In sandy loam soil, as weeds were susceptible to indaziflam, 0.25 of the recommended dose (25 g ai ha^-1) has already provided 100% control. As a result, the weed dry mass data did not fit the log-logistic model of three-parameter suggested to estimate the selectivity index. However, U. decumbens species, D. horizontalis and P. maximum were the most controlled weeds by the herbicide in this soil, where at the dose of 12.5 g ai ha^-1 showed reductions in the dry mass of 90% (Table 2).

Table 2

Sugarcane IACSP95-5000 in soil with a sandy loam texture showed a 10% reduction in dry mass (ED₁₀) at a dose of 137.83±81.48 g ai ha^-1 and a 50% reduction in dry mass (ED₅₀) at a dose of 746.01±171.59 g ai ha^-1 (Table 2). In the clay soil, a higher ED₁₀ was exhibited using 353.23±109.37 g ai ha^-1 and ED₅₀ at 671.07±80.62 g ai ha^-1 (Table 3). Thus, the percentage of reduction in total dry matter (ED₁₀) of this cultivar in soil with a sandy loam texture occurred with a lower dose of the herbicide (Figure 1).

Table 3

Figure 1

In clay soil, the species most susceptible to indaziflam was D. horizontalis, where 90% of the species control (ED₉₀) was obtained at a dose of 25 g ai ha^-1, with no adjustment possible (Figure 2). The R. cochinchinensis was the species less controlled by the herbicide, followed by the species U. plantaginea, P. maximum, and U. decumbens, where the 90% reduction in total dry mass was obtained at doses of 193±50; 152±50; 124±150 and 94±30 g ai ha^-1, respectively. The selectivity index was higher due to the species U. decumbens being 3.76, followed by the species P. maximum with 2.84, U. plantaginea with 2.32 and R. cochinchinensis with 1.83 (Table 3).

Figure 2

In sandy loam soil, sugarcane root system was the most affected with lower doses of the herbicide indaziflam when compared to its shoot system (Figure 3). The ED₁₀ for the root system was obtained with the dose of 95.15±89.59 g ai ha^-1 and for the shoot it was 162.61±112.12 g ai ha^-1 (Table 4). The doses of the herbicide for ED₅₀ in this same soil were also higher for shoots 938.19±239.04 g ai ha^-1 and for the root system it was 530.40±147.77 g ai ha^-1.

Figure 3

Table 4

In the clay soil, the root system was more affected with lower doses of the herbicide when compared to the shoot (Figure 4). The ED₁₀ was obtained in the doses of 306.93±97.72 g ai ha^-1 for the root and 361.22±111.81 g ai ha^-1 for the shoot. However, the doses required for ED₅₀ in the root system were higher for this soil, being 837.51±111.49 g ai ha^-1 for the root and 770.09±71.07 g ai ha^-1 for the shoot (Table 4).

Figure 4

The application of indaziflam (0, 30, 60, 90, 120 and 150 g ai ha^-1) in pre-emergence of P. maximum, D. horizontalis, and R. cochinchinensis, in three soils with different physical and chemical characteristics, was effective in controlling species. P. maximum and D. horizontalis were controlled from the dose of 30 g ai ha^-1, regardless of the type of soil (Amim et al., 2014). The present study corroborates these results, where D. horizontalis species was very susceptible to indaziflam regardless of the type of soil, with all species controlled in the lowest doses. However, in clay soil species such as P. maximum and R. cochinchinensis showed a 90% reduction in total dry mass at doses of 124.39±52.77 and 193.02±48.46 g ai ha^-1 respectively; these doses are higher than those recommended for the herbicide for similar soils (Rodrigues and Almeida, 2018).

Indaziflam was selective for the IACSP95-5000 sugarcane cultivar. In the sandy loam soil, the ED₁₀ was lower than the dose required for the same reduction in clay soil. This may have occurred given that indaziflam has a positive correlation to organic matter, due to its high value of the sorption coefficient normalized by organic carbon (Koc<1,000 mg g^-1) and a high value of the octanol-water partition coefficient (Kow=2.8), contributing to greater sorption of organic matter and consequently lower availability in the soil solution, impacting on selectivity and weed control (Tompkins, 2010; Alonso et al., 2011; Sebastian et al., 2017).

