The rolling landscapes of Kentucky lead to a complex flow of water over and in the soil, contributing to spatial variability in soil resources and crop yield. Plant-available N is very mobile in the soil and subject to leaching if in nitrate form. Cover crops can take up excess soil nitrate, storing it in their tissue and then releasing slowly as they decompose.
Using cover crops could be an efficient management practice to reduce N losses in landscape positions more subjected to intensive leaching. The soil water tends to move from the top and side of the hill to the bottom of the hill so that retaining N in loss-prone positions, cover crops may reduce spatial variability in the optimum N fertilizer rate for a cash crop.
We conducted an on-farm study over two years to examine the interactive effects of cover crop practices and landscape topography on yield and the profit-maximizing N rate for corn. Two separate field trials were established in Hardin County KY during the 2019 and 2020 corn growing seasons. The fields had been in a long-term no-till corn, soybean, and wheat rotation.
The dominant soil type in the study fields is Crider silt loam. Between March 1 and August 31, the fields received 28 and 27 inches of rain in 2019 and 2020, respectively. These rainfall totals were slightly above the 30-year average for this portion of the year (26 inches).
In mid-October of 2018 and 2019, we established three cover crop treatments: a cereal rye (Rye), a cereal rye/crimson clover mixture (Mix), and a winter fallow (Bare) as randomized strips throughout the field. Note that the winter fallow was not treated with herbicides in the fall, so winter weeds were pre-sent and produced biomass. We laid our plots in three contrasting landscape positions that included a hilltop (summit), hillside (backslope), and hill bottom (toeslope).
The average topographic and soil properties of each landscape position are presented in Table 1. Following cover crop termination in mid-April, four N rates were established, which ranged from 0-240 lb N/acre. Nitrogen was applied as a split application of 32% UAN, with 37 lb N/acre applied at planting as a 2X2 (i.e., 2 inches to the side of the seed, and 2 inches below the seed), and the remainder surface applied at the V5 stage.
The corn population were 31,000 plants/acre yield and the yield was determined by harvesting 92.5 ft2 using a 2-row plot combine and yield was expressed at a 15.5% moisture basis.
Table 1. Topographic and soil properties of three landscape positions used in the on-farm cover crop research study. Soil texture and soil organic C percentages were analyzed for the surface 8 inches of soil. Click Image to Enlarge
Averaged over both years and treatments, the cover crops produced approximately 1600 lb/acre of dry matter, which was (in most cases) nearly twice as much biomass as the winter weeds growing in the Bare treatment (Table 2). The Mix and Rye treatments produced similar amounts of biomass and had a similar concentration of N in its biomass. Across cover crop treatments, the toeslope position produced 40% greater cover crop biomass than the summit and backslope positions, averaged across winter cover treatments (Table 2).
Table 2. Biomass production of winter weeds and cover crops averaged across 2018-2019 and 2019-2020 seasons. Standard deviations are shown in parentheses. The average ratio of C concentration to N concentration in the biomass of the Mix and Rye treatments were 26:1 and 27:1, respectively. Click Image to Enlarge
Figure 1. Corn grain yield as affected by N rates (0, 80, 160, 240 lb N/acre) and three soil covers (Bare representing the winter fallow, Mix representing the mixture of Clover and Rye, and Rye repre-senting the Rye monoculture) across three landscape position (summit, backslope and toeslope). The yield data were averaged across 2019 and 2020. The capital letters represent the landscape ef-fect within each N rate and averaging soil cover while lowercase letters represent the N effects within landscape position averaging different soil covers. There was no effect of the cover within N rates and landscape positions on the grain yield. Error bars represent the standard errors. Click Image to Enlarge
Figure 1 shows the corn yield response to the landscape position across different N rates (0, 80, 160 and 240 lb N/acre) under three soil cover treatments (Bare, Mix and Rye). We observed that the toeslope had higher yields than the summit and backslope positions across all N rates and soil cover treatments (differences represented by the capital letters). At 0 lb N/acre the toeslope had 51% higher yield than the other landscape positions, regardless of the soil cover.
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For the other N rates, the increases in grain yield on the toeslope relative to other positions ranged from 24 to 27%. We did not observe any signifi-cant cover crop effect on grain yield when comparing the three different cover crops under same N rate at the same landscape position.
We calculated the difference in corn yield between the highest N treat-ment and the zero N treatment in each landscape position and cover crop treatment. This was similar in most cases – 60 bu/acre – suggesting that corn responded equally to N addition across cover crops and landscape.
We determined the economic optimum N rate (EONR) for each treatment assuming three different price scenarios: 0.10, 0.15 and 0.20 price ratio of N fertilizer price to corn grain price (that is, the price of N fertilizer is 0.51, 0.77 and 1.02 $/lb and the corn price is 5.10 $/bu). Increasing fertilizer prices led to a lower EONR when the price of corn was held constant.
The EONR increased in the order of summit < toeslope < backslope, but more site-years are needed to determine the consistency of this spatial pat-tern. The Mix and Rye treatments tended to increase the EONR in all positions relative to the Bare treat-ment. The highest net income considering the grain yield at the EONR and the price paid for the fertiliz-er N was generated on the toeslope (Table 4).
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Because this research was done in a limited number of site-years with a limited number of N rates, the EONRs should not be taken as N rate recommendations. However, our results suggest that the EONR can vary due to topography and that applying N at a uniform rate may lead to an excess of N at the summit positions. Corn yield as well as the net return was greater on the toeslope relative to upslope positions.
The use of a Rye or Mix cover crop did not significantly affect corn yield at any landscape position but increased the EONR at all landscape positions. Nevertheless, net returns were numerically highest with the Mix treatment on the toeslope in this study. Previous research suggests that corn yield may respond to changes in soil properties that take longer to manifest, so additional research into the long-term cover crop benefits is needed.
Acknowledgements
We gratefully acknowledge support from the Kentucky Corn Growers’ Association, the University of Kentucky College of Agriculture, Food, and Environment, the Sustainable Agriculture Research and Edu-cation Graduate Student Grant (#2018-38640-28417), and the United States Department of Agriculture National Institute of Food and Agriculture (NC1195 multi-State project and Grant # 2020-67013-30860).
We also thank Richard Preston, Laura Harris, Gene Hahn, Josh McGrath, James Dollarhide, Chad Lee, Dan Quinn, Osei Jordan, Ernesto Reboredo, Danielle Doering, and Katie Jacobs for their contributions to this project.
