Conyza sumatrensis substantially reduced soybean yield even under low densities in both cultivars and seasons. For the early maturity soybean cultivar, infestation levels of 17.1 and 17 (parameter b) of C. sumatrensis m^-2 in the 2016-2017 and 2017-2018 growing seasons, respectively, were required to cause a 50% yield loss of soybean crop. For the late-maturity cultivar, the values were 6.3 and 7.0 in the 2016-2017 and 2017-2018 growing seasons, respectively. In the 2016-2017 growing season, the parameter "i" (proportional yield loss when the weed density approaches zero) was 12.5% for the late-maturing cultivar and 9.35% for the early-maturity cultivar. In the 2017-2018 growing season, the parameter "i" was 13.7% for the late-maturity cultivar, whereas it was 10.77% for the early-maturity cultivar, which was 22% lower compared to the late-maturity cultivar. That is, a single plant of C. sumatrensis m^-2 can reduce soybean yield by up to 13.7% (Table 1).

Table 1

Trezzi et al. (2015) indicated that 2.7 of C. bonariensis m^-2 plants could reduce soybean yield by approximately 50%, which is higher than that observed in the present study. Moreover, Agostinetto et al. (2017) found that a single plant m^-2 of C. bonariensis could reduce soybean yield by approximately 25.9%. In contrast, C. sumatrensis at densities of 13 to 23 plants m^-2 did not interfere with the agronomic performance of soybeans, in a study carried out in the Brazilian Cerrado biome during a hot and rainy summer. Under these conditions, the death of C. sumatrensis plants occurred, which can be explained due to shading by the crop (Correia 2023). In the southern region of Brazil in a subtropical climate, such as this study, 20 to 35 plants m^-2 of C. sumatrensis can reduce soybean grain yield by up to 50% (Blainski et al. 2015). Weed interference can be highly dependent on the cultivar and weed genotypes and the present environment (Roncatto et al. 2021; Caldas et al. 2023). Therefore, comparative tests can be conducted to determine the tolerance of cultivars at different environments in comparison with the weeds.

The current study identified that for both cultivars, growing seasons, and densities used, C. sumatrensis has a substantial impact on soybean yield. There are a few specific studies on C. sumatrensis, which can be contrasting depending on the soil and climate conditions (Correia 2023). Therefore, it is important to provide specific data for C. sumatrensis in a subtropical climate, given the prevalence of this species in the southern region of Brazil (Marochio et al. 2017; Ruiz et al. 2022), which reiterates the relevance of this study. Other weeds highlighted for their negative impact on soybean yield include Amaranthus palmeri (Korres et al. 2019), Digitaria insularis (Gazziero et al. 2019), and Amaranthus tuberculatus (Butts et al. 2018), which are among the most important weeds in soybean crop.

In the 2016-2017 growing season, the early maturity cultivar had a grain yield in the absence of C. sumatrensis was 3,992 kg ha^-1. For the highest level of infestation (10 plants m^-2) the average yield reduction was 58%, with a grain yield of 1,677 kg ha^-1. Moreover, the late-maturing cultivar had a grain yield of 3,861 kg ha^-1 in the absence of C. sumatrensis. The maximum loss (48%) was observed at the highest level of infestation (10 plants m^-2), with a grain yield of 2,008 kg ha^-1 (Figure 2).

Figure 2

In 2017-2018, the grain yield for the late-maturing cultivar was 4,311 kg ha^-1 without weed infestation, with maximum loss (57%) for 10 plants m^-2 with a grain yield of 1,854 kg ha^-1. The early maturity cultivar produced 4,019 kg ha^-1 in the absence of infestation. Under the influence of 10 plants m^-2 the yield was reduced by 73%, with a grain yield of 1,085 kg ha^-1 (Figure 3).

