Scientists in New Zealand have been given the go-ahead to raise two herds of genetically-modified cows - but they cannot introduce any human genes into the animals.

Instead, the cows' own genetic material will be altered to change the content of their milk. Some of the cows will produce milk rich in casein, the main protein component of milk which can be made into a wide range of food and industrial products.

Other cattle will produce milk with lowered levels of beta lactoglobulin, which causes allergies in some people.

Ag Research, which will rear the animals at its Ruakura campus near Hamilton, had put forward a proposal for engineered cattle that would express a human protein in their milk which could be used to develop a new treatment for multiple sclerosis.

But the country's Environmental Risk Management Authority (ERMA) has deferred a decision on the request after receiving complaints from those such as the local Maori who have cultural objections. ERMA wants to take a closer look at the issues involved in introducing synthetic human genes into cattle.

Strict controls

The authority has also laid down strict controls for the experiments it has approved.

"The cattle will be kept in close containment and all the genetically-modified material, and the milk and waste products, will be kept on site or destroyed," said ERMA New Zealand Chief Executive, Dr Bas Walker. "This is a field test for a limited period of time, under strict conditions, so there is a negligible risk to the environment or public health."

The authority said the genetic modifications would involve 'knocking out' some cow genes and the 'doubling up' of others.

It will be several years before any commercial products are available.

ERMA has already given approval to the developers of Dolly the cloned sheep, PPL Therapeutics, to breed a flock of sheep in New Zealand that incorporate a human gene that will hopefully lead to a drug capable of treating the lung tissue disease cystic fibrosis.