Glyphosate-resistant black pickerel, the producer's new challenge in weed management

Weed resistance to herbicides in Brazil was for the first time reported to ALS-inhibiting herbicides (1993), exactly with picão-preto, at the time described as bidens pilosa. However, it was later identified that, in Brazil, infesting agricultural crops, there is a predominance of two species of the genus Bidens, B. pilosa e B. subalterns, with small morphological differences and susceptibility to herbicides. At that time, the glyphosate-resistant soybean crop was not yet being cultivated in Brazil, and the challenge for the producer was great. However, when the use of glyphosate-resistant crops was approved in 2005, the problem of controlling black walleye was immediately resolved.

In 2023, black pickerel will once again be a problem for grain producers in Brazil, as cases of resistance to glyphosate have been reported. We have developed research in the past that shows that populations resistant to ALS inhibitors have the same adaptability as susceptible populations. Therefore, once these populations are selected, they remain in the weed community for a long time. It is likely that the resistant populations of black walleye, now reported to be resistant to glyphosate, still have inherited resistance to ALS inhibitors, therefore restricting their alternative control by this mechanism of action.

As resistant populations of black walleye are still restricted in terms of distribution in the country, prevention is the best recommendation we have for the producer. Its prevention begins with desiccation. Among the herbicides recommended for desiccation, hormonal ones (2,4-D, triclopyr and fluroxypyr), ammonium glufosinate and PPO inhibitors (saflufenacil) stand out. Following desiccation herbicides, it is necessary to associate good pre-emergent management.

Among the pre-emergents that are effective against walleye, flumioxazin and sulfentrazone stand out. In pre-emergence, it is important to apply herbicides before the seed germination process begins, as rapid germination and the large amount of reserve provide control “escapes” if the seed has already started its emergence process. In this sense, "apply and plant" can be a good alternative; or, if "plant and apply", this must be within 24 hours after sowing. In any case, the best results are obtained when the herbicide is activated in the soil before the weed emerges.

In the post-emergence period, after soybeans have become established, PPO inhibitor herbicides are also recommended, such as medosafen and lactofen, in addition to photosystem II inhibitors (bentazon). In black walleye populations that are not resistant to ALS inhibitors, chloransulan can be used. In post-emergence we will also have the option of auxin herbicides (2,4-D or dicamba) for crops resistant to these herbicides respectively, however dicamba is currently only registered for pre-sowing application.

Changing to a herbicide with a different mechanism of action, which is still active on the weed population, is an immediate recommendation that every consultancy presents; as the integration of appropriate tactics based on adequate knowledge of weed biology and herbicide mechanism of action is necessary for long-lasting management of herbicide resistance. Therefore, it is important that the producer adopts a production system that disfavors the selection of glyphosate-resistant walleye.

In a management program for a weed that has recently been reported to be resistant to glyphosate, as in the case of black pickerel, it is important to consider the use of all management options; cultural, mechanical and herbicides available for effective control, in each situation, and employ some assumptions of good management practices (GMPs):

• Diversification in relation to management, focused on preventing seed production, and reducing the soil seed bank.

• If you are leasing areas, choose areas free from resistant weeds.

• Use seeds from crops free from resistant weed seeds, although black pickerel is a seed that is very easy to eliminate during processing.

• Constantly monitor the areas to check for the presence of resistant weeds.

• Use a diversification of action mechanisms for the herbicides recommended for the area, and that are effective on black pickerel, even if there is no resistant biotype in your area.

• Always use the dose indicated in the leaflet, never subdosing or exceeding the recommended dose.

• Use crop management practices that favor rapid leaf canopy closure.

• If appropriate, use mechanical management practices such as mowers and harrows.

• Be careful not to cause seed dispersion on the farm.

• Manage late-infested weeds, as these are normally resistant to herbicides; and they enrich the seed bank for the next crop cycle.

• Avoid the entry of black walleye seeds from neighboring areas or roads and highways.

