#18-134 IPPA


Date Full Report Received


Date Abstract Report Received



Primary Investigator:

Agricultural nutrient management practices are an important component of on farm efforts to improve profitability and reduce environmental impacts. Incorporating cover crops into existing crop rotations is one way to reduce nutrient losses from the field. Previous research has shown that cover crops are generally effective for reducing nitrate-N losses from subsurface drainage systems. Using cover crops together with manure has the potential to provide synergetic benefits to soil and cropping systems, as shown in Figure 1.

Figure 1. Schematic showing how manure and cover crops can be mutually beneficial in cropping systems.

However, questions remain as environmental performance and crop yields in systems where cover crops and manure are used together. Due to manure storage capacity issues, weather, and other factors, some manure is typically fall-applied before soils have cooled to below 50°F. This is the point at which soil biological activity slows considerably and there is less risk of N being lost before it is taken up by the following crop. This research explores whether a cereal rye cover crop can mitigate the negative effects of early-fall applied manure.

Nitrification inhibitors are another option for trying to better match the timing of nutrient availability to nutrient demand by the crop. These inhibitors work by delaying the conversion of ammonium-N to nitrate-N, but the water quality benefits from nitrification inhibitors used with fall manure are uncertain. Gypsum soil amendments have also been used to try to reduce the loss of dissolved P through subsurface drainage. This is a significant concern in areas near the Great Lakes, but the effectiveness of gypsum for P loss reduction in Iowa has not been evaluated.

A four-year study was started in the fall of 2015 at the Northeast Research Farm near Nashua, Iowa to evaluate these techniques. The objectives of the study were to evaluate the effects of liquid swine manure application timing, cereal rye cover crops, Instinct® II nitrification inhibitor, and a gypsum soil amendment on grain yields and on nitrate-N and Total Reactive Phosphorus (TRP) losses via subsurface drainage. The research was done on thirty-six 1-acre plots outfitted with a subsurface drainage water quality monitoring system. Treatments included Early Fall Manure (EFM) with and without a cereal rye cover crop and Late Fall Manure (LFM). These plots were managed as no-till corn-soybean rotations and received 150 lb N ac-1 from liquid swine manure. The EFM was applied as soon as possible after harvest, whereas the LFM was applied after soils had cooled to below 50°F. There was also a spring urea-ammonium nitrate (UAN) treatment that received 150 lb N ac-1 from UAN as a sidedress application. Additional treatments were managed with tillage in a continuous corn system. These included LFM, LFM with Instinct® nitrification inhibitor, LFM with gypsum applied every other year at 1 ton ac-1, and Spring Manure (SM). These plots received 200 lb N ac-1 from liquid swine manure.

Results showed that EFM with a rye cover crop resulted in significantly lower 4-yr average nitrate-N concentrations in drainage water and compared to EFM without a cover crop in both corn and soybeans. Four-yr average N uptake in aboveground cereal rye biomass averaged 88 lb N ac-1 prior to corn and 61 lb N ac-1 prior to soybeans, suggesting that significant recycling of both residual soil nitrate-N and manure N occurred. There were no significant differences in 4-yr average nitrate-N concentrations or N losses in continuous corn treatments receiving either LFM or SM. Average TRP concentrations ranged from 7 to 40 µg P L-1 and there were no significant differences between treatments. The nitrification inhibitor showed promise for improving yields when applied with fall manure but it had no detectable effect on water quality. Biennial fall applications 1 ton ac-1 of gypsum had no observable effects on water quality or yields. Delaying manure applications from early fall to late fall resulted in a 40 bu ac-1 advantage in rotated corn yield averaged over 3 years. Delaying manure application from late fall to spring resulted in a 38 bu ac-1 yield advantage in continuous corn averaged over 3 years. Due to adverse weather conditions, manure applications were delayed for the 2019 cropping season. In rotated corn, LFM with a cover crop out-yielded LFM with no cover crop by 14 bu ac-1, and SM out-yielded LFM by 18 bu ac-1 in 2019.

These results show that the cereal rye cover crop provided significant water quality benefits and did not affect average corn yields. Cover crop N uptake was substantial in an early fall manure system. This recycling of N by the cover crop led to a reduction in N loss via the drainage system, and the yields observed in this study suggest that the cover crop works well with fall-applied manure, especially if that manure is applied before soils have cooled to below 50°F. The results also indicate that delaying manure application so that it was applied closer to the period of crop nutrient demand resulted in significant corn yield increases.

The data from this study indicates that delaying fall manure applications until soils have cooled to below 50°F can help to protect water quality and enhance yields. If fall manure must be applied earlier, including a cover crop can help mitigate N loss to the environment. While application in the spring presents logistical challenges and potential for compaction if soils are wet, yield data suggests that the economic benefit might justify moving to a spring pre-plant or side-dress manure application system where feasible.

For more information about this research, contact Dr. Matt Helmers at mhelmers@iastate.edu 515-294-6717 or Brian Dougherty at brian1@iastate.edu 563-583-6496

Key Findings

• A cereal rye cover crop provided significant water quality benefits and did not affect average corn yields.
• Cover crop N uptake was substantial in an early fall manure system.
• Delaying manure application to better match crop nutrient demand resulted in significant corn yield increases.