About 10 years ago, the master farmer from Dinnenried in Baden-Württemberg noticed that blackgrass was rampant on 14 out of his 79 hectares of arable land. Despite herbicide spraying, it was almost impossible to control the weeds on the heavy clay soils.
More and more farmers are struggling with herbicide-resistant weeds and grasses; and, in Germany, this is especially the case with blackgrass. It is unlikely that new groups of active substances will be developed in the near future. But with the right management, the risk of resistance can be significantly reduced, so the focus is once again on arable and plant production measures. A diverse weed flora without individual, dominant weeds is the goal.
Mr Heine started in 2014 as a demonstration farm for integrated pest management and found the necessary support there. Since then, plant protection expert Bernhard Bundschuh from the Agricultural Technology Centre Augustenberg (LTZ) has been supporting the farmer and agricultural teacher in controlling meadow foxtail on the land.
Resistance remain undetected for a long time
Mr Heine’s blackgrass is a prime example of the typical course of resistance development: Initially, individual plants developed the ability to survive a normally effective herbicide treatment. At this time, they are hardly noticeable in the population, which is why the problem is often only recognised late.
“Maybe we underestimated the danger in the first years. We thought we were well positioned, chemically,” Mr Heine recalls. In addition, he had good experience with the herbicides used, so that he attributed less importance to the necessary change of active ingredient – a major reason why some farmers still use the same herbicides year-round. Herbicides are considered a cheap and effective form of weed control. However, they should be used with caution, as any use can promote resistant weeds. Resistant plants then multiply rapidly and soon colonise entire fields. “With the experience I have today, I now pay even more attention to changing the active ingredient every year,” explains Mr Heine.
Following Mr Bundschuh’s advice, the farmer now always applies herbicides at the highest application rate – preferably in the morning or evening when humidity is high. “I always say: It’s better to get one treatment right than to get it half right twice,” emphasises the plant protection expert. Reduced application rates only increase the resistance problem.
Using crop rotation
The development of a resistant weed population can be avoided or slowed down if the herbicide’s modes of action are switched throughout the crop rotation. In the case of a close cereal crop rotation, as in Mr Heine’s case, the DLG recommends alternating between ACCase and ALS inhibitors to control blackgrass. Both groups of active substances should account for a maximum of half the applications across the crop rotation.
For this to succeed, a diverse crop rotation is needed, alternating spring and winter crops and cereals with leafy crops. In trials, resistant blackgrass did not occur in crop rotations with more than 30% spring tillage. Without spring tillage, however, foxtail developed resistance on 40% of the fields. One-third spring tillage in the crop rotation is therefore considered advisable to significantly reduce the risk of resistance.
In recent years, crop rotations have been increasingly tightened on many farms for operational and market reasons. While Mr Heine used to consistently cultivate cereals and oilseed rape, he now alternates more and more between winter and summer harvesting. In between, he always incorporates a cover crop or maize to reduce the proportion of winter cereals in the rotation. This allowed him to alternate the available herbicide active ingredient groups more consistently, creating better conditions for a diverse weed flora.
Finding spring crops which Mr Heine could either grow and sell profitably or feed to his 200 sows was not easy. This year he is focusing on spelt. Potatoes would also do well in the rotation, but unfortunately do not suit the heavy soils or the farm overall. “If it doesn’t fit, you don’t grow it just to get a grip on blackgrass,” explains Mr Bundschuh. After all, resistance-preventing measures are not free. For sustainable implementation on farm and fine-tuning, you need not only brains but also a market for alternative products.
Tillage is vital
“Crop rotation is the first point in resistance management, tillage the second,” explains the crop protection expert. If you move the soil once more before sowing, you can achieve a lot – be it with the harrow, cultivator or plough. This is because blackgrass only germinates close to the soil surface. If it is buried, it can no longer grow up.
Mr Heine now only uses the plough every two to three years in favour of the harrow. “It’s faster with the harrow. And minimum tillage reduces water evaporation compared to ploughing,” he explains.
The stale seedbed has also proved its worth. After each harvest, the farmer carries out a blind sowing: Every three weeks, from harvest to sowing, he moves the soil a little to kill any emerging weeds. In this way, the weed seedlings emerge before the crop and can then be controlled effectively.
Sowing late is the trump card
With the same aim, it is recommended to sow winter cereals as late as possible in case of problems with blackgrass. ”It is said to be sufficient to postpone the sowing date until mid to late September. But in practice this is often not possible at all,” remarks Mr Bundschuh. Mr Heine also takes this to heart. In 2020, he sowed spelt in the last week of October. “It was already pretty wet by then,” he remarks. “I always have to find a compromise. Sow late, but not too late either. Otherwise I won’t be able to get onto the field.”
No light for the blackgrass
The denser the leaf and root system of the crop, the better it can assert itself against the competition. Therefore, seed rate, crop density and sometimes the variety influence weed emergence.
Mr Heine has found that intercropping and undersowing further reduce blackgrass. The best crops are mixtures that shade quickly, and he now wants to make more use of this. For the next rapeseed crop, vetch and Alexandrin clover will be undersown on resistant areas, which will completely cover the soils early on. This has worked well on the farm before. “In spring, however, it is essential to check whether all the plants have really frozen over the winter,” mentions Mr Bundschuh. In this way, one saves a herbicide treatment in autumn and benefits from the nitrogen that the catch crop has fixed.
What could also help to prevent resistance in the future is an even more precise spraying technique. Last year, Mr Heine replaced his 30-year-old sprayer. Now he is not just restricted to using the favourable weather conditions at night for spraying, because the device can read GPS data. “I can also use higher application rates and still cover a large area,” says the arable farmer enthusiastically. “I expect that we will be able to work much more precisely in the future.”
What has proved its worth
Mr Heine sees the combination of late sowing, mechanical weed removal and varied chemical control as an effective trio. And the efforts of the past years are bearing fruit: The blackgrass problem has been significantly reduced – both Mr Heine and Mr Bundschuh agree on that.
Of course, all measures must be well co-ordinated and applied at the right time. This requires a decisive step: Out the door and into the field. “I am now even more consistently out on the land, looking at what is actually needed and when is best to do it,” reveals Mr Heine.
At a glance
Resistance management in meadow foxtail is based on four main pillars:
- Rotation of the herbicide active ingredient groups
- More diverse crop rotations
- Targeted tillage
- Later sowing of winter cereals