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Later sowing can help control herbicide resistant weeds

Resistance to herbicides in weeds may be at the expense of other plant characteristics and may provide ideas for alternative control methods. Small changes in cultivation technology may help to reduce the problems of herbicide-resistant weeds in the fields, according to research from Department of Agroecology at Aarhus University.

2020.04.08 | Camilla Brodam

Photo: Lars Kruuse AU Foto

Weeds do not perish easily, because they are good at adapt to cultivation conditions. An example is that many farmers today are experiencing problems with resistance to the herbicides that are usually used to control the unwanted plants in the field. Farmers and consultants have the opportunity to submit samples to the university if they suspect herbicide resistance, where they will receive either a yes or no. Researchers from Department of Agroecology at Aarhus University, along with former PhD student Eshagh Keshtkar from Tarbiat Modares University in Iran, have studied these plants and investigated whether there are differences in the two biotypes of a weed species - the resistant and the sensitive, and whether such differences can be exploited to combat the resistant biotype. 

“It is not the first time studies have been done on these biotypes. Previously, we have looked at seed production and growth rate, but in this study, we have chosen to look at germination,” explains professor and supervisor of Eshagh Keshtkar, Per Kudsk from Department of Agroecology.

The fitness of the plants

According to researchers, fitness for the plants is a very important subject, which cannot be overlooked.

"There is a theory in the field of plant ecology that resistance to pesticides does not come without cost to the plant," says Per Kudsk.

In other words, if a plant has acquired resistance to an herbicide, it may be at the expense of other properties of the plant. In other words, they will have a lower fitness level and will not be able to cope or be as strong as the herbicide sensitive plants, if the farmer does not use the herbicide in question.

“But there are a lot of studies, where researched have not been able to detect a reduced fitness. In fact, resistant plants seem to have pretty much the same fitness as the sensitive ones. This means that resistance is not something that goes away by itself because you stop using herbicides, the resistant biotypes will do just as well as the non-resistant biotypes. In this project, we therefore chose to investigate the fitness in detail, to test whether there should be any little differences, which had not yet been discovered. And whether they could be exploited to control herbicide resistant weeds,” explains Per Kudsk.

Resistance in several ways

There are two types of resistance. The most common is what is called "target-site" resistance. Here, typically, it is a single mutation in a major gene in the plant, which means that it goes from being sensitive to herbicides to being completely insensitive. Non-target site is another form of resistance. Here, researchers believe that there are mutations in several less important genes, each of which makes a small contribution to reduce sensitivity, and the more of that kind of mutations a plant has, the less sensitive it will be to herbicides.

“It has a practical significance because the first type of resistance is usually very specific to certain herbicides, and here we can use other herbicides to perish them. However, "non-target-site" resistance, which is the one we have chosen to work with, is typically shown by the fact that the plant can withstand a number of the herbicides farmers use," says Per Kudsk.

Non-target-site resistance is most often due to the ability of the weed plant to break down the herbicide, and the reason why this type of resistance is problematic is that almost all herbicides are degraded via the same mechanism in the plants. If a biotype first acquires this property, it will be able to degrade virtually all types of herbicides.

"That means you often have no way to chemically fight them, which is why the issue of differences in fitness is really important for these biotypes," says Per Kudsk.

Sprouting may provide a solution

The researchers chose to look at several different traits of the weed species of foxtail grass to assess whether there was a difference in fitness that could be exploited in the control of the resistant plant in the field. First, they looked at the phenotypic evolution, that is, how quickly the different biotypes come from one stage of growth to another, but without finding any significant differences. But when the researchers looked at the seeds' germination rest, they found a difference.

"We found that the resistant biotype has shorter germination rest than the sensitive one," explains Per Kudsk.

Germination rest ensures that the seeds do not germinate immediately after they hit the soil. They must be "kickstarted" by something from the outside before they sprout. 

"For this weed species, the seeds are dropped during the month of July, and if they sprouted immediately, they would be destroyed in the tillage that occurs in the fall before sowing the next crop, and thus they would not produce seeds and carry on the genus,” says Per Kudsk. The weeds have adapted to the grain's life cycle so that it only germinates when the grain is sown in the fall. 

"But it turns out that the resistant biotype has a shorter germination rest, it wants to sprout earlier," says Per Kudsk.

The researchers therefore found that if a farmer, who has problems with this type of resistance in his field, exposes the sowing for a little while, a greater proportion of the resistant than of the non-resistant seeds will germinate and be destroyed in connection with the tillage before sowing. It will help reduce the problem of the resistant plants.

“we found different fitness in the plants, and it turned out that it is also something that can be utilized in cultivation of the fields. By changing the cultivation technique a little bit, one can favor the sensitive biotype at the expense of the resistant one. And in this way, we may in the long term reduce the proportion of resistant plants in farmers' fields,” says Per Kudsk. The next step could be to investigate whether this is generally for all non-target-site resistant plants or only the population studied.

Behind the research

Collaboration partners: Tarbiat Modares University in Theran, Iran and Christian Ritz from Copenhagen University – statistical help

Financing: European Union Seventh Framework Programme (FP7/2007-2013), Aarhus University, Iranian Ministry of Science, Research and Technology (grant no. 4250135253)

Conflict of interest: None

More information:Differences in growth, development and innate seed dormancy of susceptible and fenoxaprop-P non-target site resistant black-grass sub-populations” was published Crop Protection in March 2020. It is written by Eshagh Keshtkar, Solvejg K. Mathiassen, Majid Agha Alikhani, and Per Kudsk.

Contact: Professor Per Kudsk, Department of Agrecologi at Aarhus Universitet. Email: per.kudsk@agro.au.dk Tel.: +45 8715 8096

Research, Agro, DCA