The impact of weed competition on soybeans: how to increase productivity and reduce losses

Soy is the most produced oilseed in the world and, due to its great global demand, it is essential to understand the factors that guarantee maximum crop productivity. Among them, we can highlight weed competition as one of the main challenges, as it directly interferes with the development and productivity of soybean crops.

The competition between crops and weeds is called weed competition. This situation can directly impact productivity, generating competition for essential resources such as water, light and nutrients between the main crop and weeds. Furthermore, it affects indirectly when it increases production costs, reduces the quality of the final product, makes harvesting difficult and favors the emergence of pests and diseases (EMBRAPA, 2023).

This competition stands out mainly in the initial phases of crop development, causing productivity losses that can exceed 80% and, in more severe cases, make harvesting unfeasible.

Weed plants have characteristics that favor their development, such as rapid germination and initial growth, abundant root system, great capacity to absorb water and nutrients from the soil, high efficiency in water use, high production of propagules and efficient dissemination, making competition more aggressive.

Productivity losses



Weeds can compete with soybeans throughout the cycle, causing losses in productivity that can vary between 29% and 90%, as described by Blanco (1974). In cases where no type of control is carried out, Embrapa estimates that losses can exceed 90%. On average, losses in grain production due to invasive plants are estimated between 13% and 15%, therefore, understanding the impact of weeds and acting strategically and assertively is essential for success in the field.

When to manage?



There are times when soybean crops are more sensitive to competition. This interval, known as the Critical Interference Prevention Period (PCPI), corresponds to the phase in which control practices must be adopted effectively. If efficient management is not carried out during this period, there will be quantitative and/or qualitative losses in crop production. It occurs between the Pre-Interference Period (PAI), where weeds are present in the crop, but do not cause significant damage to soybeans, and the Total Interference Prevention Period (PTPI), post-emergence period, where the crop must be develop free from weed competition, in order not to promote losses in productivity.



It is important to highlight that the critical period varies according to the cultivar, type of soil and climate, sowing date, among other factors. Furthermore, it is described in days or weeks and not based on the crop's development stage, which reduces its applicability.





Identifying the main weeds in soybean crops



In the current scenario in the southern region of the country, we can highlight some plants that cause major losses in soybean crops, one of the main ones being Horseweed (Conyza spp.), causing a loss of up to 90% in productivity when not controlled.

Caruru (Amaranthus spp.) is difficult to control due to its high seed production, rapid development, long period of dormancy in the soil and difficulty in early identification of the species, in addition to being resistant to glyphosate.

Source: Reproduction/internet

The milkweed (Euphorbia heterophylla) is an example of a broadleaf that hinders the development of soybeans and the interference of its latex in the harvest of infested fields depreciates the harvested product and also affects its final price. In addition to milkweed, Morning Glory (Ipomoea spp.) also makes harvesting difficult as it has a climbing habit and spreads easily through the soil.


Management strategies to reduce weed competition

Reducing weed competition in soybeans requires the integration of several management strategies. For example, the use of cover crops, such as millet or sunn hemp, in the off-season, forms a layer of straw that makes it difficult for weed seeds to germinate, such as bittergrass (Digitaria insularis). Furthermore, it is important to rotate herbicides with different mechanisms of action, aiming to minimize the resistance of problematic species, such as Amaranthus spp. A common practice is to alternate herbicides such as Glyphosate (EPSPS group) with Diclosulam (ALS group), reducing the selection of resistant weeds, such as Pigweed and Horseweed.