Use of lower rates of nitrogen can cut input costs without hurting yield.
Researchers in the Minnesota Department of Agriculture were perplexed. After interviewing 600 farmers in the state, they found that many were applying 30 lbs. more nitrogen than what the University of Minnesota recommended.
Most of these cases were farms where manure was applied in addition to commercial fertilizer. "On those farms, it was common to see overapplications of 30 to 70 lbs./acre," says Bruce Montgomery, unit supervisor of special projects with the department.
The observation meant one of two things, Montgomery says. Either the university recommendations needed to be changed or growers needed to be convinced to adopt current recommendations, which take into account proper manure and legume crediting.
"Many of the farmers were using recommendations dating back to the late '70s and early '80s," says Montgomery. "But many of the University of Minnesota recommendations have dropped anywhere from 30 to 60 lbs. over the past decade."
Field-scale test. To validate the current recommendations, the Min-nesota Department of Agriculture, University of Minnesota Extension Service, Minnesota River Educa-tional Initiative and Brown-Nicollet Clean Water Partnership decided to conduct a field-scale test. They chose Rob and Janice Meyer's farm near St. Peter, MN. The city is experiencing elevated nitrate levels in its public water supply.
The Meyers' field was an ideal test site because the tile lines are arranged in a grid pattern with no tile inlets, which enabled researchers to isolate the field and know that whatever came out the main tile came from that field.
"I think all farmers are stewards of the land," says Rob Meyer. "And if something is coming out through the lines I want to be the first to know because it is wasteful and costing us money."
For five years, starting in 1995, the Department of Agriculture monitored all of the nutrients and pesticides that were applied to one 60-acre field and then measured how much of the input was lost through the tile lines. Yield was also measured.
Varying rates and timing. The first year Meyer applied his normal rates of nitrogen on corn - 140 to 150 lbs. plus the nitrogen contained in the diammonium phosphate he applied. Tile line measurements after application showed nitrate concentrations as high as 20 to 30 ppm.
Following researchers' advice, Meyer lowered his rates to 110 to 120 lbs./acre the next time he planted corn. He also changed the timing of his application from fall to spring based on the university's Best Management Practices (BMPs) for Nitrogen Use.
Just by changing the rate and timing, they were able to reduce the nitrate levels found in the tile lines to 10 and 15 ppm, a 40 to 50% reduction, without reducing yield, according to David Pfarr, University of Minnesota extension agent.
"That to me is positive," says Pfarr. "It shows that we can get back to good old BMPs and university recommendations, and we can reduce the amount of nitrate coming out of our tile and thus into our ditches and rivers."
In 1997, on a small parcel of the same field, researchers applied four different rates of nitrogen - 60, 90, 130 and 160 lbs. - in strips and measured yield to find how different nitrogen rates affect yield. The optimum yields were found around the 90-lb. rate, which falls in line with university recommendations. How-ever, researchers say that because of a wide error variance in the data, more years of testing are needed before deciding on one optimum rate for the Meyers' field.
Herbicides also measured. Researchers also measured the amount of herbicides that came out of the tile lines. For each of the five years, different compounds were applied at their recommended rates. The compounds tested were trifluralin (Treflan), imazethapyr (Pur-suit), metolachlor (Dual and Dual II), nicosulfuron (Accent), dicamba (Banvel) and acetochlor (Surpass).
The two low-application-rate chem-icals, Accent and Pursuit, were barely detectable in the tile lines, according to Paul Wotzka, hydrologist with the Minnesota Department of Agri-culture. "The typical application rate for Accent is .03 lb./acre versus 2 lbs./acre for metolachlor," he says. "So there is 100 times less mass of the product going on the field to begin with. The fact that we can control weeds by putting that little of a compound on has been an eye opener for a lot of people."
Based on these findings, Wotzka recommends the use of low-use-rate chemicals like Accent and Pursuit over higher-rate alternatives. "Accent is a great compound from the standpoint that we hardly ever see it in the tile lines," he says. "And the farmer has been very pleased with the performance of that compound."
Nationwide application. Uni-versity of Minnesota's Pfarr says the study results have application for all Midwest farmers because of the low cost of nitrogen fertilizer and the tendency to bump up rates.
"I think it is important for farmers to continually challenge themselves on the rates," Pfarr says. "It is easy to slip to the upper end on the rates and feel a comfort level there."
The researchers hope to survey 50 to 60 more sites in southern Minne-sota using the same rates and protocol to further validate the findings.
Other states, such as Iowa and Indiana, have conducted similar studies.
Nitrogen and BMP recommendations are different for each state based on rainfall, soil types, topography and other factors. For more information on the current university recommendations in your state, contact your county extension agent.