University of Maryland Extension

Offing Cover Crops for Weed Suppression: Featuring the Roller Crimper and Other Mechanical Contraptions

Author: 
!Associate Professor and Extension Specialist, $Graduate Student and *Faculty Research Associate University of Maryland, Department of Entomology

!Associate Professor and Extension Specialist, $Graduate Student and *Faculty Research Associate
University of Maryland, Department of Entomology

 

No-Till (NT) Cover Crop Termination

Cover crops are an integral component of conservation practices. As part of a vegetable or field crop’s rotation system, cover crops are typically planted from late summer to early fall and terminated in the spring. The killed residue then acts as a dying mulch on the soil surface. Cover crops used as part of a money (cash) crop rotation system can provide a variety of ecosystem services, including enhanced soil quality and health, reduced nutrient losses via leaching, and pest suppression. In no-till cropping systems, producers generally use a “burn down” herbicide to off (terminate) their cover crops prior to planting their money crop. No-till and other conservation tillage practices may provide beneficial services that cannot be obtained from conventional tillage. No-till farming can reduce soil erosion, labor and fuel inputs, allow the build-up of soil organic matter and improve other soil features. In addition, NT cover crop rotation system may be a valuable tool for managing weeds. Research has shown that NT cover crop-soybean systems achieve greater weed suppression than intensively tilled soybean habitats.

While burn-down herbicides are common, there are non-chemical alternatives for offing cover crops that are compatible with no-till production. Mechanically offing cover crops limits soil disturbance and can help reduce synthetic chemical use. Mowing (e.g., bush hog rotary, flail mowing) is probably the most popular method for mechanically terminating cover crops. Another gadget, which is not as well known, is the undercutter-roller. This is a specialized implement, designed to slice shallowly through the soil and cut cover crop roots underground. It consists of a V-plow sweep blade mounted on a toolbar, followed by a rolling harrow to crimp and roll the cover crop as it falls to the ground, creating a thick and uniform mat of residue. Weed suppression was shown to be greater when an undercutter was used to off a cover crop versus a mower.  

Enter the Cover-Crop Roller Crimper

One of the latest thingamajigs available for offing cover crops in no-till operations is the cover-crop roller-crimper (RC). The RC has made using cover crop residue to suppress weeds in NT operations more practical for organic farmers. While the act of terminating a cover crop with a roller-crimper, or what we refer to as “Big Crimpin”, has not yet gone viral, it is steadily picking up steam. Though, the RC is sold at a limited number of establishments, its rapidly rising stardom should result in it being more widely accessible in the future. Roller crimpers are reported to have been adapted from equiment used in southern Brazil and Paraguay that lays cover crops down while crimping. A RC is essentially a cylinder with protuding fins that rotates on a lengthwise axis as it moves over the soil. It consists of a hollow drum (with the option of being filled with water for increased weight) and blunted blades meant to maximize force against cover crops without cutting their stems. The implement crimps the vascular tissue which leaves plants intact and attached to their roots. This results in a flattened uniform layer of mulch that persists on the soil surface since decomposition is slowed. Subsequently, weed suppression lasts longer than in mowed mulch habitats. In a study comparing rolled and flail-mowed rye, rolled rye had significantly fewer weeds during year 1, but were similar among the two tactics the following year. Still, RCs do not require an energy intensive power take-off (PTO) drive and thus require less fuel and energy than mowers; Additionally, RCs operate at much faster tractor speeds than flail mowers in effectively terminating cover crops. In addition, if a crimper is mounted on the front of the tractor, there is the potential to terminate the cover crop and plant the money crop in the same pass. Partially for these reasons, RCs are thought to be a more sustainable investment for offing cover crops.

Integrating Big Crimpin’ with other tactics

To make the practice of cover cropping economically viable and more effective, additional management tactics may need to be added. Weed suppression through the use of cover crops should be used as part of an integrated weed management (IWM) program that includes numerous methods to help suppress weeds including ecological approaches. Timing of cover crop planting and termination can be used as an important tool to predict quantity and quality of cover crop biomass. Additional tactics could include adjusting crop row spacing and plant density, selecting competitive crop cultivars, adapting a high-residue cultivation system, and decreasing weed seed bank through intensive weed management in other crop sequence phases. Combining an effective herbicide with a roller crimper could offer the benefits of earlier and excellent cover crop control along with weed suppression. However, this may quicken residue breakdown. Multi-tactical weed management programs including rotational tillage (tilling soils every two years or less often), diverse crop rotations, and enhancement of money crop competiveness can play a role in IWM programs involving RCs and will be most effective if such acts concomitantly help deplete the weed seed bank. Further, any management tactic that limits weed seed return to the soil is a vital component of IWM because it helps deplete weed seed banks and ultimately reduces weed pressure and management cost in the long run.

