CHICAGO (Reuters) - America’s two biggest independent seed sellers, Beck’s Hybrids and Stine Seed, told Reuters they are pushing U.S. environmental regulators to bar farmers from spraying dicamba weed killer during upcoming summers in a potential blow to Bayer AG’s Monsanto Co.

Limiting spraying of the chemical to the spring season, before crops are planted, would prevent farmers from using the herbicide on dicamba-resistant soybeans that Monsanto engineered. The seeds are sold by companies including Beck’s and Stine.

Last summer, after farmers planted Monsanto’s dicamba-resistant soy seeds en masse, the herbicide drifted onto nearby farms and damaged an estimated 3.6 million acres of non-resistant soybeans, or 4 percent of all U.S. plantings.