After corn silage harvest is complete, one of the next jobs is spreading manure. Two important things to consider for fall manure application are timing and cover crop management. The best scenario would be if we did not have to apply manure in the fall for next year’s corn crop.
With fall manure application, significant nutrient losses are almost inevitable because the manure is lying on the field for a long period of time. However, there are some important things we can do to reduce these losses.
One of the most important things we can do is establish a cover crop where manure will be applied in the fall or winter. Having a cover crop will generally double the recovery of fall- and winter-applied manure nitrogen (N) by next year’s corn crop. Also, if you plan to apply manure during the winter, state regulations require that you either have greater than 25% residue or an established cover crop.
In bare corn silage ground, a cover crop is the only option for meeting this regulation. Under Act 38 (PA’s Nutrient Management Act), the following conditions meet the definition of “winter”: December 15 to February 28; frozen ground (4 inch depth); or snow-covered ground.
Manure can be applied before or after cover crop establishment. Generally, the priority should be getting the cover crop established, and then spreading manure. This gets the cover crop established in a timely fashion in order to ensure good cover going into the winter. It also delays manure application, which reduces the time the manure is exposed for loss.
The cover crop is able to start growing, and when manure is applied, the cover crop will be ready to take up the manure nutrients and hold them against loss. Delaying manure application will possibly get us into colder weather; in this scenario, the manure nutrients will not be released as rapidly, reducing the exposure of the available nutrients to loss.
The main downside to applying manure after cover crop establishment is the potential for smothering if high solids manure is applied at a heavy rate.
In some cases, applying manure before cover crop establishment is necessary – if the storage is full, for example. This will delay cover crop planting until the manure and the soil have dried out enough for planting.
Also, with early fall manure applications when the weather is still warm, there is much greater potential for manure N to volatilize unless it is quickly incorporated by tillage or rain. The loss of N at this time of year is similar to what we see with surface applications in the spring. Most of this loss occurs within the first 2 days after application, so if incorporation is not immediate, then there will be little or no N benefit from fall tillage.
This creates a dilemma because, while fall tillage may reduce N loss, it opens us up to other losses, mainly through erosion. The erosion factor can be reduce if a cover crop is established, but now we are looking at harvesting silage, spreading manure, tilling the soil, and finally planting the cover crop.
This could get us pretty late into the fall before all of this is done. Low disturbance injectors would be a good alternative for this early fall application, because they get the manure incorporated immediately, no tillage is required, and the cover crop can be planted much sooner.
A few closing thoughts on fall manure applications. Pay attention to soil conditions. Driving heavy manure application equipment on wet soils can result in severe compaction that may take years to rectify. This is another reason to get the cover crop established and delay manure applications until better soil conditions are present.
Also, make sure your nutrients are in balance. If you are applying manure this fall for next year’s crop, take that into consideration when you plan your spring and summer fertility program. If manure is applied in both the fall and the spring, higher rates of additional fertilizer may not be needed.
In the Agronomy Guide, Tables 1.2-14 (Manure nitrogen availability factors for use in determining manure application rates based on planning conditions) and 1.2-15 (Factors for calculating manure nitrogen availability based on time of application, incorporation, field history, and manure analysis with ammonium and organic N fractions) can be used to calculate the amount of nitrogen available for the following year’s crop.
Examples of the calculations are found immediate following those tables in the Agronomy Guide. Manure management plans and nutrient management plans also have these calculations in them.
Consult your local Extension Office or Conservation District for assistance with these plans if you do not already have one.