The Moorland Association says the centuries-old tradition of grouse shooting is at risk from an EU spinach ruling.
The shooting season starts on Monday and runs through to December and includes moorland in the Upper Calder Valley.
But, the association says a ban on the only effective bracken control herbicide - Asulam - will lead to the devastation of wildlife and destroy land management.
Asulam was banned by a EU committee on the food chain and animal health following safety concerns over aerial spraying by spinach growers in continential countries.
The EU ruling last December means this summer is the last Asulam can be sprayed on land and the association warns vast tracks of countryside will be vulnerable to massive bracken infestation.
Association vice-chairman George Winn-Darley, said the government-approved chemical had been used successfully and safely for 35 years.
“This will not only adversely affect biodiversity, including red list endangered species, but radically impact on grouse management. Three quarters of the world’s heather moorland is found in the UK. Without Asulam, we would have already lost 50 per cent of it.”
Game cannot be shot on Sunday, which means the glorious twelfth gives way to the thirteenth this year.
Kevin Jaundrell is head keeper on the 12,000 acre Walshaw and Lancashire Moor which includes Calderdale. He said several species including grouse would suffer from the encroachment of bracken. He said; “Bracken can spread at a vast rate. I can understand concern about Asulam going into the food chain but it wouldn’t when used for the control of bracken in the uplands.” Campaign groups are now trying to find a way forward to allow Asulam products to be available in the future and are expected to apply for re-registration.