THERE have been a couple of interesting cases of multi-breed births in both sheep and cattle reported in the farming press in the past couple of years.

The most recent one concerned three Blackface ewes that gave birth to twins comprising both a cross-bred and a purebred lamb, as well as a mixed-breed set of triplets. The mixed-breed multiple births were the result of a Scottish Blackface ram breaking into a field containing a batch of Blackface ewes that were running with a Bluefaced Leicester ram. The three ewes must have been in heat at the time and served by both rams, with the result that they conceived to both.

Mixed-breed births are rare in sheep as most ewes are mated to just one breed, but when different sires are present, these types of pregnancies are fairly common.

I used to see an odd one every other year when I was farming, after a "love-struck" ewe had managed to jump into the field where I kept several breeds of rams. Obviously two ova, or eggs, were fertilised by sperm from two different breeds of rams.

Much the same can happen in pigs. Their ovulation occurs between 38 and 42 hours after the onset of oestrus (coming into heat), and takes about 4 hours. As the sperm cells can survive longer than the ova, the optimum time for service is prior to ovulation.

Unfortunately it is not possible to predict this with any accuracy, so to overcome the problem the sow or gilt is mated, or artificially inseminated (AI'd), at least twice during the "standing heat" period.

Where two services are possible, larger numbers of piglets born per litter result, due to fertilisation of a greater number of eggs. The eggs are shed more in the latter half of the heat and also over a longer time than the sperm remain viable.

I remember when I was a student being asked by a lecturer how to prove that a second service lead to more piglets. After staring vacantly at the lecturer he replied: "Use a white breed of boar for the first service, and then a black breed for the second one. The number of black piglets in the litter is the bonus from a second service."

Multiple births are rare enough in cows, but generally the likelihood of one having twins is somewhere between 1 and 7 per cent, compared to humans where the chances of twins are 3 to 4 per cent. The chances of cows having triplets is exponentially more rare, but do occur from time to time.

A couple of years ago there was a report of a Holstein Friesian (those black and white dairy cows) in Lincolnshire that had given birth to three separate breeds of calves at the same time - a Limousin, a Simmental and a Charolais.

The cow had been AI'd with mixed-breed semen, which contained the three different breeds to extend the period over which an insemination remains viable - giving a slightly higher chance of pregnancy success.

AI has allowed livestock breeders to make huge advances in genetics. Perhaps one of the most interesting has been the development of "sexed" semen, where male and female sperm are separated.

Inseminating dairy cows and heifers with "female" semen that impregnates them with heifer calves is becoming increasingly popular. It allows dairy farmers to speed up genetic progress and produce an adequate supply of heifer replacements, with the prospect of a lucrative income stream from selling the surplus. Another advantage is that heifer calves tend to be smaller than bull calves and that leads to an easier calving.

There is also an unfortunate downside to AI. An example of what can go wrong with modern dairy breeding programmes was the emergence of Complex Vertebral Malformation (CVM), a genetic disease that results in abnormal, deformed calves that are either spontaneously aborted, or die shortly after birth.

CVM is caused by a recessive gene that has been traced back to the bull Carlin-M Ivanhoe Bell who lived in the 1980s. His semen was used for two decades in international Holstein breeding for the exceptional milk production he passed on to his daughters. Because of the wide international usage of that bull the CVM gene is found in Holsteins throughout the world. By the turn of the 21st Century more than 30 per cent of the best Holstein sires in both Denmark and Japan were CVM carriers.