Chemical management of weeds in corn hybrids
Abstract
Corn stands out among the cultivars because of its high importance in food, animal feed and raw materials for energy production. The quantity and quality of the harvested grain can be reduced, mainly due to inadequate weed management. Nowadays, weed control is accomplished with the use of herbicides as pre-emergence and postemergence, applied alone or in a tank mix. This study aimed to evaluate the efficiency of the control of weeds and phytotoxicity of the herbicides that were applied in tank mix or isolated in the corn hybrids, Formula TL® and SYN 7B28. Two experiments were conducted: in the 2012/2013 season (Formula TL®) and 2013/2014 season (SYN 7B28). There was a low initial phytotoxicity of all the tested herbicides in the initial evaluation; however, from 14 days after treatment, it was minimal. The weed control of Ipomoea indivisa, Urochloa plantaginea and Euphorbia heterophylla in both experiments was better, in general, when using the tank mixtures of atrazine + simazine + tembotrione and atrazine + simazine + mesotrione, respectively. The application of atrazine + simazine + tembotrione in the SYN 7B28 hybrids provided, with the exception of the thousand-grain weight, the best expression of grain yield components, combining a low phytotoxicity and best control of the weeds. The Formula TL® hybrid presented the highest mass of a thousand grains and yield with the application of nicosulfuron + tembotrione. The yield average difference of the herbicide treatments and the unweeded control showed an increase of 27.66 and 34.59% in the grain yield of the corn hybrids, Formula TL® and SYN 7B28, respectively.
Corn stands out as the most produced cereal in the world: in the season 2014/2015, world production was 1.00879 billion t (United States Department of Agriculture 2015). Brazil produced in the same harvest 84.672 million t that were sown in an area of 15,692,000 ha (Companhia Nacional de Abastecimento 2016), the third-largest producer. In this context, the cultivation of corn has economic, social and cultural relevance, besides its importance to the Brazilian agricultural sector, which contributes to the food security of agricultural properties, especially for small producers.
The corn has been grown in many properties, whether large or small, and at different technological levels, which often causes a reduction in the quantity and quality of the harvested grain due to improper management, especially in relation to weeds. In corn, the estimated yield losses due to weed competition are ≤85% if no control method is adopted (Galon et al. 2010; Carvalho et al. 2007).
Among the weeds that are found in maize stands out the Ipomoea sp. (morning glory), Urochloa plantaginea (Alexander grass), Euphorbia heterophylla (wild poinsettia), Digitaria spp. (Jamaican crabgrass), Bidens sp. (cobbler's pegs), among others. These weeds compete with corn for environmental resources, release allelopathic substances and can host pests and diseases. As a result, there is a loss of productivity and quality of the harvested grain (Galon et al. 2010).
In this context, control strategies of weeds in corn are needed and the chemical method is the most widespread and used, mainly attributed to its practicality in application, effectiveness and lower cost when compared to other control methods (Timossi & Freitas 2011).
However, nowadays, farmers have been optimizing the applications of herbicides by the use of tank mixtures in order to reduce the number of applications, as the mixtures increase the spectrum of control or they even control tolerant and resistant weeds to a particular herbicide. The combination of herbicides for weed control proves to be advantageous because it increases weed control and reduces considerably the damage to crops (Carvalho et al. 2010). It also can reduce the dose, enabling a more rational use, reducing the cost to the producer, environmental impact due to the lower volume used, as well as less contamination of the applicators (Leal et al. 2012).
The herbicides or their mixtures can have direct and indirect effects on the growth and development of cultivated plants (Rizzardi et al. 2003; Feng et al. 2005), such as changes in the absorption of nutrients, intoxication symptoms, deregulation of plant defense mechanisms to certain stressors, cellular oxidation, among others, which are not noticeable and not widely considered when the phytotoxicity on plants is evaluated (Rizzardi et al. 2003; Feng et al. 2005), as they also can cause losses in grain yield (Cavalieri et al. 2012).
Similarly, the genotype and climatic conditions can influence the responses to the application of different herbicides and mixes for weed control. Research that has been carried out with other crops, such as sugar cane (Costa et al. 2012), wheat (Galon et al. 2015), rice (Pinto et al. 2008) and soybean (Zemolin et al. 2014), demonstrated different responses of genotype to the application of herbicides, with frequent problems of phytotoxicity and, sometimes, the loss of crop productivity or impairment of the quality of the final product. However, it is noted that, for corn, few studies have evaluated the influence of the main herbicides that are applied by producers, alone or in tank mix, on the agronomic characteristics of the cultivar.
