NC State Extension Publications


Interest in cover crop mulches has increased out of both economic and soil conservation concerns. The number of tractor passes required to produce corn and soybeans organically is expensive and logistically challenging. Farmers currently use blind cultivators, such as a rotary hoe or flex-tine harrow, two to five times, followed by two to four between-row cultivations (see “Weed Management,” Chapter 7). Repeated cultivations can be extremely effective but are highly sensitive to weather. During wet springs, cultivations are missed, leading to weedy fields. Soybeans are particularly troublesome when blind cultivations are missed. Soybean plants cannot have as much soil thrown up to their stems as corn, meaning in-row weeds missed by blind cultivation remain in the field until harvest time.

Killing Cover Crops Without Herbicides

Rolling cover crops has proven to be the most effective and inexpensive method for organic farmers to kill cover crops. Multiple designs for roller-crimpers exist (Figure 8-1) to accommodate flat or bedded plantings. Free designs are available at the Rodale Institute’s website. The design advocated by Rodale is manufactured and sold by I & J Manufacturing of Gap, PA. Roller-crimper designs are engineered to maximize force on cover crop stems and to minimize vibrations on the tractor frame. Even so, farmers have reported occasional success using preexisting equipment such as cultipackers and smooth rollers. A combination of the right cover crop growth stage and hot weather can result in a good kill without having the optimal equipment.

Cover crops must be at least flowering or in the early stage of seed set before they can be killed with a roller. Each species has its set of characteristics to look for in determining when it can be roll-killed. Typical times and the growth stage characteristicsare listed in Tables 8-1 and 8-2. Same day rolling and planting has been advocated by some institutions. The roller can be mounted on the front of the tractor with the planter in the rear. While reducing trips, the soils are significantly drier at planting with the sameday approach. Researchers in Alabama, Georgia, and Maryland recommend rolling and waiting for rainfall to replenish soil moisture before planting. In North Carolina, only same-day planting has been tested so far, with good germination in soybean in seven out of nine trials, one of which had to be replanted to prevent total loss. Rolling and planting on different days presents a problem for farmers without GPS guidance. Rolling and planting must occur in the same direction, making the issue of what pattern to use when rolling and how to mark the passes with the planter a significant one.

Figure 8-1 shows roller crimper designs: (a) Roller crimper for elevated beds: one row and two furrows; (b) roller for elevated beds: two rows and three furrows; (c) two-stage roller/crimper designed to operate with smaller tractors (40HP power source); (d) typical roller for flat planted crops.

Figure 8-1.

Figure 8-1.

Images provided by the NC Organic Grain Project.

Cover Crop Bloom Chart for North Carolina

These tables estimate the time of full bloom or ideal time for rolling the crop to fully kill the cover crop so that corn or soybeans can be planted into the rolled mulch.

Table 8-1. Roll times for legume cover crops. Adapted from Parr et al., 2011.
Late April/
Early May
100% flower

Crimson clover

AU Robin
AU Sunrise

Hairy vetch AU Early Cover
Late May Early pod set Hairy vetch AU Merit
Steve Groff
Late May Early pod set Winter pea variety unstated from NC
Early June Early pod set Common vetch variety unstated from NC
100% flower Berseem clover Bigbee
*Timinig can vary in unusual years

Table 8-2. Roll times for rye (Secale cereale L.) cover crops. Rye should be rolled during the early milk to soft dough growth stages.
Late April Wrens Abruzzi
Wrens 96
Early May Aroostook
Mid to late May Rymin
*Timing can vary in unusual years.


Several features of a no-till planter are essential for such a high residue environment:

  • Straight edged no-till coulters. Fluted edges lead to hair-pinning of the cover crop into the seed trench. Straight edges with bubbles or flutes are ok and help to loosen the germination zone on some soils.
  • Heavy duty down-force springs or a pneumatic down-force system is often needed.
  • Some planters may need extra weight mounted on the frame. If fertilizer or insecticide boxes are unused, they can be filled with sandbags.
  • The best type of closing wheel for this system is still being debated. Even with the heaviest of closers, sealing the trench can be difficult at times.

