A brown cow has been kneeling in front of Byambasuren's home on the Mongolian steppes for two days, its eyes blank and its head almost touching the ground. "It can't get up," said Byambasuren, a Mongolian herder with close-cropped hair and ruddy-cheeks.

"It will die any day now." When it does, the skinny animal will be dragged to a nearby pile of carcasses of livestock which have died from hunger in the harshest winter Mongolia has experienced for 30 years. Before the first blizzards came in September, the family of seven adults and 13 children had 600 head of livestock, enough to meet their needs. Now they have only 200 sheep and goats and a couple of cows and they too may all be doomed. "We have no hope for them," said Byambasuren' wife, who bears the same name as her husband. "We have already driven them 50 kilometres to find better pastures and they are too exhausted to go any further."

They had erected their four round canvas tents known as gers, the traditional dwellings of Mongolian nomads, about 250 kilometres south-west of the capital, Ulaan Baatar. This is one of the areas worse affected by what Mongolians call the zud, a winter which brings disaster. There are different kinds of zud, a white zud when snow is too deep for livestock to reach the grass, an iron zud when impenetrable ice cover forms on the surface, and a black zud when drought leaves a shortage of frozen water supplies.

Today Mongolian herders are suffering the effects of multiple zud, the greatest disaster that can befall them, caused by the worst drought in memory last summer, followed by a record 24 heavy snowfalls and bitter cold with temperatures dropping to minus 46C. It was so cold that in some places the tails of cows froze solid and then simply fell off. Its effects can be seen driving across the yellow steppes, where the sun has burned away most of the winter snows.

Everywhere lie dead cows, horses, ponies, sheep, goats and camels. Some had clearly perished just the previous day, when a raging wind came tearing down from the north. Most died in the last two months, and their bones had been picked clean or they had been skinned for wool or cashmere.

Others were savaged by wolves. "This winter is a holiday for wolves, with the livestock too weak to run," said my guide, Enkhe. From a distance the steppes look desolate, but the vast yellow and grey valleys and jagged rocky outcrops are teeming with life which have thrived in the zud. Marmots and their smaller cousins, the pikkas and field mice, scamper around everywhere. They have appeared in plague numbers this year, contributing to a disastrous overgrazing of the already withered pastures. Pipits and larks flit around, and hawks and golden eagles glide low overhead, feasting on the mice. But for the livestock there is nothing only short, dry stubble.

"I am 28 and I have never experienced anything like this," said Byambasuren (the wife). "The first big snow came in September, a month early. It froze so hard that the livestock could not graze and we ran out of fodder. Many families around here have lost all their animals and now they just help each other find lost horses and cattle." She spoke philosophically about nature's cruelty. "It's not people that have died, it's animals," she said.

But their turn may come. "Without our livestock we have nothing. It is the only resource we have. I am very afraid our life as a nomadic people is in danger." Losses occur every winter in Mongolia but not on the scale of the past few months. From 33.5 million livestock registered in 1999, at least two million have died, affecting half a million people. The figure could rise to five million by the end of May. April and May are the harshest time for herders, when there is no grass, and snowfalls alternate with high winds and dust storms, and animals are giving birth.

A United Nations Disaster Management Team reported last month that widespread malnutrition and even mass starvation will occur among Mongolian herders if food aid is not made available from this month onwards, and that mass deaths of livestock will continue to early June.

The Mongolian government is providing emergency assistance from its small reserve of animal fodder, the World Bank has pledged $1.3 million and the Red Cross has made an emergency appeal for $2.5 million. The Byambasuren family and other nomads in the steppes here on the fringe of the Gobi desert have heard nothing of this. The zud has demoralised the usually good-humoured nomads.

Custom requires that households supply food freely to all visitors - all they expect in return are news and good stories - but now they are ashamed at having next-to-nothing. Depression has become a critical ailment.

"Herders and their families are emotionally attached to their livestock," said the UN report. "Deaths of their animals from starvation affect them almost as deeply as the death of human friends." The stories they tell now are of epic tragedies. One boy drove 100 horses 500 kilometres to the north but it did not keep them alive. He returned with only his riding horse.

Young men today rove the steppes looking for lost animals. "Many of them come here at night for shelter and I can't turn them away," said a nomad called Tanchiv, against a background of loud bleating from his 700 sheep and goats which he drove from a worse affected area to within 150 kilometres of the capital, angering local herders by denuding the winter pastures. "I have used up 75 kilograms of meat in nine days."

The loss of horses and camels mean a loss of transportation. People are no longer able to go for medical help to the little hospital at administrative centres like that called Ondorshireet, a collection of concrete buildings 50 kilometres from the nearest paved road in southern Mongolia. There the schools are empty as most children are too malnourished or cannot travel. A crisis looms here as bereft herders come to camp nearby in the hope of medical help and food. Food shortages are also filtering through to the poor in Ulaan Baatar as prices inevitably rise. In Mongolia nomadic families with less than 100 animals are considered to be living in poverty.

The government says 54 per cent had less than 100 livestock before the zud. They all face being wiped out. The loss of larger animals means there will be no cow or horse dung for heating next year, the sole source of fuel. The bigger animals die first because they cannot eat the stubble that sheep and goats can feed on. Culturally, Mongolians cannot passively watch their animals die and give precious resources to keep the weaker ones alive, when helping the strongest survive might be more prudent. Herders are now using their own food supplies in a desperate effort to keep them alive.

Byambasuren has 100 lambs enclosed in one ger, and the family is giving them precious milk, sugar and flour to keep them alive. But they may have too little to prevent them joining the piles of carcasses nearby, unless aid in the form of fodder arrives soon.