Cover crops potential to reduce manure N losses
With the increase in the extent of tile drainage and row crop production in recent years, nitrate-N levels in surface water and groundwater have been persistently level or trending up in agricultural areas of Minnesota. Manure is a major source of nutrients in Minnesota’s crop production, due to the large numbers of livestock in the state. Organic and ammonium forms of nitrogen in manure are converted to nitrate in the soil at rates determined by temperature and moisture. Precipitation can then move nitrate-N down through the soil profile, into tile lines and out to surface water, or directly into groundwater where there are no living roots to take it up. Manure application presents an additional problem, because the logistics of emptying manure pits and applying manure in the fall can result in application earlier than when the recommended soil temperature of 50 degrees F or lower is reached. Nitrification occurs rapidly above 50 degrees F, so nitrate-N becomes available for leaching by rain water in the fall or early spring.
The benefits of incorporating cover crops into diversified cropping systems are well documented, including reduced soil erosion, soil quality improvement, disease suppression, weed suppression, and reduced N-leaching. Research in Southwest Minnesota showed that cover crops have the potential for reducing N losses for cropping rotations wherein the primary crops are harvested before mid-September and planted after mid-May. The authors predicted that a winter rye crop can reduce drainage nitrate-N losses by 6.6 pounds of nitrogen per acre. In Iowa, another study concluded that combining manure application with crop rotation systems using cover crops can increase nutrient capture without lowering corn yield. Winter rye has been identified as a reliable cover crop in the north central region, and it is well known as a nitrogen scavenging cover crop that can protect the soil from erosion, and also provides organic matter. It remains to be determined if synergisms are sufficient to overcome available moisture for establishment, potential nutrient constraints for the following crop, and logistical issues related to harvest, manure application, and cover crop seeding in Minnesota.
Cover crops can trap nitrate-N before it leaves the soil profile and increase soil organic matter. Historically they have been little used in Minnesota because of problems with cover crop establishment after late fall corn grain harvest and yield depression of the following crop. Sufficient growing degree days usually remain after corn silage or soybean harvest to allow for cover crop establishment. Research on cover crops in Minnesota has recently been accelerated, but not in combination with manure application. Liquid manure can be injected directly into an established cover crop, which can then trap nitrate-N converted from ammonium or organic nitrogen. In the spring, cover crops are typically terminated by herbicide and left on the surface, where there are likely N losses to volatilization. If the cover crop is terminated and incorporated by tillage before corn planting, most of the N in the plant tissue may be available for the following corn crop.