Cover crop soil nitrate-N conservation after seed corn
click image to zoomFigure 1. High clearance male row destroyer with cover crop seeder. In 2010, University of Nebraska began evaluating the planting of a cover crop to manage nitrate-nitrogen in the soil following seed corn production. The goal was to use cover crops to utilize the nitrogen during the offseason to minimize potential leaching.
In 2010, 2011, and 2012 we set up a replicated study where we planted different cover crop mixtures in a seed corn field in Merrick County. One goal was to select cover crop species which would winter kill, in order to eliminate the cost of herbicide or tillage in the spring to destroy the cover crop. Cover crop species also were selected with fall cattle grazing in mind. Cover crop seed was broadcast over plots at the time of male row destruction in late July, using a Herd orbital spreader (Figure 1).
- No cover crop
- Turnips, radishes, rapeseed, spring barley, oats
- Turnips, radishes, rapeseed, spring barley, rye.
Cost for cover crop seed ranged from $5.25 per acre for the turnips to $24.11 per acre for the blends. With input costs in mind an additional treatment of turnip/radish was added in 2011 at a cost of $7.65 per acre. Male row destruction and seeding took place August 9, 2010 and Aug. 11, 2011.
click image to zoomFigure 3. Turnip cover crop, November 2010. Figures 2-4, taken November 10 before the first frost, show 1) no cover crop, 2) turnips only, and 3) a cover crop blend, respectively. We observed every year that rye and oats had reduced emergence within their respective blends. Broadcast seeding resulted in better stands of turnips, radishes, rapeseed, and spring barley.
click image to zoomFigure 4. Cover crop blend of turnips, radishes, rapeseed, spring barley, and rye, November 2010. Cover crop species or blends varied somewhat in 2011 and 2012, but treatments with and without a cover crop were standard in all years. Fields were grazed by cattle beginning in the fall and through the winter. By mid-spring each year, cover crop biomass had disappeared, either from grazing or winterkill. Consequently, soil residual nitrate-N conserved by cover crop uptake in the fall was returned to the soil in manure or urine, or in decomposing cover crop biomass, through the winter and into the spring.
Every year we collected soil samples to a depth of three feet after cover crop emergence in the fall and continuing into spring until after soybean planting. Figures 5-7 illustrate trends in soil nitrate-N as influenced by cover crops. Overall residual nitrate-N levels were much lower in fall 2010 than in 2011 or 2012. Still, there was a significant effect of cover crop presence on soil nitrate-N. Overall nitrate-N levels reflect both management of the preceding seed corn crop and weather effects (temperature, moisture) on N cycling in soil. For example, the steep increase in residual nitrate-N levels for all treatments in the spring of 2012 reflects ideal conditions for N mineralization. In all years, we see an increase in soil residual nitrate-N between November and late April or May the following year where cover crops were planted. This reflects primarily the accumulation of N deposited by cattle or decomposition of cover crop biomass. There also may have been some accumulation of nitrate-N resulting from mineralization of soil organic matter, though in 2011-2012 and 2012-2013 the no-cover treatment did not increase over the winter, and in 2010-2011 the no-cover treatment increased only slightly, suggesting that mineralization and loss of N was about equal over winter.
click image to zoomFigure 6. Cover crop effects on soil residual nitrate-N fall 2011–spring 2012. T=turnip, R=radish; RS=rapeseed; SB=spring barley. Comparing results from soil samples over years, we can definitely see the value of having a cover crop present. In all cover crop treatments, soil nitrate-N declined significantly in the fall compared to the no-cover crop treatment. In fall 2012 grazing varieties of turnip and radish were compared to ordinary varieties. Grazing varieties tend to produce more vegetative matter and smaller bulbs. However, there did not appear to be any difference in how these varieties influenced soil residual nitrate-N.
The use of a winter cover crop following seed corn production appears to be a good practice to minimize the risk of nitrate leaching over winter. In addition, cover crops provide an attractive grazing resource for the fall. From our studies there is no evident advantage to cover crop blends over a single species. The use of turnips alone, or turnips combined with radish, significantly reduced soil nitrate-N levels in the fall, and at much lower cost than more expensive blends including rapeseed or spring barley.