Eliminating herbicide resistance is extremely difficult once it has gained a foothold on your farm, but with the correct management practices it is possible to eradicate the problem in a few years.

“The best solution is to avoid getting it in the first place,” says Hestia Nienaber from South Africa’s Agricultural Research Council.

Good crop management and an integrated weed-management programme are a farmer’s best defence against herbicide resistant weeds. Instead of over-reliance on herbicides alone, farmers can use ‘traditional’ weed-control techniques, combined with herbicides.

CROP ROTATION

Crop rotation is one of the most effective ways of reducing the risk of herbicide resistance, says Nienaber. It is effective, because different crops are planted enabling the producer to use herbicides that have different modes of action.

The Herbicide Resistance Action Committee (HRAC) points out that as farmers experiment with crop rotation, they may find that certain crops differ in their ability to compete against weeds. A highly competitive crop will have a better chance of restricting weed seed production.

“Soya and sunflower are popular rotation crops in some areas, but farmers should work out what the best rotation system would be for their farm, depending on cash flow, available machinery, the area and soil type,” says Nienaber.

Also read: Sunflower as a rotation crop

Select a crop that can be planted and harvested using existing machinery on the farm. “It must match your current production machinery, or else you will have to buy more equipment, leading to massive output costs,” she says.

Some farmers borrow equipment, or use contract harvesters but, warns Nienaber, this can increase the risk of introducing herbicide-resistant weeds onto the farm.

Some farmers are not keen to rotate crops because to do so would require additional management skills and knowledge of other crop types, such as the fertiliser requirements and diseases to which these crops are prone to.

Also read: Mechanisation: A short guide for new farmers

INTERCROPPING

• Intercropping is another option to suppress weed growth and could work well in vineyards and orchards.
• Farmers can continuously grow cover crops between rows in orchards and vineyards.

PLOUGHING

• Non-chemical weed-control methods – such as regular ploughing – can make a major difference if farmers are trying to reduce weed seed banks.
• Ploughing prior to sowing helps to bury non-germinated seed and controls emerging seed.
• Burning stubble can also reduce weed seed fertility.
• Farming areas where minimum or no-tillage practices are in place, are more susceptible to herbicide resistance as a result of an over-reliance on herbicides for weed control.
• No-till farmers should consider ploughing their fields every 5 years to suppress continued weed seed germination.

Also read: Conservation agriculture: No-till and controlling weeds

CHEMICAL CONTROL

The problem is that farmers tend to use the same herbicides each season, because there aren’t many options available for an extensive herbicide rotation programme.

The HRAC recommends that, where possible, farmers avoid continued use of the same herbicide with the same mode of action in the same field, unless it is integrated with other weed-control practices.

The use of pre-crop emergence herbicides can make a big difference to a farmer’s weed-control strategy, says Nienaber.

“Pre-crop emergence herbicides should be applied after planting seed, but before the crop pushes through the soil into the seedling stage,” she explains.

If farmers find that weeds continue to grow prolifically after the crop starts growing, they should apply post-crop emergence herbicides at the correct stage for spraying.

Also read:
How to effectively use knapsack sprayers
How to calibrate a boom sprayer

“It is also a good idea to remove as many weeds as possible to create a well-prepared and clean seedbed before sowing,” says Nienaber. This will give crop seedlings the best chance to grow without competition from weeds.

The HRAC recommends that, if resistance is detected, farmers should clean all planting, cultivation and harvesting equipment to avoid transfer of resistant weed seed.

WEED CONTROL

• Always adhere to label recommendations. Never make unregistered mixes.
• Apply herbicides at the correct rate.
• Regularly monitor crops so that resistant areas can be identified quickly. Immediately apply a spot spray to the area.

POST-HARVEST GRAZING TO CONTROL WEEDS

In SA’s Western Cape Province some producers have employed a novel weed-control strategy. They plant grazing such as oats, lucerne and rye grass for sheep or cattle grazing.

The animals keep the grass short, which prevents seed from maturing and sprouting. Doing this requires extra management, because farmers must ensure that good grazing practices are in place and need to design camps to ensure that fields aren’t overgrazed.

• This article was written by Wilma den Hartigh and first appeared in Farming SA.