In late August, a field day on the Francis Childs farm at Manchester, Iowa, turned out to be a meeting of the minds. Two of the sharpest, when it comes to corn production and nitrogen management, belong to Childs and to Fred Blackmer, Iowa State University professor of agronomy.
Blackmer, a guest speaker at the event, focused on ways to increase the profitability of nitrogen (N) fertilizer use while decreasing environmental problems. Childs, who holds the world record for corn yields at 442 bushels per acre, shared what he’s learned about managing N.
Eager to learn how to attain higher yields, the crowd of 200 or so farmers listened intently and asked plenty of questions.
How much fertilizer is really needed for corn production is always a timely topic, but especially last fall. N prices are rising, with prospects of even higher prices this year, and the threat of increasing regulatory action to protect the environment is ever present.
What’s The Right Rate?
Can you get higher yields without jeopardizing water quality? Both Blackmer and Childs think so. It starts with figuring out the right rate of N to apply, they say, and granted, that’s like aiming at a moving target.
Blackmer began the discussion by asking, “How much fertilizer N would you apply for corn following soybeans in a field where no manure had been applied in recent history and yields averaged 170 bushels per acre over the past five years?”
Everyone was asked to answer by marking on a form that listed N rates from zero to 270 pounds N per acre in 10-pound increments. The results were tabulated (see chart) and it’s fair to say there’s an awful lot of variability in N application rates.
The average rate the farmers selected (141 pounds of N per acre) was less than is usually recommended by using the “yield goal” method to try to figure out how much N to apply.
The most widely used recommendation in Iowa (yield goal times 1.2 minus 40 pound N per acre for soybean credit), for example, calls for 164 pounds N per acre in the field situation described in Blackmer’s survey question. In answering the survey, 80% of the farmers selected lower rates than called for by the recommendations.
These numbers illustrate that environmental problems associated with N fertilization probably will not be reduced by programs that encourage farmers to develop N management plans using the commonly accepted recommendations based on yield goals and credits.
It should be noted that a yield goal of 170 bushels per acre is realistic for many corn producers today. It was not a realistic yield goal when today’s commonly used recommendations were developed many years ago.
It is likely that the corn production practices and hybrids used today require less fertilizer N for each bushel than was required when the recommendations were developed years ago, says Blackmer.
Why So Much Variation?
It’s not possible to explain all the reasons why farmers select different rates of fertilization for a yield level of 170 bushels per acre. It should be noted, however, methods and times of fertilizer application tend to vary greatly among farmers, and optimal rates tend to vary greatly with time and method of N application.
Research by ISU agronomists over the past decade has shown that 100 pounds per acre of fertilizer N is usually about optimal if fertilizer N is applied after the crop is 6 inches tall. “The on-farm precision farming trials we’ve conducted are especially convincing,” says Blackmer.
Substantially higher rates are needed when fertilizer N is applied in fall or during winter. The higher application rates are needed to compensate for losses of fertilizer N that occur during spring rainfall events.
The old recommendations based on yield goals and credits do not acknowledge the great importance of time and method of N application on optimal rates. The application rates recommended are high enough to compensate for losses that occur with most fertilization practices today. These recommendations pose a barrier to improving N management practices; they suggest time and method of N application aren’t important.
“Corn producers and society as a whole do not benefit from N management guidelines that fail to acknowledge the important effects of time and method of N application on optimal rates needed to attain high yields of corn,” says Blackmer.
Effect Of Yield Level
Farmers at the field day asked Blackmer if Francis Childs’ N application rates are reasonable. They’re pretty high rates, but Childs is growing very high yields.
Do optimal rates of fertilization vary with yield level? “Obviously, a field and management system that produces more than 450 bushels per acre needs much more fertilizer N than is needed where only 150 bushels per acre are produced,” says Blackmer.
Treatments that exceed 450 pounds per acre are reasonable in studies to explore yield potentials under conditions where yields greater than 400 bushels per acre have been attained. Because factors other than rate of N application limit yields in most Iowa cornfields, however, merely increasing rates of N application usually can't increase yields.
Research in Iowa indicates time and method of fertilizer application has much greater effect on optimal rates of N fertilization than does average yield level in fields that yield less than 200 bushels per acre. The results suggest that many farmers could increase their yields and profits by paying more attention to how and when their N is applied.
Reprinted with permission of Farm Progress Companies