When to Terminate Cover Crops
Knowing when to terminate cover crops starts with knowing why you planted them. Choose each cover crop for one job — nitrogen supply, compaction relief, weed suppression or grazing — and set the kill date by that job and by the crop that follows. For maize after cereal rye, terminate 10 to 14 days before planting, while the rye is shorter than about 20 cm (8 in). For a legume grown for nitrogen, terminate at early flowering. In dry regions, terminate early enough for rain to refill the soil before sowing.
The species decides what a cover crop can do. The termination date decides what it costs. A cover crop does not stop working when its job is done. If it stays alive, it keeps taking water and nitrogen from the soil, and the next crop pays for that.
Why do cover crops sometimes reduce main crop yields?
Cover crops reduce yields mainly when they are still growing close to planting, using water and holding nitrogen the main crop needs. A 2023 satellite study in Global Change Biology (Deines and colleagues) covered fields in six US Corn Belt states and found cover crop adoption linked to average yield losses of 5.5% in maize and 3.5% in soybean.
That average hides a wide range. A 2019 review in Global Change Biology by Abdalla and colleagues found an average primary-crop yield loss of about 4%. However, mixes of legumes and non-legumes raised yields by about 13%. The difference between those outcomes is mostly management: species choice, mix, and above all timing.
Match the cover crop species to one job
Pick the job first. The four options below use seed that a standard grain drill or planter can sow, or that farmers can broadcast by hand.
Nitrogen supply: hairy vetch or mucuna
Legumes are the only cover crops that add new nitrogen. In temperate systems, hairy vetch is the most reliable choice. A 2023 meta-analysis in Heliyon found that hairy vetch holding 95–150 kg N/ha raised following maize yields by 9–36% where no fertiliser was applied.
In the humid and sub-humid tropics, mucuna (velvet bean) does the same job. In eastern Uganda, a study in Nutrient Cycling in Agroecosystems recorded mucuna producing 8.2 t/ha of dry matter and 170 kg N/ha over 22 weeks in the lower-potential zone. In southern Benin, IITA work summarised by FAO found mucuna supplied more than 100 kg N/ha to the following maize.
Hairy vetch drilled with cereal rye is a practical mix. The rye supports the vetch and holds nitrate over winter, and both seeds pass through the same drill.
Compaction relief: forage radish
Tap-rooted brassicas push through dense layers better than grasses. In a 2010 Plant and Soil study at the University of Maryland, Chen and Weil found that under high compaction, forage radish had more than twice as many roots as cereal rye at 15–50 cm depth. The channels it leaves help the next crop’s roots reach deeper water.
There is a limit: a follow-up study by the same team found more deep maize roots after radish but not higher grain yield. Sow it with oats for more ground cover. In cold climates it dies over winter, so no termination is needed.
Weed suppression: cereal rye, before soybean
Weed control from cover crops depends on mulch mass, and mass takes time. Work by Mirsky and colleagues at USDA-ARS, published in 2013, set about 8,000 kg/ha of cereal rye biomass as the level for season-long weed control without herbicide. In North Carolina, Smith and colleagues reported in Weed Science (2011) that rolled rye above 9,000 kg/ha prevented soybean yield loss from weeds, while rye at 4,450–6,606 kg/ha left yields 628–822 kg/ha below weed-free plots.
That amount of biomass needs a late kill, around rye flowering. Soybean tolerates this well. Maize does not, because late rye ties up nitrogen at the time maize needs it. Use rye for weed control ahead of soybean, and use a different species or an early kill ahead of maize.
Grazing: brachiaria with second-season maize
In Brazil’s second-season (safrinha) maize, Embrapa has developed a system that sows Urochloa (brachiaria) ruziziensis between maize rows with an adjusted soybean planter. After maize harvest, cattle graze the brachiaria through the dry season, and it is then desiccated before soybean. Embrapa Soja reported a 10% higher soybean yield after maize with brachiaria than after maize alone in a Londrina trial.
An Embrapa technical chapter reports a 12% maize yield reduction from the intercrop on one high-fertility São Paulo site. Place fertiliser only in the maize row, as Embrapa advises, to limit competition.
