Herbicide-resistant pigweed: options for integrated management

Amaranthus hybridus, a weed popularly known as caruru, has been one of the biggest concerns for soybean producers in southern Brazil. Since 2018, when resistance to the herbicide glyphosate was confirmed in crops in Rio Grande do Sul (RS), a gradual increase in the infestation of areas has been observed, harvest after harvest. Resistance can also extend to ALS-inhibiting herbicides such as chlorimuron, diclosulam, imazethapyr, among others, reducing management options.

With a physiology that guarantees rapid growth, associated with emergency flows, pigweed plants that escape control during desiccation or emerge before soybeans will have a great advantage. In the conditions of the southern half of RS, greater seedling emergence has been observed from the month of November onwards, when temperatures tend to rise after the winter period. A study analyzing the economic damage of this weed to soybeans in RS indicated productivity losses of around 4 to 8% per hectare (Zandona et al., 2022). 

The problem of glyphosate-resistant pigweed (A. hybridus) has occurred since 2013 in Argentina (Heap, 2023), with the first report in 1996 for resistance to the herbicides chlorimuron and imazethapyr. More recently, cases reported in corn and soybean crops also indicate resistance to 2,4-D and dicamba and, also, medosafem (Protox enzyme inhibitor) in 2021, which causes great concern to technicians and the productive sector in that country. In the USA, there is a record of resistance to FSII inhibitors such as atrazine, metribuzin and simazine (Heap, 2023), in addition to the problem with the giant pigweed (A. palmeri), considered a quarantine pest in Brazil. Looking at the history of these countries, we ask if it is possible to prevent this disastrous scenario from happening in Brazilian crops? We know from practice that it is only possible to delay the evolution of resistance to herbicides. However, it is necessary to adopt an effective integrated management program, that is, to use associated practices that involve actions beyond chemical control.

In order to reduce the spread of cases of resistant populations of pigweed, based on seed production, which increases the problem for the following harvest, some points must be highlighted:

1 - Carry out good desiccation of the area where the soybeans will be sown: the association of alternative molecules to glyphosate becomes important and complements the control of plants that may have already emerged: glyphosate + saflufenacil + 2,4-D (3,0 L/ha + 50 g/ha + 1,5 L /ha) has proven to be a good alternative, followed by a sequential application (10 days later) of glufosinate ammonium (2,0 L/ha); options involving clethodim such as the pre-formulated association clethodim+fluroxypir (1,2L/ha) and glyphosate (3,0L/ha) or clethodim + glyphosate + saflufenacil (0,5 L/ha + 3,0 L/ha + 70 g/ha), both followed by a sequential application of ammonium glufosinate, prove to be efficient especially when there are reports of glyphosate-resistant ryegrass in the area, which can interfere with soybean establishment.

2 - Use pre-emergent herbicides: The adoption of this practice has proven to be fundamental, since there are limited options for post-emergence management of pigweed in soybeans (practically only falesafem). Good results from pre-emergence applications have already been observed with Protox inhibitors, shoot growth inhibitors, FS2 inhibitors, for example, and associations – flumioxazine, sulfentrazone + diuron, s-metolachlor, flumioxazine + imazethapyr, among others. However, the limitation observed in the use of these products is the need to reach the region where pigweed seeds are in the soil. There is a need for incorporation (mechanical or irrigation/rain) so that the product comes into contact with the layer of soil where the pigweed seed bank is located. Especially in the Campanha do RS region, the months of November, December and January tend to be quite dry, with the absence of significant rain, which can hinder the efficiency of these products. Having irrigation is the greatest guarantee of efficiency of this tool.

3 - Post-emergency management: This action complements the use of pre-emergents. As already mentioned, there are limited options for pigweed in this application modality (practically falesafem). The Protox enzyme inhibitor molecule has been used successfully, however, for pigweed plants up to 10cm tall in post-emergence. The use of post-emergent controls new emerging flows of pigweed. New technologies are arriving on the market involving the use of dicamba and 2,4-D, allowing more management options for soybean cultivars using the technologies in question. At the moment, only 2,4-D technology allows for post-emergency use. However, in areas with very high infestations, the use of pre-emergent may not yet be ruled out, providing greater security and reducing the pressure and/or responsibility on the post-emergent. It is worth remembering that there are already reports of resistance to 2,4-D and dicamba in Argentina.

4 - Crop rotation: Crop rotation with corn as well as summer pastures (forage sorghum, millet, sudangrass, among others) allows for rotation of herbicide molecules. Mechanisms of action that involve the inhibition of FS II (atrazine, simazine) or the HPPD enzyme (mesotrione) can help control and prevent new entries into the soil seed bank. 

5 - Prevent the spread of seeds: cleaning machinery used in harvesting is essential, since the spread of pigweed is widely favored. Studies have already evaluated and indicate that cattle, sheep and birds can be dispersers of viable pigweed seeds, even after they pass through the digestive tract of these animals. Therefore, grazing animals should be prevented from coming into contact with pigweed plants with seeds or even receiving hay contaminated with seeds. 

In addition to the strategies mentioned, as part of integrated management, it is possible to practice complementary actions, starting with what comes before farming, from a production system perspective. Livestock farming is an activity inherent to Gaucho culture, especially in the southern half of the state of RS. The areas used to grow grain crops in the summer, especially soybeans, usually have ryegrass and/or oat pastures in the winter, mainly for fattening beef cattle. A previous study carried out at Embrapa Pecuária Sul indicated that moderately grazed ryegrass residue (4,4 t/ha) served as an important physical barrier to pigweed emerging in soybeans in succession; this comparison was made to the residue grazed with greater intensity (2,5 t/ha) and also to an area prepared for conventional soybean cultivation (Lamego et al., 2022). 

In systems integrated with crops and livestock, so that the crops in succession take advantage of the advantages of livestock farming practiced in winter, it is essential to adapt the animal stocking rate to the availability of forage; Excessive grazing harms pasture growth in addition to favoring soil compaction, which harms crops. Therefore, observing the technical recommendations for each forage crop and adjusting the animal load is essential. We know that the “straw” coming from cover crops that preceded the crop is essential in a conservation system such as direct planting, in order to preserve the soil, retain moisture, and even act as a barrier to the emergence of weeds. Grazing residue may also contribute to the control of weeds in succession crops, in regions where livestock farming is practiced. 

Final Words

Herbicide-resistant pigweed is a serious problem and the evolution of new cases should continue to occur, if only the exchange of chemical molecules is observed. Care must be adopted in the form of management programs, aiming to avoid new cases of resistance. The use of pre-emergent herbicide to manage resistant pigweed has been efficient and important.

However, the vision of the production system, involving the phase prior to farming, as in the case of livestock farming in regions of RS, can favor integrated pigweed management. To do this, it is necessary to observe animal stockings that optimize the use of pasture and also allow residue that can be part of the management of resistant weeds, to be complemented by the other strategies highlighted.

*Per Fabiane Pinto Lamego (in the picture), from Embrapa Pecuária Sul, and Marlon Ouriques Bastiani, from Três Tentos Agroindustrial