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Soil Nitrate at Harvest of Forage Winter Cereals is Related to Yield and Nitrogen Application at Green-up

Sarah E. Lyonsa, Quirine M. Ketteringsa, Greg Godwina, Karl J. Czymmeka,b, Sheryl N. Swinka, and Tom Kilcera,c
a Nutrient Management Spear Program, Department of Animal Science, Cornell University, Ithaca, NY, b PRODAIRY, Dept. of Animal Science, Cornell University, Ithaca, NY, cAdvanced Agricultural Systems, LLC, Kinderhook, NY

Introduction

Double cropping with winter cereals provides many benefits to forage rotations, including soil erosion control, addition of soil organic matter, nutrient recycling, and boosting home-grown forage inventory. Previous work on winter cereal cover crops (green manure, not harvested for forage) in New York suggested that, in most cases, 20-30 lbs nitrogen (N)/acre can be credited to the next crop (see Ort et al., 2013). When the winter cereal is harvested for forage instead of terminated as a cover crop, about 50 lbs N/acre is removed for every 1 ton DM/acre harvested, but some N remains in the crop stubble and roots. For a winter cereal yielding about 2 tons DM/acre there will still be an estimated 0.7 tons DM/acre in the root and stubble biomass (see Long et al., 2013), meaning about 20-30 lbs N/acre could become available over the growing season as the residue decomposes. Due to the time it takes to decompose this biomass, residual N from winter cereal roots and stubble is not likely sufficient to offset starter N needs if the next planting occurs shortly after the winter cereal is harvested. However, when fertilizer N is applied to winter cereals at dormancy break, there is usually some nitrate remaining in the soil as it is likely that not all of the N applied is taken up by the winter cereal. In some situations, the residual nitrate from a spring topdress fertilizer application may meet early season corn needs and substitute for a starter N application. This summary aims to address how to credit N to the next corn crop from fertilized winter cereals harvested as forage.

Field Research

As part of a state-wide study investigating spring N needs of winter cereal forages, 63 on-farm trials were conducted in New York from 2013-2016. Trials had five rates of N (0, 30, 60, 90, and 120 lbs N/acre) applied to farmer-managed forage triticale, cereal rye, or winter wheat at green-up in the spring to determine the most economic rate of N (MERN). The forages were harvested at flag-leaf stage in May each year. For seven of these trials in central New York (in 2015 and 2016; two cereal rye, five triticale) we took soil samples (0-8 inches) at green-up and at harvest and analyzed them for soil NO3-N (KCl extractable NO3-N). While nitrate exists in the soil as NO3,  we measure it in the lab as nitrate-N, or NO3-N, which represents the total amount of N in the nitrate molecules, not the total nitrate in the sample. Planting dates for the seven trials ranged from September 15 to October 10, and harvest dates ranged from May 11 to 21. All seven sites had a manure history and were in corn prior to planting of the winter cereal. Winter cereals were drilled at seeding rates ranging from 100 to 125 lbs/acre. Here we report on the NO3-N levels in the soil at harvest of the winter cereal as an indicator of spring starter N needs for corn.

Results

Figure 1. Nitrate-N at harvest of two cereal rye trials and five triticale trials in New York (2015-2016). Nitrogen fertilizer at the indicated rates was applied at dormancy break. Different letters indicate significant differences between N rate treatments.

Across all seven trials, soil NO3-N following harvest increased with fertilizer N applications at green-up (Figure 1 and Table 1). On average, NO3-N levels at harvest were 12, 13, 18, 27, and 40 lbs NO3-N/acre for the 0, 30, 60, 90, and 120 lbs N/acre treatments, respectively. For the two trials that had yields of 1 ton DM/acre or less (#58 and #62), left-over soil NO3-N was considerably higher, especially when N was applied above the MERN (Table 1). The trial that had the highest yield at 3.1 tons DM/acre (#63) had the lowest residual soil NO3-N at harvest for all N rates even though the MERN for this site was 0 lbs N/acre (crude protein increased with N addition). These data suggest that residual soil NO3-N is related to yield; the higher the yield, the lower the left-over soil NO3-N at harvest. Applying N at rates above the MERN can also result in elevated soil nitrate concentrations.

Conclusions and Implications

Nitrate as represented by NO3-N in the soil at harvest ranged (averaged across sites) from 12 lbs NO3-N/acre for the 0 lbs N/acre treatment to 40 lbs NO3-N/acre for the 120 lbs N/acre treatment. Based on the results here, when N applications are close to the MERN for the site, soil NO3-N at harvest is approximately 10 lbs NO3-N/acre, the level typically measured at the start of the growing season at sites without winter cereals in the rotation. Left-over NO3-N in the soil is related to yield of the winter cereal; higher yield means lower residual soil NO3-N at harvest, while low yield (less than 1 ton DM/acre) can result in high soil NO3-N levels. The basic recommendation for corn is to apply a small amount of starter fertilizer at planting (20-30 lbs N/acre). This is under the assumption that there is not a significant amount of NO3-N available from other sources at that time. This winter cereal research suggests that there can be a sufficient amount of soil nitrate at harvest that could be available to the subsequent corn crop, especially when winter cereals are fertilized at green-up at levels that exceed crop needs and/or where yields are depressed for other reasons. The most direct way of determining soil nitrate at winter cereal harvest is to soil sample for NO3-N. Fields that will be planted to corn where soil nitrate levels exceed 20 lbs NO3-N/acre following a winter cereal forage harvest may not respond to starter N. Test strips can help improve these decisions over time.

References

Acknowledgements

This work was supported by Federal Formula Funds, and grants from the Northern New York Agricultural Development Program (NNYADP), the USDA-NRCS, and Northeast Sustainable Agriculture Research and Education (NESARE). We would also like to thank participatory farmers and farm advisors for assisting with the trials, including Cornell Cooperative Extension educators, consultants, NRCS staff, and SWCD staff. For questions about these results, contact Quirine M. Ketterings at 607-255-3061 or qmk2@cornell.edu, and/or visit the Cornell Nutrient Management Spear Program website at: http://nmsp.cals.cornell.edu/

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