Corn after a cover crop: Dialing in nitrogen requirements
It can be very hard to sometimes predict what the effect of the cover crop will be on nitrogen requirements for the next corn crop,” said Charlie White, a Penn State associate professor and Extension specialist in soil fertility and nutrient management.
Pennsylvania corn growers facing this conundrum can use a data-driven decision support tool developed by White and his colleagues to provide nitrogen recommendations. It’s designed to take into account the variables cover crops create, like carbon-to-nitrogen (C:N) ratios from specific cover crops, that previous models overlooked. Traditional N recommendations, like 1 lb. N/bushel of corn, do not hold up well in modern no-till and cover crop systems, according to White.
During his Northeast Cover Crops Council presentation, White used an on-farm trial to illustrate how the tool works. The trial was conducted in Manheim, Lancaster Co., PA, by doctoral student Andrew Lefever. In 2023, this farm planted corn into a cereal rye cover crop which was killed at the tillering growth stage. Post-termination, liquid swine manure was injected at 5,000 gallons/acre.
To obtain N recommendations for this field, White entered a series of inputs into the tool: average corn yield goal (bushels/acre); clay, sand and soil C percentages; soil C:N ratio; winter-killed cover crop biomass N (lbs./acre); spring cover crop biomass N (lbs./acre); and spring cover crop C:N ratio.
White’s primary method for quantifying cover crop biomass N is to use a handheld GreenSeeker sensor to measure normalized difference vegetation index (NDVI) values by walking over the cover crop canopy. The NDVI measurement is converted to pounds of N per acre using a calibration developed from hundreds of different cover crop types.
Researchers are developing tools that use satellite or drone NDVI or normalized difference red edge (NDRE) metrics to predict cover crop N content. This tool is currently in development but is expected to be released within a year.
Visual estimation is also a potential method that “could be adequate in some cases,” White said.
The C:N ration is determined using a table based on historical databases rather than laboratory measurements.
The last inputs entered into the decision support tool are the price of corn (bushels/ acre) and cost of N (dollars/lb.).
The tool suggested an economic optimum N rate of 146 lbs./acre. After accounting for the 113-lb. manure credit from the swine manure and 35 lbs. of starter fertilizer that the farmer wanted to use, the calculation showed a negative balance of -2 lbs./acre. This means the calculator indicated that no additional sidedress N was needed.
Historically, the grower’s standard sidedress rate was 85 lbs. N/acre, but since the tool recommended zero additional sidedressing, the grower did not sidedress. Despite this change in management, the grower exceeded their yield goal of 225 bu./acre, actually achieving 229.
The recommendations that sidedressing was unnecessary were also confirmed using a pre-sidedress nitrate test (PSNT). PSNT is an in-season soil nitrate test used to determine if additional fertilizer N is needed for corn, conducted on soil samples collected just prior to sidedressing. Like White’s decision support tool, the test has been updated to account for cover cropping and no-till systems.
Overall, these on-farm validation trials showed that using the new tool and the updated PSNT could lead to an average reduction of 48 lbs. N/acre, saving farmers approximately $43/acre. These figures are based on a $0.75/lb. of N cost and a $4.50/bu. corn price.
“Nitrogen and corn prices are constantly fluctuating, but prior to development of this tool, these costs were not incorporated into recommendations about how much to fertilize in Pennsylvania. This tool helps growers to see the economics and realize when they can save money,” said White.
White stressed that the decision support tool is currently calibrated only for Pennsylvania. He suggested that growers use Penn State’s Agricultural Analytical Laboratory for soil tests. The lab uses elemental combustion analysis, a method for measuring total C and total N content in soil samples, which is essential for calculating the soil C:N ratios used in the recommendation tool.
If a grower wishes to use a different lab they should confirm that it uses the elemental combustion analysis as there is significant variability in how different commercial and university labs measure and report soil organic matter and C content.
The N decision support tool can be found at extension.psu.edu/nitrogen-recommendations-for-corn.
by Sonja Heyck-Merlin
