The Truth

Reliance on herbicides alone is not sustainable and weed control. Strategies should be based around cultural methods. Increasing the diversity of farming systems is required.

Stewardship of existing herbicides is vital, since very few new modes of action have been discovered in over 20 years. Farmers trust farmers! Allowing farmers to help communicate positive stories/messages can deliver great results

“Doing the same thing over and over and expecting a different result” is often quoted as Einstein’s definition of insanity. It could easily be argued that is exactly the practice that farmers and agronomists have found themselves following in recent years. This is because herbicides in the past were highly effective, cheap and easy to use. But reliance on herbicides alone has contributed to the widespread herbicide resistance problems that we are seeing today. If you look at the problem simply, herbicide resistance is nature’s way of telling us herbicides alone are not sustainable and introducing more diverse weed control methods is required to disrupt the weed’s life cycle.

So, how are farmers and agronomists dealing with the challenge of herbicide resistant weeds? To investigate this further I visited a number of farms in Timau, Narok, Uasin Gishu, Bungoma as well as attending numerous industry events discussing how to manage herbicide-resistant blackgrass. I chose to visit these areas because by many it is seen to be the home of cereals. I found Narok to be one of the most intriguing counties that I visited. It gave me the opportunity of seeing how farmers react to a land that has been used since colonial times. It led to over 60% of cropping land being placed in wheat production and the rapid development of herbicide resistance in a number of weeds.

I found that farmers and agronomists were actively looking for better ways of dealing with herbicide resistance, with the momentum moving to more cultural controls of weeds rather than relying on synthetic chemistry. This is particularly important since very few new herbicidal mode of action has been discovered for over 20 years, and even if a new mode of action was discovered today it would take many years to work its way through the regulatory process before reaching the market.

To put it simply: herbicide resistance is a problem that is not going to go away, but it is certainly manageable!

Herbicides and resistance
The accepted definition of herbicide resistance is the one given by the Global Herbicide Resistance Committee, which is: “Herbicide resistance is the ability of a weed biotype to survive an herbicide application, where under normal circumstances that herbicide applied at the recommended rate would kill the weed. In contrast, plant tolerance to a particular herbicide is the inherent ability of that plant species to survive and reproduce after treatment with that herbicide.”

Herbicide resistance is broadly broken down into two groups: target site resistance and non-target site resistance. Herbicides work by binding onto enzymes and inhibit the metabolic process within the plant, leading to death. In target site resistance the binding site within the plant has been altered by mutations meaning that the herbicide can no longer bind to the enzyme and will lead to the plant surviving. Target site resistance is a total resistance and affects herbicides that are in the same chemical group. All other forms of herbicide resistance fall into non-target site resistance, and this may most commonly come into a group known as enhanced metabolism resistance, which is where the plant can detoxify the herbicide faster than it reaches the target site within the plant.

Herbicide resistance in Kenya
Currently we have six different herbicideresistant weeds that are an issue in farmland in Kenya. Three are grass weeds: and three are broadleaf weeds: The most problematic and widespread herbicide resistance in Kenya would be blackgrass, followed by ryegrass. Although herbicide resistance is predominantly in grass weeds, the three broadleaf weeds should not be ignored since herbicide resistance in the broadleaf weeds is almost entirely down to relying on the same mode of action at reduced rates year on year.

Herbicides – weed control of choice for the last 50 years
Ever since people have cultivated soils to grow crops, farmers have had to deal with weeds. Weeds are an issue because they compete with the crop for light, water and nutrients. Prior to the introduction of the first herbicides in the late 1940s which started with 2-4D, a highly effective broadleaf weed herbicide, the only methods of weed control would be cultural methods such as cultivation, hoeing, rotation, rouging etc.

Resistance testing
The benefits of herbicide resistance testing are obvious. Without testing for resistance to herbicides then you can never be 100% sure that you have resistance. Inadequate control of weeds from a herbicide can come from a number of factors involving poor sprayer set up, and can include: water volume, coarseness of spray, sprayer speed and boom height. Climatic factors also come into play, such as: temperature, soil moisture and speed of weed growth and growth stage.

Chemical groups and labelling Why should you ask: “do you know and understand what mode of action you are using?” To put it simply, the mode of action describes how an effective herbicide controls susceptible weeds through disrupting or blocking an enzyme in the weed. Therefore, to avoid herbicide resistance, alternating modes of action and not relying on one particular herbicide group is a very good idea. All herbicides that work in a similar pattern are grouped together by the Herbicide Resistance Action Committee (HRAC).

