As fields get harvested many farmers are making fall manure applications. Managed properly, manure can be an excellent source of valuable nutrients, but it can also pose risks to water quality when applied long before a crop is present to take up the nitrogen provided by manure. This ICM Blog will discuss considerations on how to maximize the use of manure nutrients for crop production and minimize potential water quality impacts by taking a closer look at an on-going research project at the Northeast Research and Demonstration farm near Nashua, IA.
While current soil temperatures at 4-inch depth are at the 50°F or cooler across the state, that isn’t always the case in the fall. Research has found that biological activity in the soil slows considerably when soils at the 4-inch depth are at 50°F or cooler. Bacteria in the soil convert ammonium (NH4+) from manure or fertilizer to nitrate (NO3-1), which is readily available to crops. The nitrate molecule carries a negative charge and is very soluble in water. It does not adhere well to the negatively charged clay particles in the soil and consequently can be readily leached from the soil. The warmer soils are when manure (other sources of N like anhydrous ammonia) are applied, the more rapidly the ammonium will convert to nitrate and increase the risk of losing the N prior to when the corn crop needs it next year.
In 2016, ISU established two projects to evaluate the impact of different cropping management practices on subsurface drainage water quality and crop yields.
One study evaluated manure application timing (early fall vs. late) and cover crop effect on water quality in a corn-soybean rotation. Early fall manure went on as soon as soybeans were harvested, while late fall manure went on after the soils trended below 50°F. This study’s cropping management practices included:
- Early Fall Manure (EFM150NT)
- Early Fall Manure with cereal rye cover crop (EFM150NT+R)
- Late Fall Manure (LFM150NT)
- Spring UAN (UAN150)
The second study evaluated the effect of manure application timing (late fall vs. spring) in continuous corn. This study’s treatments included:
- Late Fall Manure (LFM200)
- Late Fall Manure + gypsum (LFM200+G)
- Late Fall Manure + Instinct (LFM200+I)
- Spring Manure (SM200)
For more information on these treatments and trial setup, please read the research report found here.
Cereal rye cover crop reduces nitrate loss
Data from 2016 to 2018 show that a cover crop can help reduce nitrates in tile drainage when manure must go on early. Comparing the early manure application without a cover crop (EFM150NT) and with a cover crop (EM150NT +R) shows that the cereal rye cover crop reduced nitrate-N concentrations in subsurface drainage water in both corn and soybeans (Table 1).
Table 1. Annual nitrate-N losses via drainage water from each treatment. Treatments with the same letter within year are not significantly different.
Cover crop takes up nitrogen
Cover crop aboveground biomass samples were collected in the spring just prior to termination and analyzed for N content. There were substantial differences in cover crop growth and N uptake in the aboveground biomass directly over where manure had been injected (Fig. 1, red bars) compared to between the injection bands (Fig. 1, green bars).
Averaging the N uptake both in and between the manure injection bands across the 3-yr study, the cover crop took up about 95 lb N/ac on plots receiving 150lb N/ac from manure. Comparatively, the plots going to soybeans where no manure was applied took up about 58 lb N/ac. These results suggest that the cover crop took up significant residual soil N following soybean and likely took up N from the manure itself. Cover crop N uptake in a previous 8-yr study on these same plots was only 13 lb/ac prior to soybeans and 21 lb/ac prior to corn in a spring UAN sidedress system.
Figure 1. Aboveground N uptake by a cereal rye cover crop directly over the manure injection band (red bar), between the manure injection bands (green bar), and before soybeans (blue bar - no manure applied).
This uptake of N by the cover crop is one of the reasons for the lower nitrate-N concentrations in drainage water that we see in Table 1. We also tend to see more water coming through the drainage system in the cover crop plots, suggesting that more water is infiltrating rather than running off the surface and may be reducing surface soil erosion as well.
Delaying manure application improves corn yield in rotated fields
While we cannot say for certain when the N from the cover crop was released or where it ultimately went, but we can say that it did not impact average corn yield. The 3-yr average shows no significant difference in corn yield in the early fall manure plots with and without a cover crop (Fig 2). This suggests that a cereal rye cover crop is a good fit to improve water quality and have a negligible effect on crop yield when managed appropriately.
While the cover crop did not affect yield, the timing of manure application had a big impact on yield in the corn-soybean rotation research (Fig. 2). Late fall manure averaged 40 bu/ac greater corn yield than early fall manure over the 3-yr period. While these results may be optimistic, other research has shown a 10 to 15 bu/ac advantage with delaying manure application until soils have cooled below 50°F. There is a significant economic benefit to later manure application that consistently shows in research results.
Figure 2. Average corn yield data (2016-2018) (EFM150NT: Early to mid-October manure application, EFM150NT+R: Early to mid-October manure application and a cereal rye cover crop, LFM150NT: Early to mid-November manure application, Spring UAN: UAN application 3 weeks after planting). Note: All plots received 150 lb N/acre. Treatments with the same letter are not significantly different.
Delaying manure application improves corn yields in continuous corn
This study also looked at timing of manure application and the effect of using a nitrification inhibitor in continuous corn. In this study, when Instinct® (nitrification inhibitor) was used with late fall manure applications, there was a 16 bu/ac yield increase over the late fall manure with no inhibitor (2017 and 2018 data only) (Fig. 3). Additionally, moving the manure application from late fall to spring resulted in a 38 bu/ac improvement in yield. The gypsum treatment had no impact on corn yield.
Similar to the study in rotated corn, we see a large increase in yield with delayed manure application. Just as early fall manure is at higher risk of N loss compared to late fall manure, we see that same risk with late fall manure compared to spring manure. The closer we can match manure application to plant N demand, the less risk we have of losing that N (and our money) to the environment.
Figure 3. Average yield data for 2016 through 2018 crop years for continuous corn (LFM200: Early to mid-November manure application, +G: Gypsum application, +I: Instinct nitrification inhibitor, SM200: Spring Manure. Note: All plots received 200 lb N/acre. Treatments with the same letter are not significantly different. Instinct plots were planted to soybeans in 2015. Statistical comparison for the Instinct treatment includes 2017 and 2018 yields only.
While waiting to apply manure until late fall or spring carries risks, so does early application to bare ground. This research shows the potential for cover crops to improve water quality if manure must be applied early and an economic incentive to delay application to late fall or spring. It’s worth putting some thought into how you might incorporate cover crops and shift to later manure applications on your own operation.
See the links below for more research and resources on manure: