INTERFERENCE OF INCREASING DENSITIES OF SOURGRASS IN GRAIN SORGHUM

Sourgrass has a high competitive potential, resulting in yield losses of several crops. With the increase of grain sorghum cultivation in the Cerrado, mainly in the second crop, studies are needed to assess the interference of sourgrass on this crop. The objective of this work was to evaluate the interference of increasing densities of sourgrass on the sorghum crop under Cerrado conditions. The field experiment was carried out in a randomized block design, with four replications. Five treatments, composed of densities of 0, 2, 4, 6, and 8 sourgrass plants per m-2, were evaluated. To simulate the densities of sourgrass, this species was planted in the predecessor crop (soybean) according to the predicted density in each experimental unit, and later, at the time of sowing the sorghum, the sourgrass plants, which were clogged, were cut, in order to simulate the condition provided by mechanized harvesting. During the evaluation of the effect of the treatments on the sorghum crop, the stand and plant height, stalk diameter, panicle length, mass of 100 grains, and yield were measured. The increase in sourgrass density, in coexistence with sorghum, negatively affects all vegetative and reproductive parameters evaluated in the crop. For each sourgrass plant m-2, in coexistence with sorghum, there was a reduction in yield equivalent to 445 kg ha-1. The maximum reduction in sorghum yield observed was 87%, in a condition in which the crop was developed in coexistence at the density of 8 sourgrass plants m-2.


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The production of sorghum in Brazil has grown annually, especially when considering the commercial exploitation of this crop under second crop conditions (known as "safrinha" in Brazil) (CONAB, 2021). This fact can be partially attributed to its greater tolerance to high temperatures and water deficit compared to corn crop. Despite the rusticity of sorghum, in order to obtain higher yields, it is essential to manage agents that have the potential to reduce the yield of this crop, especially weeds .
In this context, the interference of  (Braz et al., 2016). Furthermore, the presence of weeds in the reproductive phase, creates an environment with high humidity, favoring the emergence of pathogens in reproductive structures, which culminates in the depreciation of the final quality of the grains produced, this fact, directaly reflects in the loss of yield and lower acceptance of the product by the market.
The susceptibility of sorghum to interference imposed by weeds, is mainly related, to the fact that the crop has a slow initial development, in the first two weeks after emergence (Cabral et al., 2013). Studies show that grain sorghum, in coexistence with weeds, can show a grain yield decrease ranging from 20% to 97% (Rizzardi et al., 2004;Rodrigues et al., 2010). Among the weeds that present themselves as the most restrictive for control in sorghum, monocotyledons stand out (Machado et al., 2011), since the effectiveness of herbicides becomes limited due to the morphological similarity between the weed species and the crop.
In recent years, due to the intense use of glyphosate in agriculture, and the consequent high selection pressure, reports of glyphosateresistant weeds have been increasing worldwide (Heap, 2021). Among these reports, the existence of sourgrass (Digitaria insularis) biotypes resistant to glyphosate and ACCase inhibitors stands out in Brazil. This weed belongs to the Poaceae family and is characterized by having a perennial cycle, with the formation of clumps and rhizomes, and high seed production capacity, which are easily dispersed (Gemelli et al., 2012).
Sourgrass is widespread in practically all of the Brazilian territory (Ovejero et al., 2017), infesting cultivations of the most varied crops, Cerrado.

Materials and Methods
The study was carried out in the municipality of Rio Verde, Goiás State, in an experimental farm belonging to Universidade de    The statistical analysis of the data from the experiment was performed with the SISVAR computer program (Ferreira, 2011). All data were tabulated and subjected to analysis of variance by the F test and when a significant effect was observed, the data were subjected to regression analysis (p≤0.05).

Results and Discussion
There are several factors that positively affect the development of the crop and help to obtain high yields. Among these, the crop stand stands out, which is mainly a result of the quality of sowing, in addition to the phytosanitary management practices (Pereira Filho & Rodrigues, 2015). In this sense, there     Interference of increasing densities of ...

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The whole scenario above is negatively potentialized when taking into account the higher initial development rate that the sourgrass presents at the expense of sorghum (Figure 4). In addition to this fact, there is a shortage of herbicides registered for grass control in this crop (Gemelli et al., 2012;Braz et al., 2019). In this sense, the need to adopt measures for the integrated weed management in the sorghum crop becomes evident. One of the alternatives for the management of the weed community refers to the adoption of practices related to cultural control, in which the use of cultivars with faster initial growth, reduction in row spacing, and increased plant density stand  out (Peerzada et al., 2016;Braz et al., 2019).
As for the chemical control of weeds in sorghum, as already discussed in this work, there is no extensive portfolio of herbicides registered for the crop, which makes it difficult to adopt this method in the management of the weed community. A study carried out by Silva et al. (2014b) pointed to efficacy in weed control and selectivity of S-metolachlor applied in pre-emergence of the crop, as long as its use is associated with the treatment of seeds with fluxofenim (safener). In this scenario, S-metolachlor becomes a good alternative to control sourgrass plants from seeds (Coradin et al., 2019). Despite this, this strategy is not effective in controlling sourgrass in plants from regrowth, as in the case of the present work.
Another possibility refers to reports in the literature on the potential use of the herbicide tembotrione. Despite not having a record for use in grain sorghum, it has already shown moderate selectivity potential for the crop, with differences in terms of sensitivity between hybrids, doses, and stages of application (Dan et al., 2009). However, it should be noted again that this herbicide has no record, and, therefore, studies would need to be carried out to assess the selectivity of tembotrione to sorghum in applications carried out post-emergence of the crop.
Summarizing the results generated in the present work, it is verified that sourgrass has a high potential of interference on the crop since all the growth and development parameters of sorghum were affected due to the presence and density increase of this weed. Specifically, in relation to yield, it appears that the interference imposed by the higher density of sourgrass plants provided an 87% reduction in the sorghum grain yield.

Conclusions
The increase in the density of sourgrass plants with sorghum negatively affects all characteristics evaluated in the crop.
There was a yield reduction of 445 kg ha -1 for each sourgrass plant m -2 in coexistence with sorghum.
Reductions of 87% in sorghum grain yield were observed in relation to the control when the crop was subjected to coexistence with 8 sourgrass plants m -2 .