The critical period of weed competition in corn is the phase in which the crop cannot coexist with weeds without suffering losses. In this initial stage, the plants compete for water, light, space, and nutrients, which justifies early control to avoid economic losses.
The greatest losses occur between 20 and 60 days after corn emergence, that is, between the third (V3) and twelfth (V12) corn leaf, a period in which the grain yield potential is defined. When management is not carried out correctly, weeds can reduce productivity by 12% to 100%, depending on the species present, the level of infestation, the type of soil, the climatic conditions and the phenological stage of the corn (FANCELLI; DOURADO NETO, 2000).
Main weeds in post-emergence corn
Fleece (Conyza spp.), Sourgrass (Digitaria insularis), Goosegrass (Eleusine indica), Amaranth (Amaranthus spp.) Morning glory (Ipomoea spp.) are among the main species that require attention in post-emergence corn management.
These weeds have a great capacity for competition and, in many cases, exhibit resistance to herbicides, such as goosegrass and fleabane.
Impacts of Competition on Production: Direct and Indirect Damage
The presence of weeds is one of the factors that most reduces corn productivity. Even though it is a highly efficient C4 photosynthetic pathway crop in utilizing light, water, and nutrients, this advantage disappears when corn competes with weeds of similar metabolism, such as sourgrass (Digitaria insularis).
During the critical period of interference, this direct competition deprives the crop of essential resources, resulting in:
- shorter plant height;
- reduced leaf area;
- fewer grains per ear;
- decreased final yield.
These losses can be significant even in relatively low weed populations, especially in aggressive and difficult-to-control species.
A study conducted at the Federal Technological University of Paraná (UTFPR), in Dois Vizinhos - PR, clearly illustrates the impact of goosegrass on corn cultivation.
The researchers observed that the height of the corn plants decreased by 8% when the density of goosegrass increased from 0 to 6 plants/m², going from 2.72 m to 2.50 m.
These results prove that even relatively low populations of goosegrass significantly affect the crop. In addition to reduced productivity, managing this weed is challenging because it is resistant to glyphosate and forms clumps that make control difficult. In practice, the earlier control is carried out, the smaller the economic impact.
The ideal time to control weeds in corn
The critical period of interference (CPI) is the interval in which the presence of weeds causes significant productivity losses. It is the phase in which corn cannot coexist with weeds without suffering reductions in its yield potential. Therefore, knowing this window is fundamental to deciding when to carry out control and avoid losses.
Up to 14 Days After Emergence (DAE), corn can coexist with weeds without suffering significant productivity losses (RAMOS & PITELLI, 1994).
The coexistence with weeds from 15 to 42 DAE (days after emergence) represents the most critical time interval; coexistence during this period results in a significant reduction in productivity, as it coincides with phases of rapid growth and high demand for light, water, and nutrients. Weeds that emerge at 42 DAE have a low impact on corn productivity, since the crop is already more developed. BLANCO et al. (1976) highlight that the period of greatest need for control is approximately from the 15th to the 45th DAE of corn. Controlling weeds within this window ensures greater utilization of applied inputs and preserves the productive potential of the crop.
Post-emergence herbicide alternatives in corn
Chemical control is widely used by producers due to its operational speed and effectiveness, provided that the products are correctly positioned according to the crop stage and target species.
Among the options available for corn, herbicides from the sulfonylurea chemical group (ALS inhibitor) have demonstrated good selectivity and consistent performance in the management of grasses and some broadleaf weeds, especially when applied in the early stages of the crop.
When combined with atrazine (photosynthesis inhibitor), the results are usually broader, allowing for more robust control of broadleaf weeds and contributing to complementing the spectrum on grasses.
In applications carried out after the first few days of corn emergence, different post-emergent herbicides, such as nicosulfuron, atrazine, and bentazon, have shown good performance in the management of broadleaf species, such as horseweed (Conyza spp.), morning glory (Ipomoea triloba), and milkweed (Euphorbia heterophylla), when used according to label recommendations and technical positioning.
Associations such as nicosulfuron + atrazine broaden the spectrum of control and can offer greater safety in management, especially in areas with high infestation pressure, provided that the stage of weed development and environmental conditions at the time of application are observed.
In general, well-positioned post-emergent herbicides help reduce control failures and keep the crop clean during the critical period of interference, protecting the productive potential of corn.
Text written by Eduarda Meneghello Gheller and Sílvia Fabiana Risson, students of the Agronomy course at UFSM, Frederico Westphalen campus, members of the Tutorial Education Program - PET Agricultural Sciences, under the supervision of the tutor, Professor Dr. Claudir José Basso.
REFERENCES:
BAZANELLA, R. H. de O. Productivity losses in maize (Zea mays) due to competition from different densities of sourgrass (Digitaria insularis). 2021, 27 p. Undergraduate Thesis II (Bachelor's Degree in Agronomy) - Federal Technological University of Paraná, Dois Vizinhos, 2021.
CARVALHO, F. T; PERUCHI, M.; PALAZZO, R. R. B. Efficacy of herbicides in post-emergence weed control in maize. Brazilian Journal of Herbicides. 2001, v. 2, n. 3, p. 143-148. Available at: https://www.weedcontroljournal.org/pt-br/article/eficacia-de-herbicidas-no-controle-em-pos-emergencia-de-plantas-daninhas-na-cultura-do-milho.
FANCELLI, L. A; DOURADO NETO, D. Weed Management. Guaíba: Agropecuária, 2000.
GUIMARÃES, G. B. P.; ANDRADE, P. P. Different doses of terbuthylazine in the control of morning glory (Ipomoea grandifolia) in maize cultivation. Agroveterinária, Varginha, MG, v. 7, n. 1, p. 14-29, 2025.
REZENDE, P. N. et al; Efficiency of post-emergence herbicides in maize intercropped with Urochloa brizantha cv. Marandu. Agro@mbiente Online Journal, v. 8, n. 3, p. 345-351, September-December, 2014.
VARGAS, L.; PEIXOTO, C. M.; ROMAN, E. S. Weed management in maize cultivation. Passo Fundo: Embrapa Soja, 2006. 21 p. (Online Documents, 61).
Cover photo: Up Herb/reproduction
