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Weed control in arable crops
Weed control in organic agriculture does not depend on a single strike approach. It is much more effective to consider weed control at every stage of the crop production process. This session uses the idea of 'Many Little Hammers' to illustrate this approach before looking at two of the ‘little hammers’ in more detail. (Organised by Organic Arable)
Andrew Trump (Organic Arable): Chair
Andrew Trump pointed out that weed seed bank management is a successful weed control strategy; but it is more difficult than strategies mainly focused on mechanical cultivation. Weed seed bank management involves e.g. reduction of the seed rain, encouraging seed predation by invertebrates, encouraging seed degradation in the soil, and the use of cover crops.
Jonathan Storkey showed that the use of ‘many little hammers’ can be optimized by considering the whole life cycle of weeds. Identifying most vulnerable stages enables to hit where ‘it hurts most’.
The most important weed species in organic arable systems are -
- wild oats
- couch grass
Black grass and wild oats were compared in terms of seed bank decline. Black grass declines 80 % per year, whereas wild oats are more persistent (50 % decline), and are therefore causing bigger problems: in undisturbed soils they can persist for 8-9 years.
The species also differ in maximal germination depth (wild oats 12 cm, black grass 8 cm). Increased crop seed rates will have a stronger effect on black grass than on wild oats as wild oats are more aggressive, producing fewer (but bigger) seeds.
Based on the life cycles of the two species, black grass should be attacked in the seed bank. In contrast, for wild oats there are opportunities predominantly for targeting seed production and seed shed, using timing of harvest, hand roguing, and clean machines.
John Pawsey showed that annual roguing costs for winter wild oats averaged about £34/ha per year. The use of the Garford Interrow guided hoe was successful but required a fine and level seedbed.
For the weed surfer (width 9.25 m), there is only a small time window to act because wild oat seeds become viable already 10 days after flowering. One problem reported for the use of the weed surfer was that because of unstable movement it skimmed the crops, which risked increasing disease problems, in particular bunt infection. It showed great success with charlock control, though less so with thistles or docks. So it, is only cost-effective if the roguing becomes too expensive, and it needs daily checking of field.
David Brooks showed that carabid beetles are important weed seed predators but have been in general decline over the last 10 years.
Based on data from 260 sites studied over 3 years, he showed that omnivorous carabids are more associated with weeds than previously thought. Two groups of weed species emerged from the analysis:
- weeds with spring germination and high seed weight were associated with autumn breeding, large bodied, more generalist carabid species;
- weeds with autumn germination and small seeds were associated with spring breeding, smaller, more and granivorous carabids
To encourage carabids, it was recommended to keep tussocky grass margins and hedgerows. It was highlighted that there is an unresolved tension in reduced tillage as it is good for invertebrates, but results in higher weed pressure.
The discussion that followed the presentations brought out the following points:
- Identifying most vulnerable stages in the life cycle of weeds enables to hit where ‘it hurts most’.
- A weed surfer is only cost-effective if hand roguing becomes too expensive, and it needs daily checking of field.
- There is an unresolved tension in reduced tillage as it is good for invertebrates, but results in higher weed pressure.
Speaker presentations and abstracts
Andrew Trump (Organic Arable): Many little hammers – the idea
Andrew Trump will introduce the idea of ‘Many Little Hammers’ to weed control, i.e. that we need several different approaches to weed control within our farming systems in order to manage weeds effectively.
Jonathan Storkey and David Brooks (Rothamsted): Many little hammers – hitting where it hurts! (950KB)
Farmers managing conventional systems have come to rely on herbicides almost exclusively as their means of weed control. In contrast to the big ‘one-hit’ of chemical control, however, organic systems must employ an ecological approach that uses a range cultural control options to exploit points of weakness in the weed life cycle. This has been described as the ‘many little hammers’ approach. Increasingly, these ideas are also being incorporated into conventional systems to combat the spread of herbicide resistance. Because of differences in the ecology and traits of different weed species, any given management scenario will select positively for some species and negatively for others. Using the example of wild oats, the important traits or characteristics of weeds in terms of identifying opportunities for control will be discussed in the context of developing sustainable weed management strategies.
John Pawsey (Shimpling Park Farms Ltd): Managing winter wild oats (3.7MB)
I discuss the financial implications of wild oats on an organic farm, existing methods and then a new approach using weed surfers. I show two years’ worth of evidence showing how quickly wild oat seeds become after flowering. I then discuss the results from our management of the weed, development of the weed surfers and then conclusions of the work we have done to date.