Control of the caruru-palmeri weed in Brazil

An aggressive weed, detected in Brazil in 2015, the caruru palmeri is present in approximately 12 thousand hectares of crops in Mato Grosso. With confirmed resistance to glyphosate and ALS in Brazil, Amaranthus palmeri management includes measures such as adequate use of pre- and post-emergent herbicides, use of straw through the direct planting system, in addition to intercropping and crop rotation.

Weed plants were selected from natural environmental disturbances and the development of agriculture, which modified the natural habitat of native vegetation, and thus led to the evolution of species adapted to these environments, which were then called weed plants. Often these species are disseminated throughout agricultural areas of the world by human beings themselves, who transport them to other environments, as they have ornamental, forage, medicinal and even crop characteristics and, subsequently, adapt to the new habitat, becoming weeds with variable degree of aggressiveness. Among the numerous cases, it is mentioned that in Brazil, the species Brachiaria decumbens, B. brizantha and Panicum maximum are examples of forage crops introduced since the mid-1950s, and which are currently important weeds that are difficult to manage.

The species Amaranthus palmeri (caruru-palmeri) was introduced in Brazil, and its presence was identified in 2015, in regions of the cotton production core of the state of Mato Grosso, where the succession production system of cotton, soybean crops is normally practiced. and corn. It is speculated that the caruru-palmeri may have been introduced into Brazil through the import of used grain and cotton harvesters that were contaminated with caruru-palmeri remains from outside the country, a commercial practice that is increasingly common among Brazilian producers. . After the introduction of palmeri caruru in Brazil, there was an expectation that the country's research sector would gather information about its biology, forms of dissemination, competitiveness and control, under Brazilian conditions, as this information exists for other countries, and the Weeds in general behave differently when subjected to different geographic regions in the world. According to an estimate by Indea/MT, the area infested with palmeri caruru in Brazil is currently approximately 11 thousand hectares to 12 thousand hectares, a value that has not changed much since its identification, that is, its infestation is considered under control.

The species is dioecious, that is, in a population, part of the plants will have only female flowers (“female” plants) and the other part will only have male flowers (“male” plants). This is a characteristic that facilitates the identification of caruru-palmeri, since all other species of caruru already identified in Brazil have separate male and female flowers, but on the same plant, being classified as monoecious. Seeds are produced only in plants with female flowers, but female plants can produce seeds even without parthenocarpy pollination. This fact facilitates the invasion of this species into new environments.

In the United States of North America, there are reports of populations of palmeri pigweed that were selected as resistant to at least one of the herbicides belonging to five mechanisms of action: ALS inhibitors (ALS), EPSPs inhibitors (glyphosate), HPPD (HPPD), tubulin synthesis inhibitors (Trifluralin) and photosystem II inhibitors (FSII). The management of these populations becomes even more complex, especially for populations with multiple resistance mechanisms, which has already been reported for two or three of these mechanisms of action: ALS/glyphosate; ALS/glyphosate/FSII and ALS/FSII/HPPD. Beyond the US borders, palmeri pigweed biotypes with resistance to ALS have already been observed in Israel and Argentina. In Brazil, the plant has already been researched for resistance to herbicides and published work proves that the populations studied are resistant to glyphosate and ALS. 

Some control alternatives have been successfully used in the management of glyphosate-resistant A. palmeri, among which the application of herbicides, post-emergents, PROTOX inhibitors such as medosafen, lactofen, saflufenacil or flumiclorac stands out. Furthermore, future crop traits resistant to non-selective herbicides (ammonium-glufosinate, 2,4-D, dicamba and tembotrione), which are being developed, will provide a greater range of options for the management of palmeri pigweed. It is noteworthy that for the success of chemical management of pigweed palmeri through the use of post-emergents, these herbicides must be applied in the initial stages of development of the weed, at the appropriate dose.

In relation to pre-emergent herbicides, management can be carried out using the herbicides s-metolachlor, sulfentrazone, flumioxazin, metribuzin. The use of pre-emergent herbicides has become essential for chemical management to provide good weed control effectiveness, in addition to reducing production losses resulting from weed competition at the beginning of the crop cycle. An important factor to be observed when applying herbicides in pre-emergence conditions are the attributes of the soil, such as organic matter and clay content, as well as the water condition of the soil at the time of application, that is, whether the application is being carried out in wet, dry or transitional seasons.

Furthermore, non-chemical alternatives can also be used, as is the case with the use of straw through the direct planting system, in addition to intercropping between crops, and crop rotation with green manures, which aims to reduce the pressure caused by the bank. of seeds existing in the area, as they hinder the emergence of numerous species, and make germination unfeasible over time.

The combination of more than one palmeri pigweed control method significantly increases management effectiveness and brings benefits in the medium and long term. Integrated management, in which different practices are combined, aims to obtain high effectiveness in controlling herbicide-resistant pigweed, as they provide better control, in addition to making the establishment and, consequently, the reproduction of plants more difficult. weeds in the crop and in the medium and long term, offering greater sustainability to agricultural activity.

In summary, control of palmeri pigweed is recommended mainly in the early stages of growth of the weed, given its high competitive potential with crops, and greater susceptibility to herbicides, thus avoiding losses due to early weed competition. Control of the species is also carried out throughout the crop cycle, in order to prevent escapes and, consequently, the production of seeds by the weed plant. The development of management practices that integrate the sustainable use of herbicides, in addition to adequate implementation of non-chemical methods for cropping systems affected by caruru-palmeri must, therefore, be a priority for successful management both in the current harvest and in future harvests. subsequent ones.

Acácio Gonçalves Netto, Pedro Jacob Christoffoleti and Marcelo Nicolai, ESALQ/USP

Article published in issue 211 of Cultivar Grandes Culturas.