Integrated management to control the weed Caruru palmer

The main weed of cotton crops in the USA, caruru palmer was identified in cultivation areas in Brazil in 2015. With a devastating effect when not controlled, this weed, resistant to several herbicides, has the firepower to cause productivity losses of up to 91% in corn, 65% in cotton and 79% in soybeans.

Identified for the first time in Brazil, in 2015, in cotton planting areas in the state of Mato Grosso, the weed caruru palmer (Amaranthus pameri) is the subject of important debates regarding its potential negative impact on cultures not only in this region, but also throughout the country. It is a plant resistant to the herbicide glyphosate, as well as herbicides that inhibit the ALS enzyme, with considerable speculation as to whether it could also be resistant to other herbicides and thus, what efficient control alternatives would remain for control.

The pigweed species occurring in Brazil have an annual life cycle, reproduced exclusively by seeds and, in general, are difficult to visually differentiate between species. A large plant can produce up to 1.000.000 seeds. A. palmeri is an opportunistic and competitive weed species par excellence, with high fecundity, germination and rapid growth, phenotypic capacity and phenological plasticity that allow seed production under different conditions. Studies carried out in the United States show that A. palmeri it can produce between 600 thousand and 2 million seeds per plant and take from 3 days to 8 days to germinate. 

Caruru palmer is a dioecious species, which means that, in a population, part of the plants will have only female flowers (“female” plants) and another part will only have male flowers (“male” plants). This is a feature that makes it easier to identify A. palmeri, since all other pigweed species already identified in Brazil have male and female flowers on the same plant, being classified as monoecious. Seeds are only produced in plants with female flowers, but there is an important and aggravating detail in the reproduction of the species: female flowers can produce seeds even without pollination occurring.

Caruru palmer originates from arid regions in the central south of the United States of North America (USA) and northern Mexico, and is present in several countries around the world. In recent years, this species has become the main cotton weed in the USA, due to its biological characteristics and resistance to herbicides from different sites of action. In the USA, in addition to glyphosate, there are reports of populations of A. palmeri resistant to other mechanisms of action, such as: 
• Tubulin synthesis inhibitors (K1)

It was the first report of herbicide resistance of A. palmeri in the United States. Resistance to the herbicide trifluralin was confirmed in 1989 in eight populations from different locations in North Carolina and in 1998 in the state of Tennessee. 

• Photosystem II (C)

The first reports came from the US state of Texas in 1993 regarding the herbicide atrazine. Subsequently, atrazine resistance was reported again in Texas and also in Kansas in 1995 and in Georgia in 2008. 

• ALS (A)

In the United States, ALS inhibitors have been widely used to control A. palmeri since their introduction in 1982. Most cases of resistance to ALS-inhibiting herbicides are caused by changes in the base sequence of the ALS gene, resulting in an enzyme that is less sensitive to the binding of ALS inhibitors. This altered sequence is normally inherited as a single allele with a high degree of predominance. Caruru Palmer resistance to ALS-inhibiting herbicides is widespread throughout the southern United States. Cases were reported in Arkansas in 1994, North Carolina in 1995, South Carolina in 1997, Georgia in 2000, and Florida and Mississippi in 2008. Cross-resistance to several ALS-inhibiting herbicides is common in Palmer.

• HPPD (F2)

Resistance to several herbicides that inhibit HPPD was recently confirmed in the US state of Kansas in 2012, but the mechanism and mode of inheritance of resistance is not yet known.
The management of herbicide-resistant Caruru palmer populations becomes even more complex, especially for populations with multiple resistance, already reported for two or three of these mechanisms of action: ALS/glyphosate; ALS/glyphosate/FSII and ALS/FSII/HPPD. Beyond US borders, Caruru palmer biotypes with resistance to ALS inhibitors have been reported in Israel and Argentina.

The impact of lack of control of Caruru palmer, whether due to lack of management or the fact that the plants are resistant to some type of herbicide, in any type of crop, can be devastating. These productivity losses can reach 91% in corn, 65% in cotton, 68% in sorghum, 79% in soybeans, 68% in peanuts and 94% in sweet potatoes. 

Integrated management to control Caruru palmer is essential as it is an extremely aggressive plant, capable of growing 5 to 7 cm per day, in addition to the aforementioned ability to produce large quantities of seeds that germinate quickly. Some control alternatives have been successfully used in the management of glyphosate-resistant Caruru palmer. It is possible to highlight the application of some post-emergent herbicides such as glufosinate ammonium in Liberty-Link crops, such as cotton and soybeans, 2,4-D and dicamba in resistant or tolerant crops, such as soybeans and corn, atrazine alone or in combination with HPPD-inhibiting herbicides in corn, protox-inhibiting herbicides such as falesafem or lactofem in soybean. In relation to pre-emergent herbicides, the herbicides s-metolachlor, sulfentrazone, flumioxazin and metribuzim have been used successfully. It is necessary to take into account that for successful use, the weed plant's development stage must be as small as possible for post-emergent herbicides. Likewise, soil characteristics must be observed, such as organic matter, clay content, soil moisture and time of year in the case of pre-emergents.

Several factors can contribute to Caruru palmer becoming one of the main weeds in Brazilian cultivation systems. Some of these factors are associated with weed management practices. For example, direct planting that emphasizes not disturbing the soil favors Caruru palmer with its small seeds that germinate easily on the surface of the straw. Another factor that may favor the increase in infestation is the great dependence on herbicides in the current production system based on glyphosate and ALS inhibitors, which creates a favorable environment for this weed, which has great potential for rapid evolution. of resistance. 

More studies on long-term strategies are necessary, mainly with the aim of preventing Caruru palmer from becoming resistant to herbicides other than glyphosate and ALS inhibitors. The introduction of crops resistant to other mechanisms of action will provide producers with more flexibility in chemical control, as well as greater adoption of the use of pre-mergers especially in soybean crops. However, given the great adaptability of this weed, the evolution of multiple resistance to herbicides is highly likely. 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 palmer should therefore be a priority. Essentially, the rotation of herbicide mechanisms of action is fundamental, understanding that pre-planting desiccation must be better explored, as well as the use of pre- and post-emergent alternatives to glyphosate.

Pedro Jacob Christoffoleti, Acácio Gonçalves Netto, Esalq/Usp; Marcelo Nicolai, Ronald EH Weber, Unesp, Botucatu Agrocon

Article published in issue 203 of Cultivar Grandes Culturas.