Can cover crops be effectively used as biological herbicides?

-->

Introduction

In the world of organic crop production, cover crops are critical to growing and maintaining an economically viable cash crop. A cover crop is one that is planted either in the field with the cash crop (the one being brought to market) or is planted during any period in the season when no cash crop is planted, also known as a fallow period. Cover crops are not harvested for market, they are either left in the field or incorporated into the soil by tillage prior to planting a new cash crop.

The usage of a planned cover cropping system is not only beneficial to the cash crop but is also required by the regulations established by the National Organic Program (NOP) and mandated by the United States Food and Drug Administration(USDA). The USDA requires that a crop rotation must be established using cover crops which maintains or improves soil quality, provides pest management, manages soil fertility and provides erosion control(USDA). The guidelines for crop rotation are relatively vague but a smart producer will utilize cover cropping systems to the best of their ability, as they can have very significant benefits to quality of the agricultural ecosystem as well as the producers bottom line.

Many cover crops have beneficial interactions with cash crops such as fixing atmospheric nitrogen, that would be otherwise unavailable to the cash crop or competing with weeds for resources (Sarrantoning et al, 2003). In an organic system the producer is not permitted to use any synthetic herbicides, pesticides or fertilizers. Effective cover cropping allows a producer to manipulate the local ecosystem in ways that severely decrease or inhibit weed growth, increasing economic returns, while also being less harmful to the larger ecosystem than traditional agriculture.

 One of the ways that a producer can use cover crops to directly compete with weeds is by using crops that produce chemicals which in someway reduce a weeds ability to thrive, known as Allelopathy. Allelopathic effects are those caused by compounds released from plant roots, called root exudates, which directly effect and reduce the competitiveness of neighboring plants. By using these allelopathic cover crops producers can almost completely inhibit growth of certain weeds.  This post will focus on the concept of allelopathic cover cropping, defining how it functions competitively and highlight a novel example of a specific type of allelopathic cover crop (Brassicas/Mustard family) which has the additional benefit of reducing certain pests, demonstrating the concept that cover cropping systems can be established which integrate species in ways which can provide benefits for the health of the system, beyond their primary purpose.

BUT HOW?

The idea that a cover crop which inhibits the growth of other plants would be used in combination with a cash crop seems counter-intuitive at first. Wouldn’t the allelopathic cover crop in question inhibit the growth of the cash crop? In certain circumstances this would be true, and not all cash crops can be grown with allelopathic cover crops, but with the proper combinations of cover crop and cash crop, positive benefits of can be achieved. This is possible by selecting a combination of cover crop and cash crop which does not negatively affect the growth of the cash crop, or by separating the cover crop and cash crop in time, so that the effects of the cover crop are not felt by the cash crop.

In nature, many weeds have evolved to quickly and effectively fill a niche left when the previous flora was completely destroyed and the ground is bare, such as after a fire(Schonbeck ,2013). Plants that we now consider weeds have evolved to quickly and effectively colonize these newly open niches (Schonbeck, 2013). An agricultural setting provides the perfect niche for many of what we now consider weeds to thrive. The soil is many times left bare after harvest, replicating the fire or other disturbance that would sometimes happen naturally and opens this niche for weeds to enter. Weeds are able to very effectively out compete most row crops, this is accomplished by a number of factors, but for our purposes I will focus on the size of weed seeds. Most weeds have evolved to be prolific propagators, in many cases by having very small and very numerous seeds. It is this nature that allows allelopathic cover cropping to be effective as a weed control measure.

 The most substantial allelopathic effect is one that completely stops the seed from “sprouting”, also known as germinating, and does not allow the weed to take any of the nutrients or resources away from the cash crop. However, this is not always the case with allelopathic chemicals, in many cases allelopathic chemicals reduce the germination percentage of seeds and  inhibit growth of seedling without completely eradicating the weed (Putnam et al. 1988).  Putnam et al. 1983 shows us that among other factors, seed size plays a large role in the effect that phytotoxic chemicals have on the germination of weed seeds and their resulting seedling. Simply put, the smaller the seed the larger the effect that allelopathic chemicals can have. Larger seeded vegetables were consistently far more tolerant than weed seeds and smaller seeded vegetables such as lettuce. As shown in Figure 1, Lettuce and weed seeds were all severely limited by all allelopathic residues whereas larger seeded vegetables such as cucumber, peas or snap beans thrived and in some cases outperformed the controls. Interestingly, Tomatoes were reduced by some residues such as barley and sudangrass but were comparatively unaffected by oats residues when compared to the weeds. So, like many things in ecology and agriculture, there are few "rules" which can be applied to any specific situation but general concepts can be combined with more tailored practices to achieve the desired effect.        

