Stud merino breeders have rejected the need to enshrine sheep welfare in law.

Sheep producers are being asked to comply with new welfare standards, and there are guidelines to help them.

They cover mulesing of lambs, handling of sheep and how to kill animals humanely.

But in what will be a blow to Animal Health Australia, which has drawn up the new rules, the Association of Stud Merino Breeders won't agree to back them.

Vice-president of the association, Jock MacRae, says "once these standards are enshrined in legislation, there will be no turning back".

The new standards, up for public consultation until August 5, include pain relief for mulesing and castration of lambs older than six months, and sheep to be shorn before wool gets longer than 250 millimetres.

But Mr MacRae says that, on the mulesing issue, in some years sheep producers might review whether to surgically mules an unmulesed mob of sheep, to protect them from flystrike.

"You might have a situation where somebody thinks their breeding strategy is going quite well, and determined not to mules this particular year, then 18 months down the track they have a welfare crisis.

"If this (new rules) did move forward, we have a situation where it would inhibit it."

Mr MacRae says the sunset clause, that over 12 months of age, no mulesing of sheep is allowed, would prohibit him from addressing a welfare crisis down the track.

Mr MacRae says sheep can be lost for several shearing musters and their wool can grow long.

"That sort of thing can easily happen in pastoral zones. If you have a fleece longer than 250 mm, theoretically that man couldn't shear them, because they will be breaking the law."