In response to the demand for smaller cuts of meat, several big processors have dropped the maximum deadweight carcase from 415kg to 400kg, with some going down to 380kg.
Charles Sercombe, NFU livestock board chairman, said the changes had started to have a ’massive’ effect on farmers.
"Companies have been wanting more finish and better confirmation, the consequence of this is carcases are heavier," he said.
"Farmers are really struggling to get the right conditions. They have been given three months’ notice, but that is not enough.
"I would be very surprised if there are not more changes before the end of the year."
Chris Dodds, executive secretary of the Livestock Auctioneers Association, was also concerned about the pace of the changes.
"In the last 15 years, I have seen one abattoir change their specifications half a dozen times. It does not give the producer time.
"What chance does a producer have if they are always changing the specs?"
Bigger breeds, such as Charolais, will be particularly affected. But David Benson, chief executive of the Charolais society, said some producers were feeling positive about the changes.
"It is good for us. It is going back to how they were raised traditionally," said Mr Benson.
"Calves are weaned from their mothers and go straight into finishers.
"There is a advantage with heifers. They can get to 380kg without being overfat."
More than 40 per cent of British steers slaughtered in February had carcase weights above 380kg, up from 36 per cent a year before.
Heavier carcases are concentrated in the E, U+ and –U classes, which often attract price bonuses, particularly for heifers and young bulls. The news of the reduction comes after Farmers Guardian reported an AHDB report suggested these could be offset by heavy weight penalties in future.
Chris Mallon, chief executive of the National Beef Association, said the changes were an attempt to limit the top value of these cows.
"They say it is about size of cuts but there are a lot of modern practices which mean cuts from larger cattle are still saleable," he added.
"I was at an abattoir two weeks ago with modern cutting principles. They are actually going into the same market."