Cover crops

Across 268 observations from 65 studies (1965–2015), grass covers such as rye neither raised nor lowered corn yield on average. Legume covers raised corn yield 30–33% only where fertilizer N was low or the system was no-till.

Satellite yield maps for more than 90,000 Corn Belt fields in 2019–20: an average corn yield loss of 5.5% on fields cover-cropped three or more years and a 3.5% soybean loss, versus non-adopting fields. Corn losses were larger on better soils, in cooler mid-seasons and with low spring rainfall.

In no-till corn–soybean plots at four Iowa sites (2009–11), corn yield at the economic optimum N rate was 6% lower after rye even though rye biomass peaked at only 1,280 kg/ha; the optimum N rate itself did not change. Soybean yield was unaffected.

Over 19 years (2002–20) of tile-drained corn–soybean plots in Iowa, an annual rye cover crop cut nitrate leaching 59% with minimal or no yield penalty.

Among 1,172 farmer respondents, self-reported 2019 yields after cover crops were 2% higher for corn and 5% higher for soybean. The most common seed-cost bracket was $11–15/ac.

Across 392 corn-yield observations from 55 site-years, legume cover crops raised corn yield 51% when no N was applied, a benefit that shrank as N rate rose; grass covers cut unfertilized corn yield 20%. Late termination gave a larger unfertilized-yield benefit than early termination.

Across 268 observations from 65 studies (1965–2015), grass covers such as rye neither raised nor lowered corn yield on average. Legume covers raised corn yield 30–33% only where fertilizer N was low or the system was no-till.

In a three-year southwestern Ontario rotation, summer-seeded crimson clover carried 119 kg N/ha at spring plow-down (hairy vetch 240, red clover 199). Corn after the legumes averaged 13.1 Mg/ha versus 6.4 for organic plots with no cover crop.

From hundreds of Pennsylvania measurements, crimson clover biomass at spring termination averaged 79 lb N/ac (range 53–105), against 164 (134–194) for hairy vetch and 91 (45–137) for red clover. Penn State calls its cover-crop N recommendation system highly experimental.

As a winter annual in Zone 8 crimson clover fixes 70–150 lb N/ac by mid-May; a Michigan summer-annual stand made only 50–60 lb N/ac. It overwinters dependably in Zone 8, can survive the lower reaches of Zone 6, and winter-kills in Zone 5 and colder.

The project examined nitrogen capture, root channels, weeds, and following crops. Radish released nitrogen early after winterkill; results depended on establishment, soil, and crop timing.

Species guidance covers establishment, nutrient cycling, and rotation constraints, including shared diseases with brassica vegetables.

Pooled mixture biomass was 63% greater than hairy vetch alone and 21% greater than cereal rye alone. Study conditions affected the relative response.

At the Maryland Coastal Plain site the economic N rate for no-till corn was 30 kg/ha after hairy vetch, 65 after a vetch–rye mix, 161 after rye and 201 with no cover; late-April or early-May kills consistently out-yielded earlier kills.

Between late March and early May hairy vetch added about 2 kg N/ha per day, reaching 144–203 kg N/ha in topgrowth; corn N requirement was lowest after vetch, intermediate after vetch–rye and highest after rye.

Across 55 site-years from 21 US studies, vetch–rye mixtures produced 63% more biomass than vetch alone and held 150% more N than rye alone; with vetch above 46% of the seed mix by weight, the mixture accumulated as much N as the best monoculture.

At two Maryland sites, spring N content rose from 64 to 181 kg N/ha as the vetch share of biomass went from 0 to 100%, plateauing once vetch reached about half the stand; biological fixation supplied 63% of vetch N in monoculture and 86% in mixture.

Maryland vetch–rye mixtures accumulated 130–180 lb N/ac, and late-April or early-May kills with corn no-tilled 10 days later consistently out-yielded earlier kills. Winter hardy through Zone 4; drill 15–20 lb/ac or broadcast 25–30.

Dry matter of 1,000 to 4,000 lb/ac per year depending on planting and termination dates and precipitation; oats usually winter kill in Iowa and need no spring termination. Drill 60 lb/ac PLS from mid-August to mid-September or early to late April; C:N ratio 33:1; grazing quality rated very good.

Expect 2,000 to 4,000 lb of dry matter per acre from late-summer or early-fall seeded oats and up to 8,000 lb from spring stands; late-summer plantings can absorb as much as 77 lb N/ac in eight to ten weeks. Oats winterkill in Zone 6 and colder and much of Zone 7.

Oats seeded at 10 to 60 lb/ac with radish at 1 to 6 lb/ac, planted before mid-September, produced more than 850 lb/ac of biomass at St. Paul, 2,200 to 3,000 lb/ac at Waseca and 425 to 540 lb/ac at Lamberton.

Drilled 8/15/18 at 32 lb/ac at Hickory Corners, Michigan, the Jerry oat check made 1.07 ton DM/ac of fall biomass and Pratext black oats 2.59; all oats winterkilled but left 0.55 to 1.32 ton/ac of dead residue cover in spring.

In Pennsylvania measurements at maximum spring growth before termination, the oats (spring) row averaged 19 lb N/ac in biomass with a range of 12 to 26, against 32 to 54 for cereal rye or wheat by growth stage.

Corn N-rate trials after frost-seeded red clover measured N credits of 46 lb N/ac at Janesville (2010) and 57, 75 and 100 lb N/ac at Arlington (2016, 2018, 2019), with 80 to 110 lb N/ac in the clover’s above-ground biomass; the 2016 site also had a 16 bu/ac yield drag.

Red clover under-seeded to winter wheat in Ontario provides an 82 kg/ha nitrogen credit to the following corn; across twelve locations with four wheat N rates, conventional tillage gave 48% more red clover dry weight and 6.7% more wheat yield than no-till.

In a 20-year Ontario experiment comparing seven rotations under moldboard and chisel plowing, corn yield in the corn–soybean–wheat rotation was 5% higher when the wheat was underseeded with red clover, in the moldboard system.

In Pennsylvania measurements at maximum spring growth before termination, red clover biomass averaged 91 lb N/ac with a range of 45 to 137, against 79 (53–105) for crimson clover and 164 (134–194) for hairy vetch.

Full-season over-wintered red clover can produce 2 to 4 tons of dry matter and fix 70 to 150 lb N/ac; a two-year Michigan trial logged a fertilizer replacement value of 36 lb N/ac. An Ohio farmer frost-seeds 10 to 12 lb/ac into wheat in February; winter hardy in Zone 4 and warmer.