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Fertilizer Value of Distressed Corn Harvest for Forage (08/05/21)


Last week, the fertilizer value of spring wheat and barley straw was estimated. This week, the subject is corn harvested for forage. There are at least three possible harvest methods:

- whole plants cut close to the soil surface before or at tassel, or harvested after silking because pollination was very poor

- whole plants as silage, including some grain

- ‘Stalklage’ that passes through the combine at harvest. This does not include any grain, nor is it usually cut close to the soil surface. Whatever passes through the combine.

Whole plants cut at tasseling (VT)

Several years ago, I participated in a multi-state project in which we cut whole plants at early tassel. The percentage of N in this sampling would be a good estimate of what a corn grower should expect, although they would have to estimate their dry matter weight. The P and K are estimated from the proportion of corn uptake at tassel compared to total seasonal uptake.

Percent N        1.7% 

Percent P2O5    0.46%

Percent K2O    1.5%

For each ton of dry matter of whole plants at tassel, the N would be 34 pounds per ton, P2O5 would be about 9 pounds per ton and the K2O would be 30 pounds per ton.

Corn silage nutrient removal

Although the grain in corn silage this year will likely be much reduced compared to a more normal growing season, the N, P and K content would be expected to be similar to tonnage including grain. The nitrate in this silage would also be expected to be much greater, since it did not move to grain and was not converted to protein. The following values assume 65% moisture in the silage.

Percent   N         0.63%

Percent   P2O5    0.28%

Percent   K2O     0.35%

For each ton of silage (at 65% moisture), there would be 13 pounds per ton N, 5.6 pounds P2O5  per ton and 7 pounds of K2O per ton.

Nutrient content from what comes out the combine

What comes out of the combine is neither whole plants nor is it silage. A combine usually cuts higher from the soil surface than a silage chopper, and it does not contain any grain. Also, the corn has grown to maturity, and therefore has different nutrient concentration than plants cut at tassel.  It contains cob material, and some fines are lost to the soil surface. These figures come from my 1975-76 MS work at the University of Illinois. Every once in a while, I get a chance to share these values to someone’s advantage.

Stalklage N, P and K concentrations-

Percent N        0.45%

Percent P2O5   0.11%

Percent K2O    0.8%

For each ton of stalklage dry matter, there would be 9 pounds N per ton, 2.2 pounds P2O5 and 19 pounds of K2O. Fertilizer values will vary and fertilizer prices, like grain prices, are volatile at this time.

Generally, present costs are about:

N        40 cents per pound

P2O5   66 cents per pound

K2O    53 cents per pound (as KCl)


Dave Franzen

Extension Soil Specialist