If chemicals alone are no longer enough: How can farmers try to get black grass and ryegrass under control

Hogrefe highlights an important point regarding variety selection and sowing density. In practice, these factors are often referred to as “gentle” parameters that can be implemented without major changes to the system. However, the data show that they do not generally reduce the number of emerging blackgrass plants but primarily affect ear formation – and thus seed production – by competition. In the analyses presented, the variety effect in total amounts to approximately 22%; regarding the individual data, differences between the “best” and “weakest” varieties can be significantly higher. The situation is similar with regard to plant density: higher sowing density can noticeably reduce the number of blackgrass ears, but here, too, the following applies: it is a matter of displacement, not of “no emergence”. At the same time, conflicts of objectives arise, such as competition for water and nutrients, lodging tendencies and costs. Thus, variety and sowing density are useful components, but no substitute for early control, effective soil management strategies or crop rotations adapted to the crop and the site.

From this analysis, Hogrefe deduces his “presentation-practice paradox”. Active agents are omitted, and integrated measures are required. In the field, however, restrictions apply: marketing, work management, weather windows, site limitations, erosion control, water protection stipulations, available crops, technology and, last but not least, the farm’s willingness to take risks. The key message still is: the toolkit is farm-specific, and often only a fraction of the theoretically “major” tools remains.

Another key point is the “competitor analysis”. Hogrefe classifies weeds according to their germination window and seed bank potential. In many regions, bent and brome grass are predominantly autumn-germinating weeds with lower seed bank problems and can often be effectively controlled by crop rotation and, if necessary, ploughing. This is more difficult with blackgrass, as dormancy and the seed bank create a long-term dynamic. In addition, there is a share of spring germination, meaning that crop rotation alone rarely provides 100% control. In his opinion, ryegrass is “even worse” because it germinates very flexibly in moist and warm conditions – practically throughout much of the year. These characteristics explain why integrated measures for blackgrass and ryegrass have to be combined particularly consistently and why one-off measures are rarely sufficient.

Strategies for field remediation

The second part of the presentation takes a practical approach: Hogrefe presented examples of severely affected farms that have systematically controlled their fields. The starting point always is a resistance analysis. This is to prevent farms from changing their crop rotation hoping for “new chemicals” which may ultimately prove ineffective because of cross-resistance or metabolic resistance. In one example, sulfonylureas are practically useless in cereals while other groups still show an effect. At the same time, it becomes clear that in beet, certain grass herbicides (e.g. DIM) no longer provide sufficient effect. The conclusion is not “one crop saves all”, but rather: first understand the situation, then draw up a plan. Hogrefe phrases a memorable guideline on this: rotation is not merely a change of crops, but a gateway to plant protection options. Those who want to use specific modes of action need the appropriate crop – and those who combine crops sensibly can distribute selection pressure.

One example that is particularly highlighted is a strategy in which beet fields are specifically used as control windows. The farm concentrates its beet cultivation within a single district, incorporates around 25% summer crops into a four-year rotation, and works without a plough for erosion and water protection reasons. The “game-changer” in this case is a systemic approach in beet that combines two modes of action. Hogrefe vividly describes this as “two hard blows at once” which can overwhelm the detoxification system of weeds – particularly relevant in the case of metabolic resistance components. The practical benefit is less in a single product but more in the possibility of using a highly effective combination within a single crop – one not available in cereals – thereby drastically reducing seed production on problem fields. In the system, this is complemented by late seeding in winter wheat, strong autumn soil herbicides, targeted foliar treatments during the appropriate time window and – depending on the site – other crops such as maize in which different modes of action can be used.

The control process is described in even greater detail in a case that Hogrefe classifies as a genuine site: high altitude, heavy black grass infestation, with FOPs and ALS inhibitors proving virtually ineffective. Here, the focus is no longer on “optimization” but on restructuring: at least two summer crops, followed by foliage crops, possibly glyphosate in appropriate windows, a combination of soil and foliar strategies in spring crops, and a focus on early, robust soil herbicide programs in winter crops. Winter wheat will only be reintroduced if the population has been sufficiently suppressed; otherwise, the cereal window will be closed again. Hogrefe thus makes clear that remediation has to be viewed as a multi-year process. Success depends less on “this single year” than on whether seed production is consistently prevented over several years. Otherwise, the seed bank will rebuild itself and the farm will be back to square one.

Application and water quality

At the same time, Hogrefe places major importance on issues relating to application and water quality as in practice, loss of efficacy is not only caused by resistance but also by avoidable application errors. A key point is the order in which the agents are added if sulfonylureas are used: pH reducers or water conditioners are not to be added to the water before the sulfonylurea, as this can lead to a poor solution, flocculation and residues. Allow the product to dissolve completely first, then condition the water. He also discusses the influence of pH on uptake: sulfonylureas are weak acids. Their uptake via the leaf depends on whether they are present in a form that can pass through the wax layer. As the pH rises, water solubility increases which can hinder diffusion through the wax layer. Furthermore, the charge state plays a role: for pHs in the range of the pKa value, a significant proportion of the active agent is ionised (negatively charged). For negatively charged molecules it is more difficult to pass through the plant surface which is mainly negatively charged, too. A lower pH increases the share of uncharged molecules and can improve uptake. The presentation cites a range around pH 5 as the target range for certain applications, although Hogrefe emphasises that formulations and products react differently and that the mixture has to be tested in practice.
He points out the addition of AHL (ammonium nitrate-urea solution) in certain foliar treatments as another highly practical point. The benefit is not explained as “fertilisation” but as a physical effect: AHL significantly lowers the deliquescence point of the spray coating, keeps it moist for longer and thus allows for the active agent to be absorbed over a longer period. For only what remains dissolved can be absorbed. If the spray coating dries too quickly, the active agent remains as a residue on the leaf without being transported into the plant. Hogrefe illustrates the difference with an example: mixtures without AHL require high humidity to remain moist for a long time, whereas mixtures containing AHL can remain moist even at significantly lower humidity levels

Finally, Hogrefe looks ahead to new soil-applied active agents that could potentially replace or complement flufenacet in the future, in particular cynmethylin and bixlozone. In terms of timing, 2027 is considered to be a more likely target date. Bixlozone has been approved for use in the plant protection product Isoflex since April 2026. At the same time, he warns against false expectations: new active agents are no reset button. They involve new demands regarding seedbed quality, levelling, organic matter on the surface, placement quality and variety reactions. Cynmethylin in particular could require a deeper placement making interaction between seeding technology, variety selection and tillering behaviour more important. The practical consequence is: anyone who wants to work reliably with new soil-active agents in the future has to increase the precision of their farming practices – not simply buy “a different product”.