Weed control in soybeans

Sowing time, density, environmental conditions and cultivar behavior are important factors to avoid weeds

10.03.2020 | 20:59 (UTC -3)

Weeds limit soybean productivity, as they compete for water, light and nutrients, are hosts for pathogens, make harvesting difficult and interfere with morphological characteristics, such as plant height. Sowing time, density, environmental conditions and behavior of cultivars are also important aspects in this context. 

Soybean crops are highly sensitive to interference from weeds, which cause severe damage to plant development. This occurs mainly because they compete for resources in the environment, host disease-causing agents, release allelopathic substances and harm harvesting processes.

The reduction in crop productivity due to the presence of weeds occurs due to competition for environmental resources such as water, nutrients and light, in addition to favoring morphological changes in cultivated plants. Therefore, it is inferred that crops tend to increase in height when they are under competition, as a way of standing out over other plants present so that they can maximize the capture of solar radiation.

Sowing soybeans at different times can also cause changes in plant height. However, sowing density is another factor that influences this variable, as the higher the density, the greater the final height of the plant. Characteristics of current soybean cultivars make them stand out in competition with weedy species, including rapid emergence and good soil coverage during the vegetative phase.

In weed interference studies, three periods are considered important: pre-interference period (PAI), total interference prevention period (PTPI) and critical interference prevention period (PCPI). The PAI is between ten and 33 days after soybean emergence. It is the most important time for weed management, a period where there must be efficient management measures, after which crop yield is significantly affected.

With the objective of evaluating the height and productivity of three soybean cultivars, subjected to sowing times, and whether or not they coexist with voluntary weeds throughout the cycle, work was conducted in the municipality of Frederico Westphalen, Rio Grande do Sul , on the campus of the Federal University of Santa Maria, in the 2013/2014 agricultural year. Three soybean sowing times were used (October 15th, November 15th and December 15th), used to simulate the different periods adopted in the field. The size of the experimental plots was five lines, three meters long, spaced 45cm apart. Fertilization was carried out according to the results of the soil analysis of the experimental area. The soybean cultivars and their main characteristics are presented in Table 1.

At the same time, the cultivars were implanted in their respective seasons, one area in the presence and another in the absence of voluntary weeds. In treatments with weed coexistence, no type of chemical or mechanical control was carried out. Only the crop coexisted with the voluntary growth of weed species throughout the cultivation period. The treatments without coexistence received chemical control throughout the cultivation, so that there was no competition between species. Height assessment was carried out 45 days after each sowing date, a date chosen because it was just after the period before interference (PAI), evaluating four random plants in the center of each plot, with the aid of a ruler graduated in centimeters.

The plants were harvested after the grains reached maturity, and grain productivity was determined by collecting the plants contained in the three central rows of each plot, totaling a useful area of ​​1,35m². After manual threshing, the grains were weighed and the data transformed into kg/ha at 13% humidity.

The grain productivity of soybean crops, in general, was higher in the absence of weeds. At the sowing time of October 15th, this trend may not have been observed, due to the invasive seed bank being low, to the point of not compromising the analyzed variable.

The BMX Tornado RR and TEC 7849 Ipro cultivars showed higher productivity in the absence of weed species, a behavior not observed in the TEC 5718 Ipro cultivar. These results corroborate Vazquez, et al (2014), who found a different response in soybean productivity depending on the cultivar used, and whether or not herbicides were used. In the comparison between cultivars without the presence of weeds, BMX Tornado RR demonstrated superior results to the other cultivars.

In Table 3, where data on plant height are presented, the cultivar BMX Tornado RR at the sowing time of November 15th presented higher values ​​than the other two times, both in the treatment with coexistence of weeds and in their absence. On October 15th, with weeds coexisting, the height was greater (43,32cm) compared to the presence (39,00cm) for the same cultivar. Studies show that plants tend to increase in height when coexisting with invasive plants due to a decrease in the quality of light, in order to stand out over weeds. This is relative acclimation to plant competition (Jensen et al.

When sowing on December 15th for the TEC 5718 Ipro cultivar, the superiority of this sowing time over the others was observed, however with small differences between the times, in the treatment of weeds. For the first season (October 15), the weed-free treatment of this cultivar was superior to the weed-free treatment.

The cultivar TEC 7849 Ipro showed greater height in treatments with the absence of weeds in October and December sowings. In November sowing, the cultivar reached 54,33cm with the presence of weeds, being superior to the weed-free treatment. Comparing only the cultivation with the presence of weeds of this cultivar, the tallest soybean plants were detected at the time of November 15th. The sowing date of December 15th is close to the recommended time for this cultivar, and as it is characterized by tall size and indeterminate growth, this treatment without the presence of weeds obtained a height of 63,33cm

In January, rainfall was 246,4mm, which favored cultivation, but in February the rainfall was only 16,2mm, which affected the experiment. This factor may also have influenced the response of the cultivars, mainly due to the competition between weed species and soybeans due to the little water available in February.

The presence or absence of weeds causes the soybean crop to respond differently in each situation, and may not express its maximum productive potential, thus modifying its morphological characteristics such as plant height. However, the sowing time is of paramount importance in managing the crop, which is why the zoning and cycle of each cultivar must be observed. It is also necessary to consider the environmental conditions to which the plants will be exposed, since all these factors can interfere with the development of the soybean crop.

Figure 1. Soybean cultivars exposed to weeds. Frederico Westphalen/RS, 2014.
Figure 1. Soybean cultivars exposed to weeds. Frederico Westphalen/RS, 2014.
Figure 2. Measurement of the height of soybean plants. Frederico Westphalen/RS, 2014.
Figure 2. Measurement of the height of soybean plants. Frederico Westphalen/RS, 2014.
Figure 3. Different soybean sowing times in the absence of weeds. Frederico Westphalen/RS, 2014.
Figure 3. Different soybean sowing times in the absence of weeds. Frederico Westphalen/RS, 2014.


Thaise Dieminger Engroff (Agronomy Course/ UFSM, Frederico Westphalen - RS); Braulio Otomar Caron (Agronomy Course/UFSM, Frederico Westphalen – RS); Ana Paula Rockenbach (Postgraduate in Agronomy, Agriculture and Environment/UFSM, Frederico Westphalen – RS); Elvis Felipe Elli (Agronomy Course/UFSM, Frederico Westphalen – RS); Elder Eloy (UFPR, Curitiba - PR); Luciano Schievenin (Agronomy Course/UFSM, Frederico Westphalen - RS)


Article published in issue 198 of Cultivar Grandes Culturas.

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