Fall 1999
Minimizing risk when applying manure in winter
by Jeffery Lorimor, Department of Agricultural and Biosystems Engineering
If
you don’t have to apply manure in the winter, don’t. If you
do, there are ways to lessen the risk of having the manure run off, which
results in nutrient losses and water quality degradation. The Iowa Department
of Natural Resources recommends that winter-applied manure be applied
to land with 4 percent slopes or less and that practices are in place
to minimize erosion. There are two other factors to consider to reduce
the risk of manure losses with snowmelt runoff: manure application timing
and crop residue cover.
Research at Iowa State University shows that the risk of manure losses with surface runoff can be reduced significantly by adjusting the timing of the manure application. A study in 1994 and 1995 compared four different manure application timing treatments: 1) fall incorporated manure, 2) manure applied early in the winter on frozen ground, 3) manure applied late in the winter on top of the snow, and 4) spring-incorporated manure.
The study showed that the greatest risk of surface runoff losses occurs when manure is applied on top of snow late in the winter and that losses from standing corn stalks are likely to be greater than losses from soybean stubble. In general, the more time that elapses between application and runoff, the less the risk of environmental degradation (this is true anytime, not just in the winter).
During the first year of the study, the late-winter manure was applied on top of snow on February 14. The next day a major thaw began. By the end of the thaw, the snow was gone, and much of the manure had gone with it. Ninety pounds per acre of manure nitrogen (49 percent of the nitrogen applied) were lost with the snowmelt. Table 1 shows the nitrogen concentrations in the runoff from the snowmelt.
Table 1. Nitrogen concentrations in snowmelt runoff for four manure application treatments (February 1994).
|
Date |
February 15 |
February 16 |
February 17 |
|
Parts per million |
|||
|
Fall applied |
6.7 |
8.7 |
30.6 |
|
Early-winter applied |
33.2 |
92.4 |
91.9 |
|
Late-winter applied |
1,086.0 |
739.0 |
116.2 |
|
Spring applied |
5.6 |
3.4 |
7.0 |
The rest of the year losses were minimal. When the large February snowmelt is neglected, there are no statistical differences among treatments for the year. The second year of the study, losses were minimal, and no significant differences were found among treatments.
Figure 1. Total
Kjeldahl nitrogen losses for corn stubble and soybean stubble.
Losses were higher from standing corn stubble than from soybean stubble (Figure 1). The corn stubble held deeper snow than the soybean stubble so more liquid runoff potential existed. Table 2 summarizes the overall nitrogen losses for the 2 years of the experiment for both cornstalks and soybean stubble, and includes the "catastrophic" losses of the single snowmelt event of February 1994.
Table 2. Nitrogen lost in surface runoff from corn and soybean stubble plots. Summary of 2-year averages in pounds per acre and percentage of the applied amount.
|
Fall inject |
Early winter |
Late winter |
Spring broadcast |
|||||
|
lb/acre |
% |
lb/acre |
% |
lb/acre |
% |
lb/acre |
% |
|
|
Corn stubble |
11.8 |
12.4 |
17.1 |
8.2 |
41.0 |
22.1 |
3.9 |
1.2 |
|
Soybean stubble |
1.4 |
1.5 |
2.8 |
1.4 |
19.2 |
10.3 |
2.2 |
0.6 |
Anytime manure is applied on frozen ground there is an increased risk of environmental degradation. If manure must be applied in the winter, the environmental risk can be minimized by applying early in the winter ahead of snowfall. Applying manure on soybean stubble where less snow has been captured is preferable to applying on a deeper snowpack in standing cornstalks. If manure must be applied on frozen soil late in the winter, waiting until the snow has melted off will significantly reduce the risk of runoff losses.
