Iowa Farmer Today
09-23-06
Research yields new thinking on manure, beans
By Tim Hoskins, Iowa Farmer Today
The environment might be the winner if farmers apply manure to fields before
planting soybeans and seeding alfalfa.
Michael Russelle, a scientist with the USDA’s Agricultural Research Service in
Minnesota, has been looking at the pros and cons of applying manure to legume
crops, such as soybeans and alfalfa.
“We can’t predict when manure will help a (soybean or alfalfa) crop,” he notes.
However, Russelle says there is no yield penalty for applying manure to
soybeans.
Historically, many farmers don’t fertilize soybeans or alfalfa because they don’t
respond to nitrogen application. That is because if they don’t have enough N,
they fix their own.
However, Russelle says if there is enough N in the soil, the legumes will not fix
nitrogen.
He estimates soybeans will fix 175 pounds of N per acre in the field, and alfalfa
will fix 450 lbs./acre.
Therefore, if there is plenty of N, Russelle says the soybeans and alfalfa will not
add to the N load of a field.
While the main concern might be about N in manure, he says other nutrients,
such as phosphorus and potassium in the manure, could help the soybean crop.
He says phosphorus helps the soybean plant during flowering and setting seed.
Potassium helps move sugars inside the plant and could help the plant resist
stress.
There might be other benefits from micronutrients, such as zinc and sulfur from
adding swine manure.
Russelle used liquid swine manure that had 20 lbs. of N per 1,000 gallons.
Beyond his work, Russelle says various studies have shown mixed results from
applying manure to soybean and hay crops.
The only yield penalty is from over application. Russelle says that is more of a
management issue.
As tile lines have gotten shallower and closer together, he says there are some
incidents of manure, particularly liquid manure, making its way to the tile and into
water sources.
Russelle says proper application and timing can help take care of some of the
water-quality issues when applying manure. This week, Iowa’s Environmental
Protection Commission (EPC) discussed rules about applying manure to ground
going into soybeans. Previously, the EPC asked Department of Natural
Resources staff to draft a proposal that would prohibit liquid manure application
on soybean fields, except when fields that were planned for corn were planted to
soybeans due to wet weather.
The EPC is being petitioned to consider a reduced N application rate for liquid
manure being applied to fields going into soybeans as an alternative to
prohibiting the application of liquid manure on soybean fields at their September
meeting held Tuesday.
As of press time, the results of the discussion were not available.
Iowa State University agronomist Gina McAndrews studied swine manure
collected from hoop barns and the effect on soybeans.
In her study in Boone, the solid swine manure was applied before a corn crop.
However, she measured to see if there was a residual effect for soybeans after
the corn crop.
Overall, there are some detectable and residual effects on soybean growth and
yield, she says.
= Soybean plants from manure-amended plots were significantly taller and had a
thicker stem diameter than plants from the other plots.
= The manure-treated plots produced 39 percent greater leaf area than the
control in the first year and 11 percent greater leaf area than the urea-amended
plots in second year of the study.
= There was a 21 to 34 percent greater potassium concentration in soybean
plants grown in the manure-amended sites than in the other plots.
She says there was a yield gain in the manure-treated plots compared to the
control or urea-fertilized plots.