Integrating liquid manure and cover crops to measure their impact on soil health, nutrient cycling, and agronomic performance in the upper Midwest

On their own, cover crops and manure have been shown to have a positive impact on various soil biological, chemical, and physical parameters related to soil health. However, there is limited information about the integration of both practices on soil health in addition to nutrient cycling and agronomic performance. This is especially the case for the agronomically important region of the upper Midwest. A two year study was set up to measure the effects integrating cover crops and liquid injected manure have on various soil health parameters, nutrient cycling, and corn (Zea mays L.) yield. The study was conducted at the University of Minnesota Southern and West Central Research and Outreach Centers located near Waseca and Morris, MN, respectively. Three agronomic production systems were used: sweet corn-grain corn (System 1); silage corn-silage corn (System 2); and soybean (Glycine max [L.] Merr.)-grain corn (System 3). Plots were laid out in a randomized complete block design with split plots. Nutrient source/timing served as the main plot and cover crop type and planting time/method served as the split plots. Cover crops were no-till drilled after sweet corn harvest in System 1, whereas cover crops were interseeded into standing row-crops or drilled after harvest in Systems 2 and 3. Cover crops were given time to grow before applying manure. Liquid manure was sweep-injected when soils were above or below 10 °C for Systems 1 and 2 and applied only when soils were at or below 10 °C in System 3. Plots with no cover crop and no manure but received urea at the same nitrogen rate as manure in the spring prior to corn planting (SpringN) served as the control. Cover crop and corn were sampled to measure yield and their tissue was analyzed for nutrient uptake. Soils were sampled throughout the cover crop and corn growing season to measure soil mineral nitrogen across various soil depths and various soil health indicators. Applying manure when soils were above 10 °C reduced corn yield by 7 to 22% compared to SpringN but applying manure when soils were at or below 10 °C was as good or better than SpringN in System 1. In Systems 2, manure, regardless of application time, was more effective than SpringN regarding corn yield. Corn yield was not statistically different between manure and SpringN in System 3. However, there is a heightened risk of losses nitrogen from manure through leaching when applied too early as was seen in Systems 1 and 2. Therefore, it is recommended producers follow the University of Minnesota Extension guidelines to apply manure when soils are at or below 10 °C in order to inhibit nitrogen losses. The integration of cover crops and liquid injected manure did not have an impact on the various soil health indicators measured. This may stem from the short-duration of the study. Furthermore, cover crops had a hard time establishing and producing adequate biomass to have a significant impact on soil health. Long-term studies are needed to assess the true potential cover crops and liquid manure have on soil health in order to promote agricultural practices that are capable of growing the food, feed, fuel, and fiber for a growing population in a sustainable manner.