These measures seem elementary, but in practice many of them are neglected. These measures make a difference between producers, and many of them do not represent additional costs, but are simply a matter of planning and adopting good agricultural practices.

Understanding the biology of the weed plant is fundamental in the production system with the aim of reducing the seed bank over the years, and certainly one of the objectives of good management practices, but it is not an easy task. Picão-preto is a weed that has a short life cycle when compared to the crop; and produces seeds long before the crop matures. We are all familiar with the dispersion of the black pickerel, as we have certainly had the fruits of the black pickerel carried on our clothes. However, the longevity of the seeds is short, which makes it easy to eradicate them from an area in a few years.

Reducing the “seed rain” and insertions of new seeds in the seed bank is essential. Picão-preto fruits are large in size, and even so a considerable quantity of seeds per plant is produced. Therefore, one of the ways to prevent black walleye populations from establishing themselves on a property is to prevent the plants from reaching the seed maturation stage, avoiding the enrichment of the seed bank. To achieve this, the tolerance for the presence of black walleye in the area must be zero.

Preventing resistant populations of black pickerel from arriving on the farm is certainly one of the objectives of good management practice for this weed. Therefore, a fundamental practice to prevent seeds from being produced in the crop is to “start cultivation clean”. This means the need for good desiccation, combined later with residual and selective herbicides in the crop in order to completely avoid the production of new seeds of the black pickerel. One of the biggest challenges of this procedure is the cost, which in some situations is not acceptable to the producer.

Although black walleye seed is not similar to the seeds of most crops, sometimes the use of cover crop seeds or green manures, whose quality is not as highly regarded, can come from contaminated walleye seeds. Therefore, it is very important to buy suitable, quality seeds.

The practice of surveying weed infestation in the area is essential for recommending the correct herbicide, however in this survey the producer must be aware of the increase in black pickerel infestation over the years, which may indicate a selection process of plants resistant over the years, although most producers do not carry out this survey for cost reasons.

Diversity of herbicide action mechanisms in the production system is fundamental, whether in mixtures of formulated products or in tanks, or even through crop rotation. Although this recommendation is evident and everyone agrees, there are difficulties in its use due to the lack of knowledge of the mechanisms of action on the part of producers and recommenders, as well as the problem of the increasingly frequent appearance of weeds with multiple resistance to various mechanisms of action.

An aspect that is increasingly complex in the field is the fact that the producer does not follow the dose recommendation indicated on the herbicide leaflet, as well as applying the herbicide at the time indicated for the growth stage of the weed plant. Reducing herbicide doses is a common practice among producers who take into account the principle of producing more for less. Also, the application logistics of large rural properties leads the producer to apply herbicides at a very advanced stage of development of the black pickerel, which can increase the selection process for resistance.

The adoption of more competitive cultivars that can quickly close the canopy is certainly a good agricultural practice to combat glyphosate-resistant walleye. To achieve this, it is important to choose the appropriate variety for the production environment, carry out sowing at the indicated time, follow the recommended sowing rate and appropriate spacing, as well as crop rotation and good nutrient and irrigation management.

The use of mechanical cultivation may be an recommended practice in some specific situations, however, the use of cover crops and mulch is essential for controlling black pickerel, given its sensitivity to these practices. However, the economic aspect sometimes makes the adoption of these practices difficult. Finally, adopting harvesting practices that prevent the dispersion of seeds to other areas, as well as preventing the entry of seeds from borders or paths are essential.

Although herbicides continue to dominate weed control practices due to their economic advantage, the sustainability of herbicide use depends on their diversification and integration with non-chemical weed control strategies. Only through the implementation of diverse weed management practices will herbicides be conserved as a valuable agricultural resource.

By Pedro Jacob Christoffoleti (HERBtech), Francielli Santos de Oliveira (Esalq/USP), Luiz Henrique Franco de Campos (HERBtech) and Leonardo de Oliveira Semensato (UFSCar)

Article published in issue 293 of Cultivar Grandes Culturas Magazine.