Figure 3

At maximum infestation (10 plants m^-2), there was a minimum reduction of 48.7% in soybean yield. These results show the substantial influence of C. sumatrensis on soybeans, making it necessary to maintain weed density at the lowest level, or absent, because even a single plant of C. sumatrensis m^-2 has a considerable impact on soybean yield, until 13.72%. Trezzi et al. (2015) concluded that a single plant of C. bonariensis m^-2 can reduce soybean grain yield by up to 36%. Competition between C. bonariensis and soybeans between 21 and 42 days after crop emergence can represent a reduction of 21 kg ha^-1 in yield per day of coexistence (Silva et al. 2014). This reinforces the need to keep soybean crops free from the presence of Conyza spp.

Some characteristics help in explaining the high aggressiveness of Conyza spp. plants, including vigorous growth, plasticity in their life cycle, and their ability to adapt to different environments (Bajwa et al. 2016; Baccin et al. 2022). As well as when in competition, it can affect the growth and development of soybean shoots and roots (Rockenbach and Rizzardi 2020). Furthermore, some studies have also indicated the allelopathic effects of Conyza spp. on other plant species (Ferreira et al. 2020; Peralta et al. 2022). Conyza species were dominant in some environments, with the presence of few weeds of other species (Concenço and Concenço 2016). The ecophysiological characteristics of Conyza associated crop management, no-till system, continuous use of herbicides for control, and other aspects have favored the selection of resistant biotypes and dominance of this weed (Bajwa et al. 2016; Baccin et al. 2022). Therefore, this reinforces the competitive ability of this plant, even about other weeds, and helps in explaining the impact of a single m^-2 plant on soybean yield in the present study.

The aggressiveness of Conyza spp. restates the importance of effective control, which keeps the population levels of this plant close to zero. In this study, this is reinforced by the data obtained for C. sumatrensis. Thus, the adoption of herbicides in pre- or post-emergence in combinations (Cantu et al. 2021; Albrecht et al. 2022; Garcia et al. 2023; Monteiro et al. 2024), cover crops (Wallace et al. 2019; Bunchek et al. 2020; Fisher and Sprague 2022, 2023), and herbicide combinations with cover crops (Schramski et al. 2021) are fundamental for the control of this and other weeds. For example, vetch and barley crop residues were effective in suppressing C. canadensis (Campiglia et al. 2015), also maize, Urochloa, ryegrass, turnip, wheat, and black oat crop residues were effective in suppressing C. bonariensis (Lamego et al. 2013). Which, in the sum of research, highlights the need for integrated weed management.

The adoption of these and other practices for the management of Conyza spp. is not only important for controlling and suppressing low population densities but also for controlling the advance of herbicide-resistant biotypes. Therefore, the presence of even a single plant m^-2 of C. sumatrensis should not be tolerated. This condition must be sought in integrated weed management, with the aim of achieving economic sustainability in soybean crops (Bajwa et al. 2016; Riemens et al. 2022). The integration of control methods is important, with preference given to those that reduce the emergence of weeds, such as crop rotation that provides soil cover with crop residues. Hand weeding is very expensive, and it becomes unfeasible even for small areas to control aggressive weeds, estimates indicate that a rural worker needs 15 days to weed one hectare, with successive interventions in order to keep the crop free from weed interference (Van der Weide et al. 2008). The cost of manual weeding can approach US$ 200 ha^-1, a high cost per hectare, it was not considered a viable option in the economic analysis (Dominschek et al. 2021).

Thus, the adoption of herbicides in pre- or post-emergence combinations, cover crops, and herbicide combinations with cover crops are fundamental for the control of this and other weeds. However, there are a few studies on C. sumatrensis, whereas most of the interference studies have been conducted on other species of Conyza. Therefore, extensive research on different production systems and agroecosystems needs to be carried out. Comparative tests can be conducted to determine the tolerance of cultivars in comparison with the weeds.

The grain yield reduction observed for the late-maturity cultivar was 12.54 and 13.72% per plant of C. sumatrensis, in the 2016-2017 and 2017-2018 growing seasons, respectively. The early maturity cultivar showed a reduction of 9.35 and 10.77% per plant of C. sumatrensis, in the 2016-2017 and 2017-2018 growing seasons, respectively.Conyza sumatrensis cannot be tolerated in soybean crops, because a single plant per m² has great potential for reducing grain yield.