Sex selection method could curtail trade

Dairy cows could one day be genetically engineered to conceive only female offspring, so avoiding the birth of "unwanted" bull calves which are then sold on to the veal trade.

Researchers at Cambridge are already using a technique for separating individual sperm cells to produce calves whose sex can be determined in advance. The technique, developed by the US Department of Agriculture, has been licensed to the British biotechnology company Mastercalf.

According to professor Chris Polge, scientific consultant to Mastercalf, "the technique has been very successful. More than 90 per cent of the offspring are of the desired sex."

However, the technique is currently capable of sorting only about half a million sperms an hour. For use in artificial insemination (the way in which about 80 per cent of dairy cattle are bred), about ten million sperms are required per dose. As this would require up to 20 hours sorting, it is not a viable proposition.

Professor Polge said, "we have been using sorted sperm to achieve in vitro fertilisation, and to transplant embryos into recipient cows." The idea is to increase the proportion of bull embryos from a beef breed being implanted in the wombs of dairy cows.

Dairy herds have a turnover of 25 per cent a year, so with a 50:50 probability of males and females, farmers need to breed from half their cows in order to maintain the numbers of milking cows. Pure-bred Holstein males are not suitable for beef production and so are sold into the veal trade. However, if a male embryo from a beef breed is implanted into a Holstein cow, then the offspring could profitably be reared for beef. The ultimate aim would be to produce offspring in the ratio of one female dairy calf to three male beef calves.

According to professor Polge, "It has always been a goal to control the sex-ratio in farm animal production." In the longer term, he believes that researchers might start exploring genetic methods of controlling the sex of farm animals' offspring.

In 1991, British researchers working for the Imperial Cancer Research Fund and the Medical Research Council, announced that they had discovered the genetic "switch" which diverts an embryo from developing into a female (which is the default state in mammals) and turns it into a male. Professor Polge pointed out that these researchers had applied their discovery to switch the sex of the embryo of a laboratory mouse, "so it can be done in mammals".

It may be a long route, but for British calves the road from the French and Dutch veal crates may ultimately pass through the genetics laboratory.