With nutrient pollution and water quality issues gaining attention in the media, as well as rising costs of fertilizers, retaining nutrients from fall manure is both economically advantageous and environmentally imperative.
Nitrogen is a valuable nutrient found in manure, but it is also dynamic and prone to loss. The nitrogen cycle is complex, with many pathways of loss. There are three major pathways of loss after a manure application: surface runoff, subsurface leaching and volatilization.
Nitrogen, in the inorganic forms of ammonium and nitrate, is lost from surface runoff and leaching into groundwater when temperatures are warm or when vegetation, crop residue and soil coverage are lacking.
The greatest pathway of nitrogen loss after a manure application to soil surface is through volatilization, the loss of nitrogen in the form of ammonia gas. Rates of volatilization tend to be high when temperatures are high and when a larger surface area of manure is left exposed to the atmosphere.
To prevent nitrogen loss and improve retention for crop utilization, nutrient management recommendations advise the nutrients be applied when plants are growing and can use them, typically in the growing season. This presents a challenge when manure has to be emptied from storage in the fall and there is a dearth of living vegetation.
In the Northeast, from mid-December to April is a critical period for runoff and leaching loss. It is a period of time when there is a lot of precipitation, snowmelt, freezing and thawing, and little to nothing growing in the fields.
Planting winter annuals like winter rye, wheat or triticale before a summer crop like corn has the potential to reduce the chance of runoff and leaching losses of nitrogen, enhance soil health benefits and increase forage production on farms.
However, there is a short window between fall harvests and winter to apply manure and establish a winter annual. While crops like winter rye can be planted and germinate in very cold temperatures, a delay in planting can result in poor establishment.
Prioritizing planting a winter annual in the fall after crop harvest, and applying manure afterwards, can ensure better stands of the winter annual crop. When demands for field operations slow down later in fall, then manure can be applied.
The research we are conducting at Penn State University evaluates the strategy of delaying manure applications rather than delaying planting. We applied manure early in mid-September or later in November. After a late application, when temperatures tend to be cooler and volatilization losses of nitrogen after manure applications were reduced, about 14 percent more nitrogen was available for the winter annual.
In no-till fields, manure is broadcast and left on the soil surface; with dairy manure, about 85 percent of the available manure nitrogen is lost as ammonia gas. Immediate incorporation after application is ideal but is not possible in no-till practices or requires an additional plow incorporation pass.
Therefore, we also compared surface application of manure to shallow-disk manure injection in both early and late fall to assess if injection would increase nitrogen availability to cash crops from fall manure.
Injecting manure with a shallow disk injector can minimize soil disturbance, and manure can be incorporated in one pass. Economic analyses from the Northeast SARE Sustainable Dairy Cropping Systems project found custom-applied, injected manure tends to cost more for labor and fuel, but because injected manure conserves more of the manure-nitrogen than broadcasted manure, side-dress fertilizer costs may be significantly reduced.
How the winter-annual crop is managed also plays a role in nitrogen availability to a summer crop like corn. The winter annual can be managed as a cover crop. In this system, the winter annual is terminated near to planting of the following summer crop, and any nutrients taken up by the winter annual are returned to the soil for use by the following crops as the residue decomposes. Alternatively, winter annuals are often harvested in the spring for forage prior to planting the following summer crop.
In this case, harvesting the winter annual will remove a lot of the nitrogen conserved by the winter annual. Thus, the following corn crop, after an early spring harvest of silage, will have less nitrogen available than if the winter annual was managed as a cover crop.
However, in both cases, the winter annual increases the overall N use efficiency either by utilizing the N to produce forage from the winter annual or retaining more N for use by the crop following the winter annual managed as a cover crop.
After a rye cover crop, corn silage yields were 23 percent higher than after rye silage, but total harvested forage was 8 percent higher in a system with rye and corn silage than with a cover crop and corn silage.
Also, supplemental nitrogen will likely be needed either for the winter-annual forage or for the following summer crop after a winter annual is harvested for silage, but less will be needed if the manure was injected in the fall.
We are continuing to evaluate and compare these conservation strategies and have found there are viable management practices that will address the public’s growing environmental concerns, conserve more nitrogen within crop fields and optimize the fertilizer value of fall manure.
If crop production goals are focused on summer annuals, plant a winter annual as a cover crop and inject manure later in the fall to get the most nitrogen out of the fall manure.
If getting more forage out of your fields is a higher priority, plant the winter annual then inject manure later in the fall. In the spring, an early forage crop can be harvested and followed by a summer-annual crop. Conserving and using the nitrogen from fall manure to double-crop could increase forage production on-farm and reduce feed costs. PD
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Rachel Milliron
- Graduate Student
- Department of Plant Science
- Penn State University
