Fonterra's move to limit the amount of palm kernel expeller (PKE) fed to dairy cows has come as a surprise, vets say.
The co-operative is encouraging farmers to limit the amount of PKE they use as a supplementary feed to a maximum of three kilograms per cow per day.
It says consumers pay a premium for grass-fed milk products, and it wants to increase that value by ensuring New Zealand milk comes from predominantly grass-fed cows.
The president of the dairy cattle branch of the New Zealand Veterinary Association, Jenny Weston, said the announcement probably came as a surprise to farmers as well.
Ms Weston said it would be difficult for those farmers who had come to rely on PKE as a significant part of their cows' diet at certain times of the year.
"If they are going to be asked to restrict that they are going to have to fill that hole in their feed budget with something else," she said.
"There's nothing readily available [to replace it with]. I guess there are other things like rapeseed, tapioca; farmers and contractors can grow other amounts of maize, silage and things like that.
"But nothing that's readily available and able to pick up the phone and order to have it delivered."
Ms Weston said it was unclear whether it would be possible to produce more maize for use as supplementary feed.
She said farmers would also have forward-ordered some supplements, and sourcing alternatives at this point would be difficult.
"People will find it difficult to change their system at short notice if that's what's required."