One of the sad facts of modern milk production is a large proportion of pure-bred, dairy bull calves are put to sleep by lethal injection within a few hours of being born.

Dairy bull calves grow out to have carcasses that have a lower yield of high-value cuts of beef compared to beef cattle bred from beef cows. As a result, they fetch a lot less when they are slaughtered and invariably lose money in those times when beef prices are depressed - so few British farmers are prepared to risk rearing them.

Having said that, producing beef from intensively-reared, dairy bull calves is a widespread practice throughout the rest of the EU. Dairy heifer calves, on the other hand, grow out to be valuable breeding replacements for dairy herds. Fortunately, scientists have successfully developed a technique that separates male sperm from female sperm that gives farmers the ability to determine the sex of calves born to artificial insemination (AI).

Inseminating dairy cows and heifers with "female" semen that will impregnate them with heifer calves is becoming increasingly popular as it offers many advantages.

Firstly, it allows dairy farmers to produce an adequate supply of heifer replacements for their herd, with the prospect of a lucrative income stream from selling surplus heifers. Indeed, the technique became so popular in the US that some states saw prices for dairy heifers collapse as the market became flooded with them. That led to farmers becoming more disciplined and only breeding heifer calves from their best cows, and inseminating their poorer ones with semen from beef bulls to breed beef-type calves suitable for fattening.

The second advantage of using sexed semen is that heifer calves tend to be smaller than bull calves and that leads to an easier calving. Cows and heifers don't yield as much milk if they have had to recover from a bad calving, whilst in the worst cases some are so badly injured that they have to be culled.

A third advantage, that has been recently discovered by researchers in the US, is a female calf's mother will produce significantly more milk than a cow that produced a bull calf. The trend is most pronounced in a first-calf heifer which will give more milk in both her first and second lactations if her first calf is female. If her second calf is also a heifer, the benefit accumulates, and is worth 446kg of milk over the two lactations.

This is 2.7% more production than from a cow which produced two bull calves, and there is no difference between the two in the quality of the milk.

While the researchers - from Harvard and Kansas State Universities, and from Dairy Records Management Systems knew the generally easier calving of a heifer calf was likely to have a positive effect on subsequent production, they were able to prove unequivocally that there was an additional cause.

Beginning their research with 2.39 million lactation records from 1.49 million Holsteins, they cut this database down to eliminate lactations that began with a difficult calving, and even among this easy-calving group, there was a positive effect on milk production of a female calf.

The mechanisms involved were not specifically identified, but there was circumstantial evidence to suggest the female foetus was exerting a hormonal influence on her dam.

Some dairy farmers are inseminating their heifers with sexed semen from top dairy bulls because they believe it speeds up genetic progress. They argue a heifer has more up-to-date genetics than a 10-year-old cow that was bred from the top-performers 10 years ago.

Using sexed semen isn't cheap, but it certainly has many advantages, and should have beef farmers anxiously looking over their shoulders. The main source of beef in the EU is the dairy herd, and specialist beef-breeding herds, such as we have in Scotland, are in the minority. Scottish farmers developed their specialised beef herds to graze their hills and uplands, the type of poorer land we have in abundance.

Imagine if most dairy farmers bred their heifer replacements from maiden heifers and top-performing cows, and inseminated the rest of their cows with semen from beef bulls, as is now happening on a lot of dairy farms in the US. They might even go on to use sexed, beef-semen to produce bull calves so that European production of good quality beef soared. It doesn't take a lot of imagination.