The high level of twin births in Irish cows, especially in Cos Monaghan and Louth, is being investigated by the Department of Agriculture, using DNA sampling.
In the developed world the average twinning rate is between 1 per cent and 1.8 per cent, but herds in certain parts of the State have been returning rates as high as 4.2 per cent.
Since the State introduced mandatory tagging of calves four years ago, the only route open to "legitimise" calves smuggled in from Northern Ireland has been to register them as the second twin.
According to veterinary sources, the practice was widespread all over the State until it was brought to an end by the foot-and-mouth crisis, which sealed the Border.
The monitoring of twinning levels began late last year using the Department's database, and areas of high twinning were identified by the district veterinary offices.
They found that in 1999 the twinning rate in herds in Co Louth was the highest. Out of 21,823 births, there were 908 twin births, giving a twinning rate of 4.16 per cent.
Second was Co Monaghan, which had 68,032 births, 2,557 of which were reported as twin calves, a rate of 3.76 per cent.
However, last year Monaghan's twinning rate jumped to 4.27 per cent, with 2,746 multiple births from 84,373 calvings, a figure described by one vet yesterday as "beyond belief".
Co Louth's cows did not appear to be as productive as they were in 1999 and returned only 783 twin births, dropping to 4.03 per cent.
A comparison of this rate with other counties in the State shows that Clare at 1.90 per cent, Sligo at 1.91 per cent and Mayo at 1.92 per cent had the lowest rates in the State.
Cavan had a twinning rate last year of 2.79 per cent, Dublin 3.08 per cent, Galway, 2.01 per cent and Limerick 2.56 per cent.
However, Meath at 3.44 per cent and Wicklow at 3.02 per cent, were among the highest in the State.
The Department said its investigations into the twinning rate, interrupted by the foot-and-mouth crisis, will be resumed in coming weeks, and DNA and blood samples will be taken from animals to determine the relationship between cows and calves.