Cover Crop Biomass Calculator

This calculator models how much dry matter your cover crop has actually put on using a logistic (S-shaped) growth curve rather than a straight-line guess, because real cover crop growth starts slow, accelerates through a rapid mid-season phase, and then levels off as the stand matures. Each of the five species built in — cereal rye, crimson clover, tillage radish, winter pea, and hairy vetch — carries its own maximum attainable biomass, its own growing-degree-day midpoint (the GDD value where growth is fastest), and its own growth-rate constant, based on species-level yield and nitrogen-credit ranges reported in the USDA's SARE handbook "Managing Cover Crops Profitably" (e.g., cereal rye's 10,000 lb/ac ceiling and crimson clover's roughly 100-130 lb N/ac fixation credit both track field-trial figures from that reference), so a radish reaching its ceiling around 600 GDD looks very different from a cereal rye still climbing toward 1,200. Seeding rate scales the ceiling up to a 120% cap, reflecting that overseeding buys some extra stand density but yields diminishing returns beyond the recommended rate.

From the resulting biomass estimate, the calculator assumes dry matter is about 42% carbon, divides by the species' carbon-to-nitrogen ratio to get nitrogen content, and for legumes adds a nitrogen-fixation credit that scales with how close the stand is to full maturity — so a crimson clover terminated early banks less fixed nitrogen than one allowed to flower. That total nitrogen credit is converted to a rough fertilizer-equivalent dollar value at a fixed $0.60/lb N. Treat every output as a reasonable in-season estimate: real biomass depends heavily on local moisture, soil fertility, and stand establishment success that a generic GDD curve can't capture, so use it for planning purposes rather than a substitute for clipping and weighing an actual sample.