In response to a question at a Forman field day late last week, here are a few methods to estimate cover crop biomass and N value.
Actual measurement: For those consultants whose acreage base was substantially cut this growing season, this would be a useful thing to do for acres not cropped.
Make 1-foot square frames out of PVC, metal or something substantial and cheap.
• Throw them out into the cover crop and clip or cut the vegetation to the soil surface.
• Bag the material and keep cool until you get dry them
• After drying, weigh the biomass.
If you are really scientific about the results, send them in for N analysis. If you understand the variability involved will probably overwhelm the plant analysis, assume that any green vegetation has about 4% N.
• Every 20.8 grams of dry matter from a square-foot equals 2,000 lb dry matter/acre, or
Each gram from a square-foot equals about 98 lb/acre dry matter
Dry matter is 4% N, so:
(Weight dry matter/acre X 0.04 lb N/lb dry matter) = (Weight N/acre)
According to John Moraghan’s work from about 2005 (Sugarbeet R&E Reports on the web), about 1/3 of the N you find will be available under conventional till for next year’s crop.
A rule of thumb for no-till is that about 1/5 of the total will be available under no-till for next year’s crop (my estimate).
Estimating without the work:
If you have 100% ground cover of anything broadleaf that is green and it is 6 inches tall at the time of kill, you have 2,000 lb/acre dry matter. For every inch above that, add 150 lb/acre dry matter.
For the N content, use 4% N again to calculate total N, then take the fraction appropriate for your tillage from above to determine roughly what might be expected for the crop. If the broadleaf cover crop is yellow (radish on low N soil), you will not receive any appreciable N credit.
For rye and most small grain cover crops, 8 inch height, 100% ground cover is about 2,000 lb/acre dry matter. If it is green, assume about 4% N. If it is yellow-green or yellow, you will not have any appreciable N credits.
If ground cover of either broadleaf or grass is less than 100%, make the appropriate adjustment in dry matter content.
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