BAYFIELD, Wis. -- The Natural Resources Board approved a groundwater limit for a herbicide byproduct because of health concerns.

The board voted 7-0 to limit alachlor-ESA to 20 parts per billion. Alachlor-ESA is a breakdown byproduct of alachlor, a corn and soybean herbicide manufactured by St. Louis-based Monsanto.

State tests in 2001 showed alachlor-ESA in 28 percent of Wisconsin's private wells. In creating the limit, the Department of Natural Resources cited studies showing the chemical can cause blood problems in rats.

The board adopted an identical limit last year, but Monsanto said it was too strict and Republicans who controlled the Legislature blocked it.

DNR officials said they hope Democrats, who now control the Senate, will ensure the new limit survives.