When should you terminate a cover crop?
Terminate a cover crop when its job is done and before it competes with the next crop for water and nitrogen.
| Job | Species | Termination window |
|---|---|---|
| Nitrogen supply | Hairy vetch | Early to full flowering, before pods harden |
| Nitrogen supply | Mucuna | End of the short fallow, before main-crop land preparation |
| Compaction relief | Forage radish | Winterkill in cold climates; otherwise at bolting |
| Weed suppression | Cereal rye before soybean | Flowering (anthesis), at or near planting |
| Nitrogen scavenging | Cereal rye before maize | 10–14 days before planting, rye under ~20 cm (8 in) |
| Grazing | Brachiaria | After grazing, with enough time before soybean for regrowth to be controlled |
Hairy vetch needs flowering for mechanical kills to work, because rolled or mown vetch regrows if cut too early. For maize after rye, Iowa State University extension advises the 10–14 day gap because living rye can carry seedling diseases to young maize.
What late cover crop termination costs in water and nitrogen
A late kill costs stored soil water and available nitrogen. Which matters more depends on rainfall.
Stored soil water
A 2022 meta-analysis in Agronomy for Sustainable Development covering 38 studies found that cover crops in dryland systems reduced soil water at cash crop sowing by 18%, soil mineral nitrogen by 25%, and yield by 7%. It identified about 700 mm of annual rainfall as the break-even point for yield benefits.
USDA-ARS trials at Akron, Colorado, and Sidney, Nebraska, reported through Nebraska Extension, put a number on it. Wheat lost 5.94 bu/ac for every inch of water the cover crop used that rain did not replace — about 16 kg/ha per mm. If a cover crop leaves 50 mm (2 in) unreplaced, the next wheat crop loses about 800 kg/ha.
Nitrogen tie-up
As rye matures, its carbon-to-nitrogen ratio rises. A 2023 Field Crops Research study notes that residue above a C:N ratio of about 25 can release nitrogen slowly or cause soil microbes to lock it up, and that maize after rye often yields 1.5–7% less. A 2024 Agronomy Journal study in Nebraska found late-terminated rye had a higher C:N ratio, more seedling disease and lower maize yield.
Worked example: cereal rye before maize
A farmer plans to plant maize on 10 May. To keep a 14-day gap, the rye should be killed by 26 April.
In a Practical Farmers of Iowa trial, killing rye on the day of maize planting instead of 15 days before cost 14 bu/ac, about 0.9 t/ha. At an assumed farm-gate price of US$180 per tonne, that is roughly US$160/ha.
If rain delays the kill and the rye grows past about 20 cm, add starter nitrogen at planting. Nebraska Extension, citing the Midwest Cover Crop Council (2019), recommends 34–56 kg N/ha (30–50 lb N/ac). In many markets that costs less than the yield it protects.
Frequently asked questions
Can I plant maize into a living cover crop?
You can, but it carries more risk with maize than with soybean. Practical Farmers of Iowa strip trials from 2016 to 2018 recorded maize yield losses of 2–14% when rye was terminated within two days of planting. If you plant into living rye, apply starter nitrogen and watch closely for seedling disease.
How tall should cereal rye be when I terminate it before maize?
University of Minnesota Extension advises terminating rye before it reaches about 20 cm (8 in) ahead of maize. At this height, the rye has taken up useful nitrate but has not yet built the high carbon-to-nitrogen residue that locks up nitrogen during early maize growth.
Does a legume cover crop replace all nitrogen fertiliser?
Rarely. Hairy vetch and mucuna often supply 75–150 kg N/ha, but how much the next crop can use depends on termination date, residue placement and soil temperature. Reduce your fertiliser rate by a measured credit rather than removing it, and confirm with a soil nitrate test where one is available.
Termination dates depend on your planting date, your rainfall and how fast the cover crop is growing this season. Valora Earth’s WhatsApp advisor can work through these with a farmer by voice, in their own language, from their planned sowing date and a description of the cover crop in the field.
