Scientists have created a genetically modified (GM) cow that produces milk with low levels of a protein known to cause allergic reactions in a significant proportion of children. The researchers believe it could one day lead to the sale of "hypoallergenic" milk from herds of GM cows.

The calf had been cloned and genetically engineered with an extra piece of genetic material that switched off its natural gene for producing a milk protein called beta-lactoglobulin, which is not present in human milk and causes allergies in some young children.

Tests on the cow's milk showed that it contained less than 2 per cent of normal levels of beta-lactoglobulin and was far richer than usual in other kinds of milk proteins, such as the caseins used in cheese-making. The researchers also believe the GM cow's milk will also contain higher concentrations of calcium than ordinary milk.

The cow, however, was born without a tail which is a rare congenital abnormality. The scientists believe this was a result of the cloning process, similar to that used to create Dolly the cloned sheep, rather than the GM technique used to eliminate the milk protein.

The dairy industry produces hypoallergenic milk formulas by removing certain bovine proteins with the help of digestive enzymes but the industrial-scale processing is expensive, causes the milk to taste bitter and does not always remove the offending allergens, the scientists said.

In developed countries, between 2 per cent and 3 per cent of infants are allergic to the proteins found in cows' milk so there is a demand to find ways of making milk that is safer for them, the researchers said.

A person who is allergic to milk proteins can suffer a range of symptoms, which can occur within minutes of drinking milk or some hours later. They include vomiting and gastrointestinal upsets, skin rashes and breathing difficulties.

A team, led by Goetz Laible from the University of Waikato in Hamilton, New Zealand, used a revolutionary technique called RNA interference (RNAi) for "knocking out" the cow's gene for beta-lactoglobulin. The RNAi technique uses a natural method for switching off genes without the need to generate DNA mutations within the genes.

The study, published in the journal Proceedings of the National Academy of Sciences, is one of the first examples of the RNAi technique being used to create farm livestock with novel traits. Other scientists are working on ways of using RNAi to create new strains of domestic animals that have a natural immunity to viruses and infections.

Bruce Whitelaw, Professor of animal biotechnology at the University of Edinburgh, who was not involved in the research, said the study demonstrates the power of the RNAi technique. But he added: "Whether this is commercially viable depends on how it would compare against other methods. RNAi has a long history of successful application in diverse species from plants to worms. This is the first report for livestock… Time will tell how widely applicable RNAi will be in GM livestock.