What cover crop should be used with the RC?

Most research conducted in the Northeast with respect to RCs has used cereal rye as the test cover crop. Many humans think of rye as a cover crop super hero. Rye doesn’t wear a cape on the outside of its seed coat; however, rye is extremely cold tolerant so a cape is not practical. Other super natural powers of rye include: flexible establishment date, rapid emergence, ability to develop a fibrous root system, tolerance to low fertility soils, easy uptake of available nitrogen, weed suppression, and soil erosion prevention all in a single growing season. Grass cover crops such as rye, barley, spelt and triticale have a higher C:N compared to legumes (crimson clover, Austria winter pea, red clover, etc.). As such, residue mulches persist a considerably longer period of time after termination. The persistence of the grass residue results in longer-lasting soil coverage, which protects soils from erosion while preventing weed emergence. Further, grass cover crops are more capable of retrieving soil-applied nitrogen. The ability of grass cover crops to grow in winter/spring and sequester nutrients makes them an ideal RC partner, particularly in watersheds that suffer from excess nutrient runoffs such as the Chesapeake Bay Region of the United States. However, we tend to believe that the greatest amount of ecosystem services can be obtained by using cover crop mixtures and specifically mixing grass and legume species. Many legumes can be readily terminated with a RC at the flowering stage. Unfortunately, such mixtures especially those that include a legume may not be in compliance with state cover crop cost share programs.

Limitations of Physical Termination Methods

Each physical method used for offing cover crops has rewards and shortcomings. For example, mowing (e.g., rotary, flail, stalk chopper) may result in cover crop regrowth and unequal displacement of residue on the soil surface. Uniform ground cover is important if cover crop residue is to aid in weed suppression because weeds capitalize in niche spaces and germinate in areas without residue. In the past, producers wanting to suppress weeds mostly used a herbicide to off their cover crops. Mechanical methods were not cost effective and resulting cover crop residues were less persistent and uniform. Uniform ground cover can be obtained by using a flail mower with the appropriate cutting blades, but similar to other mowing instruments that macerate plant tissue, cover crop breakdown is hastened. Studies have shown that mowed mulch decomposition rate is accelerated compared to rolled mulches. Further, flail mowing requires a low tractor speed (2 to 5 mph) to effectively mow cover crop stands unless there is little biomass. Still mowers are versatile, easily available and more commonly owned.

Other potential disadvantages of using mechanical termination methods include cover crop “pop ups” (RC) and regrowth (mower) which may affect crop establishment. This generally occurs if the cover crop is mechanically terminated at an early development stage and/or in the case of the crimper there is not enough “crimper weight”. These problems can be remedied by adding water inside the RC drum and offing the cover crop at the appropriate stage of development. For example, research has shown that it is important to delay mechanical termination of cereal rye until 50% of plants have reached anthesis (flowering) to prevent regrowth and ensure successful termination. In Pennsylvania, rye was consistently controlled at anthesis with a RC, but using a RC prior to this stage was less effective. Unfortunately, waiting for cover crops to flower can delay money crop planting. If the delay is significant, for example in the case of soybeans, yield reductions can occur. Further, soybean stands may be reduced if planted into a thick cover crop residue which decreases the seed-to-soil contact. This may be avoided by planting the soybean directly into standing cover crop and offing the cover crop while soybean seedlings are small enough to avoid being killed during cover crop termination. This tactic should reduce soybean seed-soil contact problems and subsequently improve weed management.

Recent advancements in crimper design has improved their efficiency and comfort. Tractor operators using older RC models felt they were riding an excessively vibrating dryer as opposed to big crimpin. Further, older RCs were less efficient in offing cover crops and planting into the cover crop mulch was not always easy. Despite improvements and the promise RC shows for cover crop termination, it is still not very effective in killing cover crops at earlier growth stages and is not useful for directly offing weeds; the sweep plow undercutter is likely a better bet for terminating young cover crops. Moreover, the RC is not as easily obtainable compared to other farming implements. Because a RC only crimps, it may not provide direct farm services beyond offing a cover crop and thus may not be a good investment for producers not cover cropping or using conservation tillage practices. Unlike mowers, RCs can’t moonlight with landscapers.