One thing that is very obvious everywhere I travelled was how manufacturers put the herbicide chemical group on the side of the product. I was also amazed at the level of understanding amongst agronomists and farmers about the need to rotate as many different modes of action as possible within the rotation and not to rely on one chemical group alone because that hugely increased the risk of herbicide resistance.

To make it easier it should be taken one step further and develop a five-stage colour code system for pesticide containers and boxes, using green labels for products with the least environmental impact, through to red for those that have the greatest environmental impact.

Crop walking / scouting
Large scale farmers have employed an agronomist while the medium scale consults technical representatives from agrochemical companies. Everywhere I travelled farmers seemed to really value the opinion of a professional agronomist, even if it was often only for strategic and seasonal advice. While visiting Kitale, I was lucky enough to be involved in one strategic agronomic meeting, planning for the next season’s cropping. Despite having to rely quite heavily on my host’s translation, the conversation was wide ranging from the obvious gross margin implications of changing crop rotations to how changing rotation could influence cover crop options and the ability to rotate more chemical modes of action to improve weed control. In fact it could be said that weed control was one of the main drivers of the conversation after the obvious need to maintain profitability!

Cultural control
Cultural control of weeds encompasses everything that does not come out of a chemical can. Cultural and chemical controls are often combined and used together as part of an Integrated Weed Management form of approach.

Crop rotation
Changing and extending crop rotation was probably one of the first cultural controls that farmers were willing to change in the fight against resistant weeds.

When it comes to rotation planning the key is certainly that the more diverse the better, and avoiding mono-cropping systems that often lead to repeatedly spraying the same chemistry year on year.

Mono-culture (Kitale) is perfect for the development of multiple herbicide-resistant weeds. However, with a change in altitude, and changes generations, more crops became viable. By extending their crop rotation they have been given different opportunities to tackle herbicide resistant weeds throughout the rotation, and rotate more modes of action, and not just rely on one herbicide as the main chemical control.

Typical current rotations would be one cereal crop followed by a break crop, mostly established by a no-till seeder. For example, the herbicide resistance capital of Kenya is old cereal growing areas e.g Narok ; this is partly because they were the first to move into more intensive cropping - and 80% of that was in one crop, wheat! This lack of cropping diversity led to repeat exposure of the same herbicides. Agventure group has tried to introduce Canola as a break crop. Broadening the rotation has enabled usage of a more diverse range of herbicide mode of action and, in some cases, ‘double breaks’ are used to maximise that opportunity to mix herbicides up.

Cultivations: ‘To till or not to till’ Most of the large scale growers I visited were in favour of No-Till. In these hot climates moisture conservation is key to establishing a successful crop. In other farms I saw a mixture of no-till and mini-till establishment methods.

When it comes to weed control, cultivations can often be seen as either a blessing or a curse. This is because a tine cultivator sweep or a disc cultivator can do a really good job of chopping through the roots and cutting up existing plants, but can lead to another germination of weeds which, again, can be controlled out-of-crop with a non-selective herbicide or another cultivation pass.

Cover cropping / brown manures
Planting cover crops though minimal is the ‘fashionable’ thing to be doing on farms at the moment, with some of the benefits including reduced erosion, trying to improve soil organic matter, holding nitrogen and ‘mining’ phosphate from the soil; and, by keeping roots living within the soil, improving mycorrhizal activity (depending on the species of cover crops planted) as well as trying to improve weed control through creating a mulch layer. Once terminated this cover would provide mulch that would slow down the emergence of weeds within the crop.

This reduces the cost from out-of-crop herbicide sprays which have climbed up to unmanageable costs. The massive increase in one herbicide resistance means that more herbicides had to be used to control out of- crop weeds. In some cases this makes cover crops cost-neutral because the reduced weed germination from the cover crop meant a cheaper cover crop destruction spray is required; and the farmers are starting to benefit from re-building soil organic matter again. This cover has the benefits of providing ground cover therefore reducing weed germination, as well as giving a nitrogen boost to the following crop.

Roguing
With the decline in control, increased herbicide resistance, roguing should be making a massive comeback.

Some of the farms I visited on my travels were quite large in size making labour availability an issue. The move back into rouging is probably one of the biggest signs that herbicide resistance has developed to such a high level that farmers are returning to this most ancient of weed control methods: hand pulling and removal of weeds and their seeds. In developed agricultural counties, rouging is happening on a scale that probably has not been seen since before the start of the green revolution in the early-to-mid 20th century.

Organic
During my visits, I was fortunately lucky to visit a farm in Narok. I was particularly keen to examine the ‘Farming Systems’ to investigate the process of organic conversion.

The emphasis is on Maize and bean production to replicate what is actually happening in area, but the conventional and organic plots are in different rotations because of the need to build fertility within the organic system.