Figure 1

Mustard, Not just for hotdogs anymore 

While many allelopathic cover crops such as rye, crimson clover or hairy vetch have been well researched and documented others, such as brassicas or the mustard family, have been less researched (Haramoto Et al 2004). All members of the Brassicaceae family produce root exudates known as Glucosinolates. Glucosinolates are not toxic but are biologically broken down in the root-zone or “rhizosphere” into groups of chemicals known as isothiocyanates (Haramoto Et al 2004). As shown in figure 2, Isothiocyanates can be very effective at controlling germination of many troublesome types of weed seeds. Isothiocyanates were shown to be effective against controlling crabgrass, dormant crabgrass, mayweed, pigweed, sow thistle, black grass and velvet leaf. However, the ability of members of the Brassicaceae family to control weeds is not what makes them more interesting than other well known allelopathic cover crops. 

Figure 2 

Members of the Brassicaceae family have also been shown to be effective in controlling other pests such as the fungus responsible for  "Take-all Root Rot", the fungus Gaeumannomyces graminis var. tritici, as well as root-knot nematodes cause by the parasite Meloidogyne incognito. Montford et al. 2007 shows that brassicaceae species can at least be a helpful part in a integrated system of disease management and can help reduce losses from G. graminis, which is a problematic fungal pathogen for vegetables, especially in our region. Angus Et al. 1994 shows that Brassicaceae species can also be very effective in reducing or eliminating root knot nematodes. Root-knot nematodes are a parasite which can build up in the rhizosphere over the seasons in agroecosystems and can be very economically damaging to producers, while also having few effective controls, especially in an organic system.  

There is no one combination of cover crops or any other organic system that can be applied to any field or crop in order to achieve economic and ecologically "healthy" results, but by carefully considering and researching the cash crop in question, a producer can create a cropping system that integrates useful and effective cover crops to effectively achieve their goals. The effects of cover cropping can be: increased soil fertility, decreased run-off, capturing nutrients or combating the effects of weeds and pests among others, and all of these effects can be achieved when producers effectively design their crop system. 

Bibliography

United States Department of Agriculture; Guidelines for Organic Crop Certification, https://www.ams.usda.gov/sites/default/files/media/Crop%20-%20Guidelines.pdf

Sarrantoniog, M. and Gallandt, E. 2003. The role of cover crops in North American cropping systems. Journal of Crop Production 8:53–73.

Schonbeck, M. 2013. An Ecological Understanding of Weeds. Virginia association of biological farming. eOrganic.

Putnam, A. R., and J. DeFrank. 1983. Use of phytotoxic plant residues for selective weed control. Crop Protection 2: 173–181

Putnam, A.R. 1988. Allelopathy: problems and opportunities in weed management. In Altieri, M.A. andLiebman, M. (eds). Weed Management in Agroecosystems: Ecological Approaches. CRC Press, Boca Raton, FL. p. 77–88

Haramoto, E., 7 Gallandt, E. (2004) Brassica cover cropping for weed management: A Review. Renewable Agriculture and Food Systems, 19(4), 187-198

W.S. Monfort, A.S. Csinos, J. Desaeger, K. Seebold, T.M.Webster, J.C. Diaz-PerezEvaluating Brassica species as an alternative control measure for root-knot nematode (M. incognita) in Georgia vegetable plasticulture Crop Prot., 26 (2007), pp. 1359-1368

-->

Angus, J.Gardner, P.Kirkegaard, J. and Desmarchelier, J. 1994. Biofumigation: isothiocyanates released from Brassica roots inhibit growth of the take-all fungus. Plant and Soil.

--> -->