Considerations for Fall Manure Management
By Dennis Frame, Emeritus Professor, UW-Extension
August is one of the few months when farmers can take a breath or two from their cropping programs and begin to assess how the farming systems and weather influenced this years yields. It is also a time to begin preparing for next season by planting cover crops, fall seedings and applying manure. As you start applying manure for the 2019 cropping season, there are a few things you can do to reduce the risk of nutrients moving out of the root zone.
Prior to applying manure anytime, farmers should evaluate their fields to determine the need for pre-tillage to close cracks and macropores in the soils due to dry weather conditions. This summer was dry in parts of Kewaunee and Door Counties and the heavy clay soils shrunk and opened cracks that could increase the movement of nutrients and/or pathogens through the soil. Slight tillage of the soil prior to applying manure closes these cracks and reduces the risk of liquid manure moving rapidly through the soil. The key here is slight tillage, enough to close the cracks and prepare the soil for liquid manure applications. This is not the time of year to prepare a seedbed unless you are fall planting alfalfa and a cover crop.
Fall manure applications can be broken down into two windows, early after harvesting cereal grains, early corn silage or terminating forage crops (August – October); and late after harvest of corn for grain, late corn silage or soybeans (October – December). The timing of manure applications in the fall is critical in regards to retaining nutrients for next year’s crops. Applying manure in August and September means that soil temperatures are most likely above 50 degrees Fahrenheit, and soil bacteria is still actively converting ammonium to nitrate. This form of nitrogen has the potential to move out of the root zone and not be available to next year’s crops. Take weather conditions and predictions into account prior to applying manure.
Tips to reduce the risk of nutrient movement out of the root zone with early fall manure applications, consider implementing some of the following recommendations:
- Incorporate fall applied manure and also consider using cover crops.
- Dairy manure, from facilities bedded with sand, can contain more than 15 pounds of ammonium nitrogen per 1,000 gallons. Much of this nitrogen can be converted to nitrate (NO3) by soil bacteria and lost through leaching or denitrification before the following crop season begins.
- Fall cover crops can capture nutrients in manure and keep these nutrients from moving out of the root zone. In one Ohio State study, researchers found cover crops decreased the nitrate nitrogen (NO3) concentration in soils with manure compared with the control soil by more than 70% before the field froze in the fall.
- Ryegrass is the best cool-season grass for capturing excess nitrogen. Because rye over-winters, research has shown it can capture and hold 25 to 50 pounds of nitrogen (organic form) per acre. It germinates at temperatures as low as 34 degrees so can be seeded later than oats. However, less nitrogen will be captured the later the rye is seeded. The organic nitrogen stored in the plant will eventually be released as ammonium nitrogen when the plant dies.
- Applying manure with a nitrification inhibitor can potentially help reduce the conversion of ammonium to nitrate, but don’t expect that to last for months. Inhibitors can extend the application window, but check with your distributer to determine how long the inhibition is effective.
- Keep weather conditions in mind. If you’re surface applying, don’t put it on one or two days before a large rain event.
Tips to reduce the risk of nutrient movement out of the root zone with late fall manure applications, consider implementing some of the following recommendations:
- Depending on the date of application, soils maybe at or near 50 degrees Fahrenheit, and soil bacteria may not be actively converting ammonium to nitrate.
- Soils above 50 degrees may benefit from nitrogen inhibitors, which can extend the application window and stop the conversion of ammonium to nitrate.
- Incorporate fall applied manure using as little soil disturbance as possible. Low disturbance manure injection can reduce soil loss and increase nitrogen availability.
- Depending on the date of harvest, consider planting a cover crop prior to manure application. Cover crops reduce the potential for soil and nutrient loss. Match cover crop to planting dates and apply manure to the growing crop.
- Keep weather conditions in mind. If you’re surface applying, don’t put it on one or two days before a large rain event.
The benefits of fall manure applications are that soils are normally drier in the fall than in the spring, lowering the risk of compaction. When soil compaction is an issue in the fall, the freezing and thawing of the soil help eliminate compaction issues. The fall application window can also be longer than in spring because of spring rains and the timing of planting. That does not be that rain in the fall is not an issue, but normally there are periods where manure can be safely applied.
However, with a potentially tight window for getting on fall manure in the coming months, the best policy is to be prepared. Farmers and their crop consultants should evaluate next year’s cropping plans and begin thinking about fall applications. Make sure your equipment is ready and you have adequate help to get the job properly done. Review your nutrient management plan for which crops will receive manure applications.
Have a safe and productive fall.