How does the crimper compare to the burndown system

Successful weed management in no-till farming systems depends upon the use of reliable tactics that are not second-rate to synthetic herbicides. A study was conducted in Illinois to investigate the potential of the roller-crimper to inhibit weed development in a no-till soybean system with minimal or no reliance upon herbicides. As part of the investigation, they compared “Big Crimpin” with the “Burndown” tactic. Following vetch or rye cover crop, crimping reduced weed biomass 26 and 56% more, respectively, than burndown. In contrast, burndown reduced weed biomass more in the no-cover crop control habitats, suggesting that the RC did not effectively kill weeds. Yield reduction caused by weed interference was unaffected by cover-crop termination method (RC or burndown) in soybean plots following rye, but yield lost was higher in RC than burndown treatment in both hairy vetch and control treatments. This study showed that using a RC to off a rye cover crop can reduce weeds and maintain high yields in no-till soybean. In production systems where herbicide use is an option, chemically offing the cover crop allows for termination at younger stages relative to mechanical termination and this reduces the risk of losing yield potential in some crops. However, using a RC to off a cover crop could be an important option for growers seeking a non-chemical method for managing weeds that minimizes labor, fuel costs and soil disturbances. This study found also that agronomic benefits of offing cover crops with a RC are contingent on cover crop species.

How do cover crops aid in weed management

Cover crops can aid in weed management as a living mulch by competing for resources such as sunlight, space and nutrients, and as a dead mulch on the soil surface that suppresses weed emergence. Surface mulch can inhibit weed growth by providing a physical barrier to weeds, intercepting light before it reaches weeds (reducing light availability), lowering soil surface temperature, physically blocking weed growth, and increasing weed seed predator populations. Cover crop mulches that reduce light levels at the soil surface slows photosynthesis and warming of soils in the spring. These conditions reduce weed seed germination and act as a physical barrier to their emergence. If weed management is mainly achieved by the cover crop physically blocking weed growth, cover crop biomass is critical. The level of weed suppression depends on the amount of cover crop biomass that accumulates prior to termination, with an exponential relationship between biomass and weed emergence. Depending on the amount of residue and termination method, sufficient weed control has been shown to last from 4 to 16 weeks into the season following cover crop termination.

Though biomass is critical for weed suppression, large amounts of cover crop residue at money crop planting time may be challenging to producers that direct seed. However, improvements in planter and drill technology have alleviated some of these NT planting pitfalls; and though excessive cover crop residue can be potentially challenging, more weeds may emerge in low levels of residue than in bare-ground plots. How is this possible? Low cover crop residue is not sufficient to inhibit weeds from emerging but can create environments more conducive for weed germination and emergence. Low residue can impede evaporation of soil moisture and thus provide more uniform moisture conditions for weed germination and emergence than would exist on the surface of bare-soil. Also, nitrogenous compounds released into the germination zone, particularly from legume cover crops, can stimulate certain weed species to germinate.

Some cover crops aid in weed suppression by releasing allelochemicals or allelopathic phytotoxins that are toxic to weed seeds. When residues of allelopathic cover crops decompose into the soil, phytotoxins may be released that inhibit the emergence and growth of many weed species. While allelopathic mechanisms of weed suppression have been well studied and are soundly understood for some cover crop species, there is little known about potential interactions between cover crop mixtures and weed seed germination and growth.

How to Better Manage Cover Crops for Weed Suppression

Termination technique. Cover crop choice is important, but cover crop termination technique and residue management are considered by some to be the most critical factors in successfully using cover crops for weed suppression. Cover crops can be terminated climatically (e.g., winterkill), naturally (e.g., senescent), chemically or via physical or mechanical tactics (e.g., plowing, disking, mowing, big-crimpin or undercutting). The most appropriate termination method will depend on the production objective and equipment fleet. For example, incorporation of cover crop (e.g., green manures) into the soil is most common when the goal is to increase soil nutrients. Weed suppression can be enhanced by incorporating cover crop residues that release greater amounts of allelochemicals within the soil. Tilling in the top growth of an allelopathic green manure causes an intense but relatively brief burst of allelopathic activity throughout the till depth but leaving the residue on the surface as an in situ mulch creates a shallow (less than one inch) but more persistent allelopathic zone that can last for 3 to 10 weeks depending on weather and soil conditions. Thus, no-till cover crop management offers a potential for selective suppression of small-seeded annual weeds in transplanted and large-seeded crops, whose roots grow mostly below the allelopathic zone. Thus, when weed management is a priority and allelopathy is needed to suppress shallow small-seeded weeds, termination of allelopathic cover crops resulting in maximum and extended surface residue and minimal soil disturbance has the greatest potential to inhibit weed seed germination, establishment and growth.

