As weeds become smarter at resisting efforts to control them, more diverse rotations deserve a closer look.
Resistance to both chemical and cultural weed control is developing at an increasing rate, and the number of weeds with resistance to multiple modes of action is also increasing.
This is according, Dr Harry Strek, scientific director at Bayer’s Weed Resistance Competence Center [subs- correct spelling] in Frankfurt, Germany.
Offering a global perspective to an audience of agronomists at a Bayer breakfast meeting, he explained how one plant studied in south-western Germany had three different types of ACCase mutation, one ALS mutation and metabolic resistance.
More than 250 weeds have developed resistance in 93 crops across 70 countries.
In Europe (including Russia and Turkey) there are 78 resistant species, closely behind the US with 81.
In Denmark, non-target site resistance to ACCase inhibitors became widespread when the Government ‘chopped herbicide rates in half,’ said Dr Strek.
“High use rates were reduced, Government took over registration of rates and introduced taxes based on environmental friendliness of actives. This meant some products became cheaper and other products like glyphosate went up in price.”
However, it is not just resistance to chemical controls farmers and agronomists need to be aware of, with cases being seen of weeds developing resistance to hand rogueing in rice paddies in China.
Dr Strek said: “In the beginning the rice seed would be spread in the paddy, and it was very distinctive. The weed was a different colour green and it could be picked no problem. But sooner or later it figured out ‘if I start looking like the rice plant early on I won’t get picked,’ so they were surviving and that’s resistance to a non-chemical method. If you use it over and over, selection pressure will continue - it doesn’t matter if it’s a chemical or not.”
Herbicide resistance is often very hard to see until it’s too late, he added.
"Most farms believe they got resistance from their neighbours – there is some of that. But what is underappreciated is that resistance revolution follows a checker board pattern - there’s no infection model like there is with diseases. Each field evolves differently and resistance control and weed control is field by field, not farm by farm.
“We always think we’re smarter than weeds, but somehow they outsmart us every time. There is resistance against general practices – don’t just associate it with what’s coming out of a can.”
In Germany, simple changes to the rotation have been an important factor in managing resistant black-grass.
“Adding maize to cropping rotations to control black-grass is one of the best things German farmers have done. They have a full summer crop and a completely different herbicide palette to work with.
“Understand the biology of the weed and where you can find key intervention points. For example, if you have a winter or summer annual, develop a tactic such as a summer crop in the rotation.”
In a study looking at agronomic influences causing resistance in black-grass, 30-80 plants per field were analysed in 50 fields across Germany.
It was found that more than one spring crop every six years in the rotation reduced the risk of resistance in the field by 2.5 times, while growing fewer than four winter cereals in six years reduced the risk of resistance sevenfold.
Increasing the number of crops in the rotation to four or more in six years also decreased resistance risk.
Meanwhile, a separate study of 150 farms found that mixtures of herbicides are 83 times less likely to evolve resistance compared to single products, even in sequences.