Fat cows deliver wee boys

Icecream and hot chips for a boy, celery sticks and crackers for a girl?

New data from a study of Waikato cows suggests that if women want to give birth to a boy, then hitting the high-calorie diet before conception may be just the trick. Shedding kilos beforehand may help to ensure a girl.

Scientists from the Hamilton-based dairy research organisation Dexcel and an Irish colleague have studied 18 years' worth of data from 1200 cows.

"Cows gaining weight or condition - primarily body fat - prior to conception are more likely to have a bull calf nine months later," Dexcel scientist John Roche said yesterday.

The reverse was true when it came to births of heifers (females).

The results bore out international studies and "we would expect what holds true for one mammal will hold true across the board", said Dr Roche.

Scientists were not sure exactly why the weight factor affected the sex of cows' offspring.

"It's a good question ... all we can do is hypothesise."

One idea centred around the fact that an embryo travelling to a cow's uterus sent out signals announcing its sex, and that higher glucose levels in the uterus were more favourable for the survival of male embryos.

That meant a cow losing weight before conception may have some mechanism allowing it to reject a male embryo because it was not up to supporting a bull calf's development.

Another theory was that the cow's condition before conception affected which sperm would fertilise the egg.

Dr Roche emphasised it was not whether the cow was skinny or fat at conception that appeared to influence gender.

"It's whether they're gaining or losing weight prior to conception, or the degree to which they're gaining or losing weight."

Interestingly, the research showed the fatter a cow was at calving, the more likely the next calf would be female. Dr Roche said that was because a fatter cow was more likely to be losing weight before it conceived again.