The Forage Aid charity is calling for pledges of livestock feed to be made via its website to help flood-hit farmers in north-west England.
Several farmers had been severely affected by Storm Desmond in Cumbria, with confirmed reports of livestock fatalities, said Forage Aid spokesman Ed Ford.
“At this time, we would urge people wishing to donate forage or haulage to use our online pledge system available on the Forage Aid website,” he said.
“We discourage farmers from simply turning up to affected areas with forage.
“If you are unable to donate forage or haulage but still would like to help you can donate funds through our Just Giving page,” he added.
The Addington Fund, which is focusing support on farming families whose businesses have been affected by the floods, said it was aware of one farm where 300 sheep had drowned
“At the moment, farmers are concentrating on the immediate needs of livestock – either moving sheep onto fresh grounds or dealing with flooded cattle sheds,” it said in a statement.
“Experience tells us that it will be days – if not weeks – before individual farmers are able to assess the full effect of the floods, and it could be next spring when help is needed.
“Once the floods have subsided, we welcome discussions with farmers who have been affected by this devastating weather to seen how Addington can best help.”
The NFU said access to livestock that required fodder could be limited with farm roads blocked and some bridges destroyed by rising water.
Dairy farmers had found milking particularly difficult with flooded parlours and no electricity to power equipment, it said.
Milk collections had not taken place on many farms as tankers were unable to reach farms in both counties, the NFU added.
NFU Cumbria adviser Jenny Willis said some farmers were struggling to get on to their land to see the extent of the damage because minor bridges had been washed away or were unsafe.
“There is a huge amount of debris to clear – lots of gravel, stone and rocks that have been washed down from the hills.”
She added: “We have had so many offers of help from farmers across England and Wales and the RSPCA, which has been helping feed and move livestock.”
Defra secretary Liz Truss said flood defences had protected 8,600 homes across the north of England, although the Environment Agency said 5,200 homes had been affected by floods.
But critics have already voiced concern that the government’s housebuilding policy on flood plains is exacerbating the situation.
Susan Aglionby, who farms on the northern edge of Carlisle, said repeated floods in the area underlined the need to look again at development.
Ms Aglionby said hundreds of houses were being built close to her farm – all with hardstanding parking areas for cars which raised the risk of flooding.
“People have to be very brave and make all sorts of decisions about planning,” she said. “We have to stop putting car parks down and making solid drives.”
Offers of financial help have been flooding in to the office of the Cumbria Community Foundation, with more than £250,000 given to the £1m disaster appeal.
Foundation chief executive Andy Beeforth said: “We are still in the eye of the storm, but we know that it appears the flooding is worse than 2009 in some areas of the county.
“We will be convening meetings with the major voluntary groups involved in the recovery and also with major providers of social housing to assess the effect on their tenants.”
Liberal Democrat leader and South Lakeland MP Tim Farron, who himself was caught up in the floods at the weekend, called on the government to provide additional funds to Cumbria.
“It is essential that the government responds to this crisis in a fair and forward-thinking way, by making available the funds needed to repair the damage.”