Cultivated plants, which infest crops in succession, are considered weeds, and in practice are called volunteers. Under favorable crop succession conditions, any crop can become a volunteer plant in the subsequent crop. The losses caused by volunteer plants in succession crops are not only observed in productivity, but also in the industrial quality of the raw material, difficulty in harvesting and transportation operations. The problem of volunteer plants is aggravated when crop succession occurs between glyphosate-tolerant crops, requiring diversification of control measures, especially herbicides. Therefore, this is one of the new challenges that are emerging in the Brazilian agriculture scenario, since volunteer plants are becoming new enemies of grain and fiber production crops.
Despite the benefits brought by glyphosate-tolerant crop technologies, some problems are also being generated and require producer attention. Often, due to occasional crop losses or excessive maturation of reproductive organs, grain losses from commercial crops end up occurring and these plants are left in the field. Thus, because they do not germinate constantly, they can give rise to plants called tigueras or volunteers resistant to glyphosate, interfering with subsequent crops, causing losses.
Often, the producer does not have precise information on quantifying the losses that volunteer plants cause, but the values are very worrying, mainly because cultivated plants have greater vigor than many weed plants. In the case of glyphosate-resistant corn volunteers, significant losses in soybean productivity have been observed from the infestation of 6,5 volunteer plants/m2. In research carried out in the USA, it was reported that the occurrence of 50 volunteer plants per hectare of cultivated soybeans can reduce up to 12% of the crop's total production.
Currently, one of the factors that intensified the need to control volunteer plants was the significant increase in succession cultivation, such as the planting of cotton in the second crop (safrinha), right after the harvest of the soybean crop. This strategy used by many producers is advantageous in reducing the cycle and size of the cotton plant, resulting in reduced use of inputs and greater profitability compared to other options for the second harvest. However, along with the emergence of second-crop cotton, the occurrence of volunteer soybean plants is common, leading to losses in the development of this crop, due to initial interference. As the initial growth rate of soybeans is faster than that of cotton, volunteer plants cause shading, reducing the photosynthetic rate of these plants and stimulating their etiolation. Furthermore, due to the few herbicide options indicated for broad leaves registered for this crop, soybeans can become a weed that is difficult to control in cotton.
Another problem that intensified the need to control volunteer plants was the difficulty of carrying out cultural practices such as “sanitary emptying”, which aims to reduce the incidence of pests and inoculums, through a period established in the off-season in which there should be no the presence of emerged plants. The presence of volunteer soybeans in the off-season, for example, serves as a host for the survival of the inoculum and multiplication of the Asian rust fungus, a disease that has caused significant damage to crops. Volunteer cotton plants also present problems associated with the absence of sanitary voids, since the presence of tiguera in the midst of its cultivation favors the incidence of pests, such as the cotton boll weevil.
In order for volunteers to be controlled at the beginning of the cultivation of new crops, avoiding any type of loss due to weed competition, herbicides applied in pre- and post-emergence conditions of weeds, with mechanisms of action different from glyphosate, are the main alternatives. of management. Pre-emergents have been used with advantage, as they can be applied to the direct planting system, eliminating the need for incorporation. Furthermore, due to its residual period, it prevents many reapplications from occurring, saving time, fuel and machinery, in addition to preventing soil compaction. The main precautions that must be taken with this type of herbicide are related to its selectivity, in order to prevent injury to commercial crops.
Management with post-emergence herbicides must be carried out when the volunteers are still in the first stages of development, from three to five leaves (V3 to V5). If control is carried out late, it will be necessary to apply a graminicide (in the case of volunteer corn). As for volunteer plants in soybeans and cotton, whose control is more complicated due to the lower selectivity of herbicides for these crops, control must be in the first stages of development.
Having notified the problems that volunteer plants can cause and the extent to which they have been intensifying, the control of these plants has become a mandatory legislative measure in several states of the federation. This fact highlights the importance of investigating and evaluating possible control alternatives for tiguera plants, especially those resistant to glyphosate.
Considering the three most important annual crops in Brazil, soybeans, corn and cotton, and taking into account that succession/rotation can occur in all possible combinations, the main recommended herbicides stand out (Table 1).
Table 1 - Main selective herbicides for controlling corn, soybeans and voluntary cotton, in soybean, corn and cotton crops, respectively
Volunteer plants | ||||||
Soybean | Corn | Cotton | ||||
Cultures | Pre* | Post** | Pre | Post | Pre | Post |
soybean | --- | --- | No option | Clethodin, haloxyfop, sethoxydin, tepraloxydin, fluazifop-butyl and quizalofop-butyl | No option | Ammonium glufosinate***, lactofen, flumioxazin and medosafen |
Corn | Atrazine | Tembotrione, atrazine mestotrione and 2,4-D**** | --- | --- | Atrazine | Mesotrione, Ammonium glufosinate***** and 2,4-D**** |
Cotton | Diuron | Trifloxysulfuron sodium, Pyrithiobac and ammonium glufosinate*** | Diuron | Clethodin, haloxyfop, tepraloxydin, fluazifop-butyl and quizalofop-butyl | --- | --- |
* Herbicides applied under pre-emergence conditions | ||||||
** Herbicides applied in post-emergence conditions | ||||||
*** in case the crop is tolerant to glufosinate ammonium | ||||||
**** Observe the appropriate stage of the application crop to avoid phytotoxicity | ||||||
*** for the case of the voluntary plant not be tolerant to glufosinate ammonium | ||||||
Volunteer plants
Soybean
Corn
Cotton
Cultures
Pre*
Post**
Pre
Post
Pre
Post
soybean
---
---
No option
Clethodin, haloxyfop, sethoxydin, tepraloxydin, fluazifop-butyl and quizalofop-butyl
No option
Ammonium glufosinate***, lactofen, flumioxazin and medosafen
Corn
Atrazine
Tembotrione, atrazine mestotrione and 2,4-D****
---
---
Atrazine
Mesotrione, Ammonium glufosinate***** and 2,4-D****
Cotton
Diuron
Trifloxysulfuron sodium, Pyrithiobac and ammonium glufosinate***
Diuron
Clethodin, haloxyfop, tepraloxydin, fluazifop-butyl and quizalofop-butyl
---
---
* Herbicides applied under pre-emergence conditions
** Herbicides applied in post-emergence conditions
*** in case the crop is tolerant to glufosinate ammonium
**** Observe the appropriate stage of the application crop to avoid phytotoxicity
*** for the case of the voluntary plant not be tolerant to glufosinate ammonium
Click here to read the article in issue 179 of Cultivar Grandes Culturas.
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