Low-till farming and other conservation measures designed to reduce erosion and pollution were not consistently detected in 133 watersheds across the country, a new USGS study shows.

The U.S. Geological Survey analysis raises doubts about the effectiveness of the Conservation Reserve program, where the government has paid billions of dollars to farmers over the years to replace crops with native grasses. But the Boise-based hydrologist who led the study said changes in water quality may lag behind changes on the land.

"The effects of conservation practices are not yet consistently detectable at a large watershed scale," said Lori Sprague, USGS hydrologist in Boise. "Current nutrient conditions in streams may still be reflecting agricultural practices that were in place prior to the implementation of the conservation practices."

Nitrogen from agricultural land moves slowly to streams through groundwater, so it can take several years for reductions in nitrogen inputs on the land surface to affect nitrogen levels in streams, researchers said. But low-till farming, which limits plowing while retaining crop residue on the surface may not capture as much of nitrogen from fertilizer in the soil.

The study is important now because no-till or low-till farming is used on 25 percent of the cropland in the country. Another 8 percent is enrolled in the Conservation Reserve Program.

In reauthorization of the Farm Bill, Congress is reconsidering how much land to place in the Conservation Reserve Program, which also provides wildlife habitat and other benefits. This environmentally sensitive farmland is restored to filter strips, grassed waterways, riparian buffers, and long-term vegetative covers, such as introduced or native grasses.

If changes in nutrient loss from agricultural watersheds do lag implementation of conservation practices, nutrient levels in streams may be reduced in the years beyond the scope of this study, researchers said. That includes USGS data from 1993 to 2001 paired with conservation data from that time period that has only recently become available.

This study was supported by the USGS National Water-Quality Assessment Program.