Harnessing plant power for sustainable weed and pest management

How do we tackle weeds and pests without using agrochemicals? This question is at the heart of many of the networks field labs. Farmers want to find ways of reducing their inputs and there are agroecological solutions out waiting to be translated to commercial practice.

At this year’s first virtual Oxford Real Farming Conference, we heard from three speakers on how they have sought to make use of the ecosystem services offered by plants to benefit their weed and pest management systems.

Watch the full session here or scroll down for the follow-up Q&A to find out more about buckwheat, intercropping or trap crops. 

Deceiving potato pests

Andrew Webster, a conventional vegetable and arable farmer in Lancashire talked about his field lab using trap crops to reduce PCN damage. PCN can devastate up to 80% of potato yield. So farmers are looking to non-chemical control methods, particularly with the rise in regulatory pressure. “There aren’t many viable PCN-resistant varieties out there. Our current control methods include long rotations of 7 to 8 years, the use of oil radish as a winter cover crop before potatoes, and a mustard catch crop for biofumigation.” Trap crops show greater potential to limit nematode production but lack establishment guidelines. From field lab trials in 2020, Andrew’s group found that later sowing timings, i.e. late June – July, led to poor establishment. With later sowing, weed control and rainfall patterns impacted particularly on establishment in clay loam soils. They plan to continue exploring this topic into a second year, with earlier planting and priming of the seed to aid germination.


From living mulches to intercropping

Dominic Amos, crops researcher at the Organic Research Centre (ORC) discussed several trials, including a field lab set up by organic farmers to explore wheat varieties to identify genetic traits which are more suited for farming outside of an agrochemical regime. “We found that early ground cover of the wheat crop is very important in terms of controlling weeds… Another nice finding was that a diversity of weed species reduces dominance and competition of weeds; the community essentially manages itself.” For no-till and organic farmers, under sowing with a living mulch may suppress weeds without dominating cash crop growth whilst also building soil fertility and attracting pollinators. “When thinking about how to improve cover to increase competitive ability, consider combining the traits of different crops by using mixtures and increasing in-crop diversity.” The intercropping field lab found a 73% reduction in weed biomass and increased land use efficiency when growing wheat and beans together, when compared with a monocrop of beans. Other ORC research found that increased diversity of cover crop mixtures led to an average of 37% more yield due to their ecosystem services of biomass and weed suppression.

Exploring the allopathic properties of buckwheat

Andy Dibben, an organic mixed farmer in Cirencester explained how, after taking part in a field lab on buckwheat, it is now a permanent feature of his rotations as a couch grass suppressant. There are several reasons why buckwheat may suppress couch - both have allelopathic effects and scavenge for phosphate so can thrive in phosphate-poor soils, but it may be that buckwheat outcompetes on these characteristics.

Buckwheat also casts dense shade which prevents couch from thriving. Additionally, because buckwheat likes warmer soils, cultivations take place later than other plants. In the field lab trial, Andy found that whilst buckwheat and the control of bastard fallow were both good at controlling couch, intercropping with buckwheat was “far superior” on several fronts; “germination in the following green manure phacelia crop was much better, soil structure was greatly improved, and buckwheat works better for all weather conditions.” “For best effects, it should be destroyed and incorporated into soils. It is not a silver bullet for couch grass; is part of an integrated approach, crucial to constantly outcompete couch, with green manures and harvested crops – never give it time to establish.”

Questions about trap crops, buckwheat and intercropping

One of the advantages of the new online format for the Oxford Real Farming Conference was the democratic nature of audience engagement that we saw across the week. We had loads of questions from the audience but unfortunately did not have the time to answer all of them. So, we have included answers to your questions below:

Trap crops

Q: If the trap crop suppresses root feeding nematodes, doesn’t it suppress beneficial nematodes?
A: Trap cropping doesn’t suppress beneficial nematodes as the mode of action is PCN specific. Trap crops trick PCN eggs into hatching at a time when only the trap crop is available to feed on, but don’t provide enough energy for PCN to complete its life cycle. There are many more beneficial nematodes than pathogenic nematodes in soils, so it is important that control methods are targeted. For example, certain biofumigants may adversely impact on beneficial soil organisms and natural enemies of targeted pests.

Companion cropping/intercropping

Q: Could companion cropping/mulch work to reduce flea beetle in oilseed rape?
A: (Dominic): Some farmers use nurse cropping for OSR including berseem clover, buckwheat (that is killed off in the winter) and even fenugreek that may help confuse adult CSFB by giving off volatile plant compounds and disguise the crop. Adding biodiversity may help encourage natural enemies and improve crop health. Other theoretical reasons why a mulch may help is green background confusion of the pest. Growing crops together can also help as host crop become less attractive to pests due to volatile interactions between neighbouring plants.
A well-researched example for tap cropping in Oilseed rape is with the use of black mustard, or fodder radish as trap cropping for pollen beetle, with those companion crops also being beneficial for parasitoid populations.

Q: To what extent have these trials quantified pest control in intercropping?
A: (Dominic): The example of wheat and beans from the field lab was concerned only with improved weed control from bi-cropping over monocropped beans. In another example from the field lab, one farmer grew oats with linseed to help reduce flea beetle damage.

Q: How does harvest work with intercropping?
A: Intercropping for forage/whole crop harvest makes things simpler and harvesting two grain cash crops is usually straightforward when both ripen together, but it becomes more difficult if post-harvest separation is required. In wheat and bean intercropping field lab, grain was being used for cattle feed so no need to separate. For bi-cropping a cash and cover crop as in the case of the living mulch, harvest works as normal, with the white clover mulch selected to be rather prostrate so as not to interfere with combining. Farmer trials in the past have shown trefoil to cause issues at harvest as it grows taller than white clover when undersown.

Buckwheat

Q: Do you have an indication of cost/benefit of buckwheat?
A: (Andy): For non-organic growers, reduced phosphate and insecticide bills as well as increased yields in follow-on crops means that the buckwheat far outweighs the cost of the seed. The 10% increase in onion yield equates to a £400 in income. Buckwheat seed cost and associated management is around £150. So there are definite gains.

Q: How does growing large areas of buckwheat affect beneficial invertebrate populations?
A: (Andy): Buckwheat, if allowed to flower, attracts a huge array of insects, both pollinators and predatory insects with huge benefits for surrounding vegetable crops.

Q: I tried using buckwheat on my allotment plot, but all of it got eaten. Which animals are likely to be eating buckwheat, and what can I do to protect against this?
A: (Andy): Birds love to eat the seed before it has germinated, cover with mesh till it has germinated. Rabbits and slugs will eat small plants, fence and encourage beetles which eat slugs.

Q: Can you grow buckwheat as a grain crop?
A: Yes

Q: How deep does the buckwheat need to be incorporated into the soil? And why was the buckwheat terminated after just 6 weeks?
A: (Andy): I incorporated at a depth of 20 cm in order to bury as much of plant material as possible, destroyed at 6 weeks as this allows it to flower and attract insects but does not let it set seed and become a problem weed in following crops

Q: Why did you choose Phacelia Tanacetifolia as your winter green manure crop?
A: (Andy): I use Phacelia a lot as a winter green manure, it germinates quickly and happily through crop trash, it is also very easy to work in in spring for early ground prep. If left to flower in Spring amazing at attracting pollinators and predatory insects.

Q: Is the effect of buckwheat on couch control due to destruction and incorporation, or would the same effect be found if you allowed it to die with frost?
A: (Andy): Observations in this trial were that it had best effect when incorporated, although we did not specifically investigate effects after frost destruction, it appeared not to have a significant effect while growing.

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