Weed Control

To get consistent weed control and good soybean yields, more than 8,000 lb of rye dry matter is needed (Figure 8-2). Early planting and adequate fertility are required to obtain this level of production. Even then, overly wet winters inhibit rye growth. Walking fields in March is recommended to assess whether the stand is sufficient for good rye production. A poor or spotty stand should be disked in and a clean tillage system used. If rye becomes too large before disking, it can be difficult to prepare a clean seedbed and to cultivate.

Less is known about the performance of legume–corn systems here in terms of weed control. Mixtures of legumes and rye are capable of producing more than 8,000 lb of dry matter (Figure 8-3). Whether this 8,000 lb cutoff applies to legume–corn systems is still unknown.

N Fertility in the Legume–Corn System

Legumes can be grown in monoculture or as a mixture with a small grain. Legumes are capable of providing a substantial proportion of the N demand for corn. While N production varies significantly from year to year, a good rule of thumb is that the legume can provide two-thirds of the needed nitrogen. Pelleted organic fertilizer can be applied at planting in only limited amounts with normal granular fertilizer boxes. For instance, John Deere granular boxes, with the high rate auger installed, can deliver approximately 700 lb of conventional fertilizer at the highest setting. Pelleted feather meal, as an example, is less dense and only 500 lb can be delivered. At 12 to 13 percent N, the total N that can be applied is 65 lb.

Assuming adequate legume cover crop growth, this may be sufficient. Other organic fertilizers are less rich in N, such as pelleted chicken litter, and therefore even less N can be applied. Clampco sells organic fertilizer boxes capable of putting down higher rates of granular and pelleted material than conventional fertilizer boxes. Another option is to broadcast chicken litter over the top at or soon after planting. While this practice would be allowed for feed crops, manures must be soil incorporated at least 90 days before harvest for all food-grade grains.

Figure 8-2 shows soybean yield in roll-crimped rye mulches at Goldsboro, Kinston and Plymouth, NC in 2008 and 2009. Treatments consisted of rolled rye mulch with no additional weed control measures versus a weed free check that consisted of conventional tillage with S-metoloachlor PRE and imazethpyr POST.

Figure 8-3 shows productivity of legumes and legume–cereal mixes in the Southeast from 2008 to 2010. Figures come from a pooled analysis of trials in Georgia (GA) and North Carolina (NC). Box plots were created from nine site-years for all cover crops except lupine and lupine–rye, with four to six replicates per site-year. Lupine data is from GA only (five site-years) due to extremely low productivity in NC. Crimson clover varieties: Trifolium incarnatum L. AU Sunrise in NC; Dixie in GA. Vetch varieties: Vicia villosa Roth AU Early Cover in NC; Vicia sativa L. Cahaba White in GA. Winter pea varieties: Pisum sativum L. subsp. sativum var. arvense (L.) Pior variety unstated in NC and GA. Narrow-leaf lupine varieties: Lupinus angustifolius L. TifBlue 78 in NC and GA. NC data adapted from Parr et al. 20115. Rye, (Secale cereale L.) at both locations was Wrens abruzzi grown without added N fertility.

Return to North Carolina Organic Grain Production Guide.

Figure 8-2.

Figure 8-2.

Adapted from Smith et al., 2011.

Figure 8-3.

Figure 8-3.


Associate Professor
Crop and Soil Sciences
Research Ecologist
Cooperative Extension
University of Massachusetts
Soil Science Assistant Professor
Crop and Soil Sciences
Agricultural Engineer
Research Agronomist
Plant Physiologist
Extension Associate: Tillage & Soil Management
Crop and Soil Sciences
Research Associate
Crop and Soil Sciences

Publication date: Feb. 10, 2014

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