Resistance - What do we see? What happens next?
Resistance is becoming an increasingly important issue in arable farming. Michael Horsch and Michael Braun talk about the causes, strategies and technical approaches to dealing with a growing problem.
The issue of resistance is no longer a theoretical discussion but by now affects many farms. The spread is becoming increasingly noticeable: “We think resistance will become one of the major challenges in the coming years, especially in Western Europe,” Michael Horsch says. "The problem with blackgrass is well known. What we are now facing to an increasing extent is ryegrass," Michael Braun adds. A key trigger for increasing herbicide resistance is the high share of winter crops in crop rotations.
This development is linked to two key factors: Firstly, it becomes increasingly difficult for the researching chemical industry to detect new groups of active ingredients, launch them on the market and keep them there; secondly, nature reacts with greater resisting power and forms resistance. "Resistance starts gradually. Initially, it is still possible to counteract it with higher quantities of active ingredients which initially brings partial success,” Michael Horsch says, “but eventually you reach the point where a new wave of resistance emerges. Then the quantities are increased further - with less and less effect. At the same time, the crop suffers from the high level of active ingredient stress which leads to plant and population density losses and costs yield. If the stress on the population increases, it is less able to defend itself against fungi and insects which in turn means that fungal and pest infestations increase."
Origins
The first grass resistance was observed in England about 15 years ago. Particularly in regions with a very high share of winter wheat in the crop rotation, blackgrass developed resistance against important groups of active ingredients and became a challenge on farms. To counteract this, many farmers even returned to ploughing: “Ploughing achieved a short-term success,” Michael Horsch says. However, this did not bring the hoped-for results in the long term, and the weeds came back all the stronger afterwards. Therefore, the plough was no longer a solution.
As a result, many farms extended their crop rotation and significantly reduced the share of winter wheat. However, the alternatives are very limited, especially on heavy soils. They increasingly relied on spring crops (e.g. spring barley) or maize to expand the crop rotation with additional crops. “Finally, I have the feeling that the affected farms are still struggling,” Michael Horsch assumes. As a result, many farms extended their crop rotation and significantly reduced the share of winter wheat. However, the alternatives are very limited, especially on heavy soils. They increasingly relied on spring crops (e.g. spring barley) or maize to expand the crop rotation with additional crops. “Finally, I have the feeling that the affected farms are still struggling,” Michael Horsch assumes.
However, there are also limits to the conversion of crop rotation. It is true that many farmers are able to establish clean crops using spring crops such as peas, beans, maize or spring barley combined with optimised production methods. However, these crops often generate significantly lower margins - a dilemma that puts farmers under economic pressure.
Effect of soil herbicides
When we talk about active ingredients, the current focus with regard to weed control is on soil herbicides the effect of which we have to maximise. Many environmental conditions, such as soil moisture, are decisive for the success of the herbicide application. But seeding, too, is an important factor for the success of weed control. "Evenly deep seed placement, sufficient seed cover and a crumbly field surface are the be-all and end-all. This is the only way soil herbicides can be applied after sowing safely and at the same time with minimum negative effects on the crop," Michael Braun points out.
In some regions, ryegrass is replacing blackgrass as the number one weed. Herbicide resistance in blackgrass has been known for several years. Resistance in ryegrass is more recent. At present, the problems are noticeable almost exclusively in Italian ryegrass which is also used in annual forage production, under sown crops and catch crop mixtures.
Biology of the two sweet grasses:
Blackgrass is mainly found on heavy soils and is a winter and summer annual. Germination mainly takes place in autumn at soil temperatures between 10 and 15 °C but can extend into spring. A light stimulus of just a few milliseconds is enough to stimulate germination. Elevations with 20 to 60 cm high stems develop from the twisted cotyledon. From this, up to 20 false ears per plant develop between June and July. They can produce up to 2,000 seeds. The seed can be described as precocious as it is ready to germinate in the wax-ripe stage. In the upper soil layers, the seeds are no longer able to germinate after about four years. Ploughed-in seeds can survive for up to ten years.
Italian ryegrass is a perennial. It prefers deep and warm soils with high rainfall. In contrast to blackgrass, germination takes place all year round. However, the soil temperature has to be above 10 °C. Then, elevations develop. The plants grow to a height of 30 to 100 cm and tower over the crop. In the summer months from June to August, the ears are formed. Up to 1,500 seeds per plant can develop. They can start to germinate shortly after dropping and therefore before the next crop is sown. The seeds can survive in the soil for up to five years.
Sources: Lfl Bayern, Praxis Agrar, BIO Aktuell, lk Online, TLLLR, LFULG Sachsen
The role of seeding time
The seeding time of winter wheat plays a key role in terms of resistance. “You should actually sow later,” Michael Horsch says, "but late-sown crops are comparatively weak with regard to competition. This is why many farmers on problematic weedy sites deliberately sow earlier and, thus, try to establish highly competitive populations. This means that, despite sowing relatively early, they also drill a relatively high seed rate of winter wheat in the row. This allows the crop to build up a strong competitive power in the drilled row and counteract the weed pressure there." Farmers find themselves in a conflict: on the one hand, they want to sow as late as possible to make most of the blackgrass seeds emerging before seeding. On the other hand, early seeding leads to stable and competitive populations - which in turn facilitates weed control. "The ideal is: if the seeding conditions are right, you should benefit from them and not wait for a later date. Finally, the perfect population with strong competition within the rows is important,” Michael Horsch summarises. “Another point is the efficiency that is required at the perfect time," Michael Braun adds.
Row discussion
In regions with resistant weeds, the discussion about rows becomes increasingly important again in the face of these challenges. Especially farms with high blackgrass pressure sometimes prepare the seedbed several weeks before the actual seeding. They spray the fields with a total herbicide shortly before sowing, thereby eliminating the blackgrass that has been stimulated to germinate before sowing. They then sow directly using the stale seedbed method without tillage. In this case, wide rows move less soil than very narrow rows with coulter stagger. "However, the wide row spacing always has to be considered from a very site-specific and, above all, crop-specific point of view. But we should definitely look at 20 or 25 cm row spacing to achieve precise embedding of the seed and populations with higher competition in the row to kill off the grasses," Michael Horsch says (read more on pages 6-7). If there is not enough competition between the rows, you have to think further. Basically, wider rows are required to be able to place seed with equally long coulters at the same depth to achieve even population development with high competitive power in the row. For the area between the rows, hoeing could be taken into account as a mechanical solution on the one hand and chemical plant protection with new methods on the other. For successful weed control, it becomes increasingly important to recognise the interrelations and to deduce and implement the right decisions. “Strategic thinking is important before you get started,” Michael Horsch emphasises.