Better Farming February 2017

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Better Farming February 2017 The Business of Ontario Agriculture 51
CROPS: YIELD MATTER$
What do cover crops contribute?
Let’s consider red clover under-
seeding in wheat, working with the
assumption that by fall the red clover
reaches a height of 18 inches.
From my forage testing days at the
lab we assumed as a thumb rule 200
lbs of dry matter per inch of plant
height. That assumption would
equate to 3,600 lbs of above ground
growth.
A 15 per cent retention of residues
adds 540 lbs of biomass. This more
than covers the SOM volume lost by
selling one short ton of straw (300
lbs).
Red clover makes both rotations
(corn-soybeans-wheat and
soybeans-soybeans-wheat) better just
from the SOM contribution – with-
out consideration of the nitrogen
contribution to a subsequent corn
crop.
What does manure contribute?
Manure and compost or any other
organic source is subject to the same
processes as crop residues. With
manure and other non-agricultural
source materials (NASMs), you need
to be aware of moisture content and
calculate additions based on dry
matter.
As an example, 40,000 lbs of solid
dairy manure with 25 per cent dry
matter adds only 10,000 lbs of dry
matter. At 15 per cent retention, this
manure is adding 1,500 lbs of organic
residue to cover the annual loss (due
to normal microbial respiration of
CO2) in our example of 1,200 lbs,
resulting in a net contribution of 300
lbs of SOM.
Liquid sow manure at 5,000
gallons per acre and 2 per cent dry
matter is adding 1,000 lbs of dry
matter. With a 15 per cent retention,
the manure is only adding 150 lbs to
go toward covering the annual loss of
1,200 lbs of SOM.
A word of caution when consider-
ing NASMs for building SOM: you
must always consider the nutrient
content.
Otherwise, as you apply large
amounts of NASMs to obtain organic
matter, you may over apply nutrients
which may not be needed for optimum
crop production. Most likely, that
nutrient would be phosphorus. (Even
though it may be in an organic form,
where it will be subject to the phos-
phorus cycle, it will breakdown into
inorganic ortho-phosphate ions
indistinguishable from ortho-
phosphate found in commercial
sources. Excess phosphorus is
excessive phosphorus regardless of
the original source.)
There is more to soil health than
simply the amount of residues from
the various rotations. The length of
time that soil has living roots in it
also builds better soil biology and soil
structure. One of the benefits of
winter wheat in a crop rotation is the
fact it grows for nearly 11 months of
the year.
Dr. Bill Deen, an associate profes-
sor at the University of Guelph, and
Dr. David Hooker, a field crop agrono-
mist and assistant professor at Ridge-