Pre-emergent herbicides for weed management in cotton

A culture with slow initial development, cotton suffers from negative interference from weeds, with limitations to growth, productivity, fiber quality and processing. Pre-emergent herbicides are one of the tools available to manage the problem.

Weeds constitute one of the main problems in cotton cultivation, as they directly interfere with growth, productivity, fiber quality and processing. The cotton plant is generally cultivated with wide spacing that varies from 0,80m to 1,0m between rows and has a slow initial development. These factors end up favoring the growth of weeds, especially in the first months of crop implementation. Therefore, effective control of weeds is necessary to avoid the risk of compromising the productive potential of the crop.

In this context, an experiment was conducted on the residual effect of different active ingredients of pre-emergent herbicides on the main weeds that occur in cotton crops in the southeast region of Mato Grosso.

The experiment was carried out in the municipality of Campo Verde, Mato Grosso, in the 14/15 harvest, in a commercial area with planting on 04/01/15, cultivar FM975 WS with 6 treatments and 4 replications. The treatments are presented in Table 1.

Each plot consisted of 4 rows with spacing of 0,80 m by 5 m in length. The parameters evaluated were weed population and species in 1 m2 in the useful area of ​​the plot at 14, 28 and 42 DAE and plant height at 20 and 45 DAE.

The dosage determinations were the result of discussions and recommendations from professionals, taking into account the product registrations with the Ministry of Agriculture, Livestock and Supply (Mapa) for cotton crops. A CO2 backpack pump with 110° ADI ISSO fan spray tips regulated to a volume of 130 l/ha was used for the application.

The parameters evaluated were weed population and species in addition to plant height at 20 and 45 DAE. To evaluate plant height, a single line was used in each block, evaluating the 5 plants in the central region in a single direction in each treatment so that the amount of fertilizer, which may vary from one line to another, did not cause interference. When measuring height, the distance from the ground to the apex of the cotton plant was considered. The data were subjected to analysis of variance and the means of all treatments were compared using the Tukey grouping method at 1% probability. For the quantification of the weed population, the grouping method used was the Scott-Knott method at 5% probability.

Observed results

Three weed assessments were carried out at 14, 28 and 42 days after cotton emergence (DAE), in addition to plant height at 20 and 45 DAE. The following herbs were found in the experiment: chicken feet (Eleusine indica), junk (Cyperus esculentus.), fire extinguisher(Alternanthera giroidea L. Sm.), trapoeraba (Commelina benghalensis L.), viola chord (Ipomoea triloba L.), joá-bravo (Solanum sisymbriifolium Lam.), caruru (Amaranthus deflexus L.) and Santa Luzia herb (Commelina nudiflora).

The first evaluation found the lowest number of weeds. There was a significant difference only for the control (single application of Glyphosate) in relation to the junquinho population, which presented 0,5 plants/m2, while in the other treatments there were no plants (Table 2).

In the second evaluation 28 DAE, 31 days after application of the first 5 treatments and 21 days after application of S-Metalachlor, a significant difference occurred for yerba de santa luzia and joá-bravo where the control (only Glyphosate) and Trifluralin, respectively, were less efficient in controlling these weeds (Table 3). 

In the third evaluation, at 42 DAE, there was no significant difference between all treatments (Table 4 and 5).

When evaluating plant height, no significant difference was found between treatments, indicating that the products tested do not affect the initial development of the cotton plant (Table 6).

For all purposes, only herbs that emerged after the application of the herbicides were evaluated, avoiding the influence of plants that were already on site at the time of implementation of the experiment. 

Glyphosate did not show efficient sedge control 14 days after emergence, since it is not a pre-emergence herbicide. At 28 DAE, it showed lower efficiency over yerba de santa Luzia and, like all other herbicides evaluated at 42 days, it did not differ in terms of the total number of herbs.

At 14 DAE, Trifluralin did not show efficient control over joá joá. In the following evaluations, it did not differ statistically from the other treatments, when considering the control efficiency, with significant numbers being found indicating that there was no satisfactory reduction in the population.

For Diuron, Diuron + Trifluralin and S-Metalochlor, no results were found that differed from the other treatments in any of the evaluations (14, 28 and 42 DAE). Furthermore, in the last evaluation, at 42 DAE, it was possible to verify that there was no difference between the treatments, indicating that the residual effect of the pre-emergents had already been exhausted and were statistically equal to the control.

According to the plant height assessments carried out at both 20 and 45 DAE, no differences were found in the average plant height, confirming that there was no phytotoxic effect of the tested products that would influence the height or initial development of the cotton crop.

From the statistical data presented, it can be concluded that Glyphosate and Trifluralin were less efficient up to 28 DAE, leaving a greater quantity of invasive plants to infest the area. However, during the experiment, at 42 DAE, all treatments were equal, showing that from then on none of the treatments were efficient in controlling invasive plants.

Cotton in Brazil 

The herbaceous cotton plant (Gossypium hirsutum) It is one of the main fiber-producing crops, with 35 million hectares cultivated around the world (ABRAPA, 2015). According to surveys by Conab (2015), the cultivated area in Brazil in the 2014/2015 harvest was 1.017,1 million hectares, with an average of 1,7 million tons of feather produced. Brazil ranks among the five largest producers in the world, being the first in productivity in upland cotton, third largest exporting country and fifth largest consumer (ABRAPA, 2015).

The Central-West Region is the largest cotton producer in the country, highlighting the state of Mato Grosso as the largest national producer. The state of Bahia is the second largest producer. Mato Grosso and Bahia account for 85% of the country's cotton planted area. Particularly, the culture develops in regions where agriculture is already consolidated and soils with high fertility, which ends up concentrating the producing regions. 

By Wellinton L. Figueredo, Vagner Zuconelli Mendez, Crislaine Cristina S. Martins, Mauricio Claro Silva, Rita de Cassia S. Goussain e Charles Revson Araujo, Federal Institute of Education, Science and Technology of MT

Article published in issue 203 of Cultivar Grandes Culturas.