The Washington State Dairy Federation is challenging rules meant to protect groundwater, saying it will delay fertilizing crops with manure.
Capital Press reports the state Department of Ecology rules impose statewide a formula that prohibits spreading manure until temperatures are above freezing for a prolonged period.
The dairy federation says the formula will work in western Washington's milder climate but prevent fertilizing in eastern Washington's colder climate until mid-March in some cases
The federation says that delay will deprive crops of nutrients without any benefit to water quality.
Dairies already must follow a separate manure-management law enforced by another state agency.
An Ecology spokeswoman says her agency used the best available science and broad input to develop clear, understandable rules.
Environmental groups separately have sued over the rules, alleging deficiencies.