These factors may have had an impact on the selectivity and control of the weed species evaluated in this study, since the sandy loam soil has a lower organic matter content (18 mg dm^-3) than the clay soil (42 mg dm^-3) and consequently, to reach ED₁₀, a lower dose of the herbicide was necessary. Also, in sandy loam soil, indaziflam was effective in controlling weed species with lower doses. Sebastian et al., (2017) evaluated the influence of the physical-chemical properties of the soil on the efficacy of controlling indaziflam on Kochia scoparia, and observed that a concentration of 10 to 100 times greater is necessary to promote a 50% growth reduction (ED₅₀) for soil with 16.8% organic matter compared to soil with 0.4%.

Indaziflam application (300 g ai ha^-1) did not cause injury to the sugarcane (RB867515 cultivar), in an area with sandy soil (Simões, 2018). However, from the dose of 137.83±81.48 g ai ha^-1, reductions of 10% in the total dry mass was observed. In the present study, the selectivity index was not possible to obtain for the sandy loam soil, due to the sensitivity of the species to the herbicide indaziflam in this type of soil. This can be correlated with the physical-chemical characteristics of the soil, being the herbicide more available. It may be considered that indaziflam was very selective in this type of soil, due to the control of the weed species evaluated in the first doses and because it required relatively high doses to cause reductions in the dry mass of the sugarcane cultivar. In clay soil, indaziflam was also considered selective, even with the need for higher doses to control species. A selectivity index value greater than 1 indicates that the herbicide is more selective due to the weed species (Bartley, 1993).

A reduction in dry root mass was observed in the sandy loam soil with lower doses of indaziflam (95.15±89.59 g ai ha^-1), when compared to the shoot in this same soil (162.61±112, 12 g ai ha^-1). In clay soil, the doses of indaziflam required for a 10% reduction in root dry matter were 306.93±97.72 g ai ha^-1. The reduction in biomass and morphology of the root system by indaziflam has also been observed in other studies (Jones et al., 2013; Brabham et al., 2014; Jones et al., 2015; Schneider et al., 2015).

Schneider et al. (2015) obtained similar results to the present study, using Cynodon dactylon plants, in the sand, regardless of the indaziflam dose, an injury was observed, with a root and shoot reduction of 32 and 10% up to 10 cm depth, respectively. This may be a factor that influenced the results of this study, as the sugarcane stems were planted at 8 cm depth, where the first roots may have come into contact with the herbicide. The authors also observed that the increase in clay content and especially the addition of organic matter, significantly improved the root mass at a dose of 16 g ai ha^-1 compared to sandy soil without organic matter. However, this same dose caused inhibition in the growth of the roots, without influence on the development of the shoot of the plant in the sandy and clay soils even with organic matter.

The physico-chemical properties of indaziflam suggest that the molecule can be moderately mobile in the soil (solubility in water (Sw)=4.4 mg L^-1 at pH 4.0) due to the long persistence (half-life time (T_1/2)>150 days) associated with low solubility in water (Sw=2.2 mg L^-1 at pH 7.0 to 9.0) and moderate sorption (sorption coefficient (Kd)=4.9 to 27.4 g mL^-1) (Tompkins, 2010; Alonso et al., 2011). Nevertheless, as the sandy loam soil has a lower content of organic matter (18 mg dm^-3) compared to the clay one (42 mg dm^-3), and the herbicide has a great affinity to organic matter (Koc<1,000 mg g^-1 ), and with the presence of water during the entire conduction of the experiment, indaziflam may have had more mobility in the sandy loam soil, causing greater reductions in the root system of sugarcane in this soil.

In sandy loam soil, the indaziflam dose of 25 g ai ha^-1 provided 100% control for all weeds. In clay soil, for D. horizontalis the ED₉₀ was obtained at 25 g ai ha^-1, for R. cochinchinensis at 193 g ai ha^-1, for U. plantaginea at 152 g ai ha^-1, for P. maximum at 124 g ai ha^-1, and for U. decumbens at 94 g ai ha^-1. Indaziflam was selective to IACSP95-5000 sugarcane cultivar in both soils, with ED₁₀ at 137 g ai ha^-1 for soil with a sandy loam texture and 353 g ai ha^-1 for clay soil. The selectivity index was higher than 1 for all weeds in clay soil. It was not possible to obtain the selectivity index for sandy loam soil due to species susceptibility to the herbicide.