Figure 1. Cover crops grown for a weed
suppression experiment in Armidale NSW.
This article was first printed in
Australian Organic Journal, Winter 2006,
published by Biological Farmers
cover crops: do they kill the weeds though?
University of New England
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
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
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.
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.
Gardner, B. and Morgan, W. 1993. Green Manure Crops: The
Organic Alternative for Improving Soils. Agmedia, East
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.
Jaeger, C. 2003 Organic Farming: Which Green Manure Should I
Grow? Victorian DPI, Mildura.
Madge, D. and Jaeger, C. 2003 Organic Farming: Green manures
for vegetable cropping. Victorian DPI, Irymple.
Cover Crop Resource Page by SAREP, UC-Davis has a cover crop
database, links to publications and other web sites
Dr Paul Kristiansen
School of Rural Science and Agriculture
University of New England
Armidale NSW 2351
Phone: 02 6773-3962
Fax: 02 6773-3238