSeek test. I had heard many people say that an F1 (first generation) male yak hybrid was sterile but that males that were ¾ yak or more (F2, F3, F4 etc) were fertile. This bull calf was at least an F2 and so I believed there were no fertility concerns. Fortunately Robert Stuplich, owner of American White Yak Ranch, saw my correspondence with the seller and cautioned me that there were fertility issues beyond the F1 generation.
While there have been no scientific studies in the United States on the fertility of male yak hybrids as a function of generation of backcrossing (B1=F2, B2=F3, B4=F5 etc) there is knowledge available from yak herdsmen who have been cross breeding and backcross breeding for many years. Shakoor Ali, in his book “YAK – The Cryophilic Species of Baltistan” wrote that after interviewing many yak herdsmen in the Baltistan region of Pakistan the operating understand among the herdsmen was:
Generation Male Fertility Rate Female Fertility Rate
F1 – ½ yak Sterile Fertile
B1(F2) – ¾ yak Sterile Fertile
B2(F3) – 7/8 yak Sterile Fertile
B3(F4)-15/16 yak 25%-40% Fertile Fertile
B4(F5) – 31/32 yak 50%-60% Fertile Fertile
B5(F6) – 63/64 yak 60%-90% Fertile Fertile
B6(F7) – 127/128yak 100% Fertile Fertile
While the above is based on years of observation among generations of yak herdsmen in the northern most territory of Pakistan and is a not a “scientific study” it provides valuable information to those interested in developing hybrid breeding programs or in purchasing a hybrid yak. While you may find an F2 or F3 yak hybrid bull that is fertile, what the chart tells us is that from the experience of generations of yak herdsmen who have been crossbreeding and backcrossing yaks, such a find is unlikely—and would not be a wise investment without a fertility test. On the other hand it tells us that it makes sense to investigate the fertility of F4 and up hybrid yak males.
Note: by GeneSeek DNA testing a hybrid yak the results will be able to tell you if the yak is F1, F2, F3, F4, or F5 and above. The reason the test cannot differentiate between F5, F6 and F7 is that all yaks are technically hybrids and have about 2% cattle alleles. So if you see a hybrid you really want to purchase, have it’s DNA tested by GeneSAeek first and find out what percentage yak (what generation backcross) it is. That will tell you a lot about the fertility and how many generations of backcrossing you will need to do to bring its offspring up to what is considered a non-hybrid yak.