Weeds Resistant to Herbicides Threaten Soybean Farms in Brazil

“Herbicides are not obsolete and they must be used correctly, with an adequate dose, correct recommendation regarding the target…”

Anderson Luis Cavenaghi is the director of agricultural, biological and engineering sciences at Centro Universitário de Várzea Grande – Univag, where he coordinates weed resistance monitoring projects, mainly in the State of Mato Grosso. Cavenaghi is an agronomist, M.Sc and Ph.D in plant protection from Universidade Estadual Paulista.

Anderson Cavenaghi, director at Univag

AgriBrasilis – What is the current situation of weed resistance in the State of Mato Grosso? Which crops are most affected?

Anderson Cavenaghi – The biggest problem currently is the Indian goosegrass (Eleusine indica) with multiple resistance to EPSPs [e.g. glyphosate] and ACCase [e.g. haloxyfop] inhibitors. In addition to this weed, other important ones, such as flaxleaf fleabane (Conyza bonariensis), sourgrass (Digitaria insularis) and amaranth (Amaranthus spp.) are resistant to glyphosate, that makes management difficult.

As for crops, the greatest difficulty is with soybeans, as they have the largest planted area and the fewest options for herbicides to control resistant plants in post-emergence. Even so, resistant plants will be present in the areas and will be a problem for any crop established there, such as cotton and corn, in greater or lesser proportions, depending on the species that occupies the area.

AgriBrasilis – How is resistance monitored? What is the difference between resistant and tolerant plants?

Anderson Cavenaghi – In the case of Univag, monitoring is carried out by collecting seeds in areas where there are control problems. This collection is carried out mainly by representatives of multinational companies, with whom we carry out monitoring projects in partnership.

The seeds are sent to Univag for herbicide resistance studies, that is, tests with products used in the collection area, alternative herbicides for control, dose-response curves and use of molecular biology to try to understand what happens in each case.

Tolerance against a given herbicide indicates that a species will not be well controlled, as it has never been totally susceptible, as is the case with the dayflowers (Commelina spp.) to glyphosate, for example.

Resistance indicates that there will be no control for a species that was previously susceptible to the herbicide, as happened with Indian goosegrass and glyphosate, for example.

AgriBrasilis – Which herbicides have shown the highest number of resistant plants? Is it possible to consider that some herbicides, such as glyphosate, are becoming obsolete?

Anderson Cavenaghi – Lately, the major concern, mainly because of RR crops, is the glyphosate herbicide, but the ALS inhibitors [imazaquin, imazethapyr, chlorimuron, metsulfuron, diclosulam, among others] no longer control important plants such as milkweed (Euphorbia heterophylla), black-jack (Bidens pilosa), amaranth and flaxleaf fleabane longer, because of resistance.

Herbicides are not obsolete and they must be used correctly, with an adequate dose, correct recommendation regarding the target, etc. Rotation of action sites should be practiced, among other precautions.

AgriBrasilis – What is multiple resistance? What cases are identified?

Anderson Cavenaghi – Multiple resistance is the one that has caused the most concern, because of the loss of more than one site of action in controlling the same resistant biotype. Several cases have already been registered in Brazil and I advise accessing the page www.weedscience.com to know all the cases already mentioned.

AgriBrasilis – What strategies should be adopted to manage weeds resistant to herbicides?

Anderson Cavenaghi – The main one is to use herbicides with different sites of action in the area with resistance problem, always remembering the importance of using pre-emerging herbicides in these areas.

AgriBrasilis – Why has the importance of Palmer amaranth (Amaranthus palmeri) increased in recent harvests? Could you comment on the history of this pest, since its introduction in Brazil in 2015?

Anderson Cavenaghi – The Palmer amaranth was initially found in the State of Mato Grosso, but a plant was recently found in the State of Mato Grosso do Sul, that called everyone’s attention again.

The first report, in 2015, was possible precisely because there were ongoing monitoring projects and we received A. palmeri seed for evaluation. Once identified, researchers, pesticide companies, public agencies and farmers came together to understand what was happening, inform society and fight this plant. This led to a reduction in the density of this species in the first infested properties. However, although slow, there was an increase in the number of properties identified with the species.

To fight Palmer amaranth, Normative Instruction No. 047/2015 was published, updated in 2020, through IN Indea-MT 003/2020, with rules for eradicating and containing the invasive plant. This set of actions has prevented the problem from becoming much bigger so far.



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