Cover crop biomass will vary widely depending on its growth stage at termination. The growth stage is partially influenced by timing of fall planting and spring termination. Most winter cover crops are not grown to full maturity, so achieving optimum biomass and soil coverage is difficult. Winter cover-crop biomass accumulates in spring, with low accrual early on and growth at break neck speed later in the spring. A study showed that termination date in the spring had a greater influence on final cover crop biomass than planting date in the fall. For example, a delay in rye termination in the spring resulted in an increase in cover crop biomass and an associated decrease in summer annual weed populations. Rye termination delayed from May 01 to 30 resulted in an increase in cover crop biomass from 400 g m -2 to 1000 g m-2 and an associated reduction in weed density from 36 to 24 plants m-2. In another study, delaying rye termination by 10 to 20 days nearly doubled its biomass, but did not consistently improve weed control. It was suggested that the earlier termination dates (April 24 to May 17) followed by soybean planting along with competitive rye contributed to these results. However, even at early termination, weed density and biomass were reduced compared with treatments without rye cover crop. In an effort to delay cover crop termination, the money crop could be sown into the living cover crop which is suppressed at a later date, or a shorter season money crop cultivar could be chosen which will allow it to be planted later in the season. If a cover crop has to be terminated early in the spring, another approach is to plant it early the previous fall and choose a cover crop that provides additional mechanisms of weed suppression such as allelopathic activity. Enhancing the synchrony between high cover-crop biomass and soil coverage, and weed emergence can improve weed management drastically.

Cover crop cultivar. Choosing the appropriate cover crop cultivar is critical. For instance, the problem associated with having to terminate a cover crop prior to it reaching maturity in spring may be overcome by using an early-maturing cultivar. This may allow greater flexibility of cover crop termination. Some cultivars may produce greater biomass compared to others. A study found that the rye cultivar ‘Aroostook’ consistently produced greater biomass than ‘Wheeler’.  Increasing cover crop seeding and fertilization rates are two additional tactics to increase cover crop biomass. However, applying fertilizer may be counter intuitive since cover crops are often planted to scavenge nitrogen from the soil profile. Further, it was shown that increasing rye biomass by applying poultry litter did not lower weed biomass. Weeds are known to disproportionately benefit from over fertilization, so that crop competitiveness can be increased by temporarily and spatially managing soil nutrient availability. Soybeans and other legumes that fix atmospheric nitrogen have a competitive edge over weeds when soil nitrogen levels are low.

Seeding rate. Increasing cover crop seeding rate can reduce weed biomass without an associated increase in cover crop biomass. An increase in rye seeding rate was shown to reduce weed biomass without an increase in rye biomass by time of termination. In the study, rye was planted at 90, 150 and 210 Kg seed ha-1 which is equivalent to 80, 135 and 185 lbs seed/a-1, respectively.  It is likely that the greater seeding rate increased cover crop residue coverage. It has been suggested that it is necessary to achieve 97% soil coverage with cover crop residue to reduce weed density by 75% with respect to light interception. Thus, tactics that increase the amount of soil surface coverage by cover crop residue could lead to greater and lengthy weed suppression.

Diversity. Increasing cover crop diversity may impact weeds differently as a mixture of cover crop species may more readily suppress a broader range of weed species. Using a single cover crop species is popular due to the simplicity of planting, uniform development, predictable growth stage and termination efficacy. However, mixed cover crop habitats may provide greater weed control by increasing the number of mechanisms that contribute to weed suppression. A mixture of cover crop species with complementary growth features could increase weed control by way of greater overall cover crop shoot biomass accumulation, appropriately timed degradation of residue, and a broader spectrum of allelopathic activity. Multi-species mixtures may enhance productivity, stability, resilience, and provide greater on-farm services than single species. In a study evaluating cover crop mixtures, it was found that increasing cover crop diversity increased biomass productivity in 2 of 3 study years and that diverse cover crop mixtures were more resilient following management error and severe weather disturbance.

Finishing statement

Whether conventional or organic, producers should rely on multiple weed suppression tactics. Various methods include crop rotation, flaming, weed seed predation, smother crops, competitive crop cultivars, cultivation, cover cropping and etc. Frequent cultivation is the core of many organic weed control programs. However, continuous cultivation has negative consequences on soil quality and health and increases input costs. Weed management tactics that integrates no-till practices with mechanically terminated cover crops should maximize ground coverage, minimize soil disturbances and avoid causing stand reductions of the money crop. In addition to manipulating cover crops for weed suppression, there is the potential to directly manipulate the money crop. For instance, planting soybean in narrower rows or increasing the seeding rate places greater emphasis on their competitive ability for weed suppression. This suggests that cover crops don’t have to be a standalone approach and can be integrated with other tactics to formulate a successful IWM program that is less dependent on herbicide intervention. 

 

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