This article discusses the limitations of using brassica cover crops for weed control. A brief overview of the role of cover crops is provided, followed by a short review of research looking at brassica cover crops.

Cover crops in organic agriculture

Cover crops or green manures have played a vital role in organic agriculture due to their wide variety of benefits in the farming system. Including cover crops in the crop rotation contributes to nitrogen fixation by legumes, re-mobilising unavailable nutrients (e.g. too deep in soil for cash crops, bound to clay), bulky carbon inputs from cereals, breaking pest and disease cycles, hosting beneficial organisms, and maintaining ground cover, thereby conserving soil moisture and reducing erosion.

Figure 2. Poor establishment by brassica cover crops grown for a weed suppression. Note good establishment by the ryegrass (dark green strips).

Many of the benefits that can be obtained from using cover crops are closely related to each other. For example, increasing soil organic matter with green manures provides better nutrient and water holding capacity and improve soil structure. The overall improvement in soil conditions allows for better plant vigour (outcompete weeds) and hardiness (resist or tolerate pests and diseases). The extra soil organic matter also provides more ‘food’ for the soil microbes to consume, increasing overall microbial activity and reducing the impact of harmful soil organisms, further improving crop performance.

The benefits of cover crops are many, but a successful cover crop requires good management and attention to detail. The establishment and maintenance of cover crops also needs to be closely integrated with other aspects of the production system over time and space, just as it is for a cash crop. While cover crops commonly have no direct cash benefit, the benefits, or ‘ecosystem services’, listed above provide a valuable indirect contribution to the farm budget. Many of these benefits can be measured (e.g. soil nitrogen produced, disease severity), but there are several other potential advantages with off-farm implications, such as atmospheric carbon sequestration and total energy consumption, that are more difficult to estimate.

Figure 3. Root rot in fodder radish due to waterlogging.

The cover crop needs to be well planned within the farm’s rotation schedule to maximise the range of benefits achieved, e.g. weed suppression by rye versus nitrogen gain with vetch? In many cases, a mixture of annual plants would be appropriate. Selecting crop varieties that are suited to local soil and climatic conditions often requires some trial and error, and while it is easy to stick with a regular formula, using cover crops from a mix of plant families – not just legumes and cereals – introduces a greater level of biological and agricultural diversity to the farm’s ecosystem. Other crops such as marigolds in the daisy family, buckwheat in the dock family and the various mustards, radishes, turnips and so on in the brassica family also offer unique characteristics such as nematode, weed and fungal control.

Brassica, biofumigation and bioherbicides

The use of plants from the Brassicaceae family, or brassicas, as cover crops has long been observed to have a cleansing effect on soils with certain plant pests, including root knot nematode and cereal take-all, whether through the release of chemicals (allelopathy) by the cover crop or through the absence of a suitable host for the pest. Researchers in Australia looking at the effect of brassicas in conventional crop rotations on various pests and diseases have anecdotally reported that the cover crops appeared to have a suppressive effect on weeds in the subsequent crop, presumably due to a group a chemicals called glucosinolates (GSLs). Other reports from Europe and North America have also suggested that brassicas can be used for integrated weed management due to their apparent allelopathic effect on weeds.

Figure 4. Cover crops and other pre-crop treatments at 6 weeks after sowing, Armidale NSW. BF = bare fallow (rotary hoed), GF = green fallow (unweeded), MU = mustard, RA = radish, RY = ryegrass. Although good suppression was provided by the cover crops, weeds in a subsequent lettuce crop were not reduced.

Recent work on brassica cover crops in Finland, Italy, the USA and Australia has found very little evidence of a reliable effect on weed numbers, even using high GSL varieties. Although weeds may be effectively controlled during the time when the cover crop is growing, the weed levels in the following cash crops are the same as those for cash crops grown after various fallows or other cover crop varieties. The absence of an effect on weeds is may be attributed to:

insufficient plant material grown in a season to achieve effective weed control,
incorrect timing of operations related the cover crop including termination and incorporation
the lack of persistence by the GSLs in the soil after incorporation of the cover crop, especially where cover crop residues are mulched and turned in to the soil,
disturbance of soil due to tillage practices in annual cropping

A number of indications show that allelopathy was probably not even a significant factor in inhibiting weeds during the cover crop phase. Such indications include a strong link between shading and weed levels, weeds continuing to emerge very close to the cover crop without inhibition, and the impacts were not correlated with the amount of brassica plant material added or to measured GSL levels.

Concluding remarks

The inhibitory effects of allelochemicals are very specific to particular target plants and not others. Even when some weed suppression has been observed, certain weed species are susceptible and others are not inhibited at all. This incomplete control is a normal aspect of an integrated whole-farm weed management program, where reliance is not placed on a single tool for broad spectrum weed control. However, the number of reports indicating no effect on weed control by brassica cover crops provides a warning that allelopathic crops do not necessarily offer a simple, non-chemical ‘silver bullet’ to weed control. Indeed, very careful management is required to achieve success with any cover crop. After all, weeds have defied the ‘silver bullets’ of tillage for centuries, herbicides for decades and transgenic crop manipulation for several years.

Brassicas are a suitable alternate ‘green manure’ to diversify rotations, add nutrients and improve soil structure and health. Brassica cover crops should be sown at the seed supplier’s recommended rate (or greater if the germination rate is unknown or doubtful) into well prepared seed beds in order to maximise biomass production and outcompete weeds.

RESOURCES

Books

Gardner, B. and Morgan, W. 1993. Green Manure Crops: The Organic Alternative for Improving Soils. Agmedia, East Melbourne.

McCoy, S. 2001. Organic Vegetables. A Guide to Production. Department of Agriculture, Western Australia, Perth.

McCoy, S. and Parlevliet, G. (eds.) 2001. Organic Production Systems: Beef, Wheat, Grapes and Wines, Oranges and Carrots. Rural Industries Research and Development Corporation, Barton.

Sustainable Agriculture Network (ed. 1998). Managing Cover Crops Profitably. Sustainable Agriculture Research and Education Program, Washington DC.

Internet

Jaeger, C. 2003 Organic Farming: Which Green Manure Should I Grow? Victorian DPI, Mildura.
http://www.dpi.vic.gov.au/DPI/nreninf.nsf/childdocs/-49A21C225110DAB74A2568B30004DB83-87033AD2C9D2F3F4CA256BC700836044-9B49B51BA195265B4A256DEA0027DACB-0D845C51F20E6E26CA256DBE008143EA?open

Madge, D. and Jaeger, C. 2003 Organic Farming: Green manures for vegetable cropping. Victorian DPI, Irymple.
http://www.dpi.vic.gov.au/dpi/nreninf.nsf/childdocs/-71E8091F577D52D24A2568B30004F3B2-F35B1CAC4054922BCA256BC8000291D4-7CBD1D972BE5DD0D4A256DEA0027411F-7C11EE5BCD6D3520CA256CAE0001D821?open

Cover Crop Resource Page by SAREP, UC-Davis has a cover crop database, links to publications and other web sites
http://www.sarep.ucdavis.edu/ccrop/search_ccrop.html

Contact

Dr Paul Kristiansen
School of Rural Science and Agriculture
University of New England
Armidale NSW 2351
Phone: 02 6773-3962
Fax: 02 6773-3238
Email: paul.kristiansen@une.edu.au

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