Thus, works that address the application of herbicides in a tank mix in order to reduce the dose and increase the action spectrum have significant importance. Therefore, the objective of this work was to evaluate the injuries and efficiency of herbicides that were applied alone or in tank mixtures for the chemical management of weeds that infest corn.
Materials and Methods
This study consisted of two experiments that were conducted in two separate years, both at the field in the experimental area of the Agricultural State School Ângelo Emílio Grando in Erechim, Brazil, (27°39′44. 1′S, 52°18′15.2′′W, 670 m a.s.l.), according to Köppen classification as a “Cfa,” characterized as subtropical humid. Experiments I and II were installed in the years 2012/2013 and 2013/2014, using the corn hybrids Formula TL® and SYN 7B28, respectively. The soil of the experimental area is classified as Oxisol (Embrapa – Brazilian Agricultural Research Corporation 2013).
The experimental design that was used for experiments I and II was the randomized block design with four replications. The treatments that were tested on the corn hybrids, the doses and the mode of application and adjuvants used are described in Table 1. The herbicides that were used were atrazine + simazine (Simtrac 500, 250 g ai L−1, SC, Atanor, Resende/Rio de Janeiro/Brazil), nicosulfuron (Nicosulfuron Nortox, 40 g ai L−1, SC, Nortox, Rondonópolis/Mato Grosso/Brazil), tembotrione (Soberan, 420 g ai L−1, SC, Bayer, Leverkusen/Renânia do Norte-Vestf´lia/Alemanha) and mesotrione (Callisto, 480 g ai L−1, SC, Syngenta, Basiléia/Suíça). Commercial Aureo (methylated soybean oil) (Bayer/Leverkusen/Renânia do Norte-Vestf´lia/Alemanha) and joint oil (mineral oil aromatic hydrocarbon) adjuvants were used. In these experiments, by mixing herbicides in the tank, half the dose that is recommended by the manufacturer was used, except for the commercial product, Simtrac 500, which is already a premix, so half the commercial product dose was used. Thus, the producer has lower production costs and a lower environmental impact, as there are few studies evaluating the dose reduction of herbicides for weed control in corn.
| Treatment | Dose (g ha−1) | Adjuvant | Application time | |
|---|---|---|---|---|
| Commercial product | Dose (L ha−1) | |||
| Atrazine + simazine | 1250 + 1250 | Joint oil (MOAH) | 0.75 | Pre-emergence |
| Atrazine + simazine | 625 + 625 | Joint oil (MOAH) | 0.75 | Postemergence |
| Atrazine + simazine | 1250 + 1250 | Joint oil (MOAH) | 0.75 | Postemergence |
| Nicosulfuron | 60.0 | – | – | Postemergence |
| Tembotrione | 24.2 | Aureo (MSO) | 1.00 | Postemergence |
| Mesotrione | 76.8 | Joint oil (MOAH) | 0.75 | Postemergence |
| Atrazine + simazine + nicosulfuron | 625 + 625 + 30 | Joint oil (MOAH) | 0.75 | Postemergence |
| Atrazine + simazine + tembotrione | 625 + 625 + 12.1 | Aureo (MSO) | 1.00 | Postemergence |
| Atrazine + simazine + mesotrione | 625 + 625 + 38.4 | Joint oil (MOAH) | 0.75 | Postemergence |
| Nicosulfuron + tembotrione | 30 + 12.1 | Aureo (MSO) | 1.00 | Postemergence |
| Nicosulfuron + mesotrione | 30 + 38.4 | Joint oil (MOAH) | 0.75 | Postemergence |
| Tembotrione + mesotrione | 12.1 + 38.4 | Aureo (MSO) | 1.00 | Postemergence |
| Unweeded control | – | – | – | – |
| Weeded control | – | – | – | – |
- Aureo, methylated soybean oil; MOAH, mineral oil aromatic hydrocarbon; MSO, methylated seed oil.
The cultivation system that was adopted for the two experiments was tillage and the groundcover, composed of oat + turnip desiccated with glyphosate (1440 g ha−1) (Monsanto/Louis/Missouri/EUA) 7 days before the sowing of the corn. The sowing of the corn hybrids was carried out by using a seeder/fertilizer, at a spacing of 0.65 m between rows and four seeds per m1 in density, yielding a population of ~55,000 plants per ha1. The hybrid, Formula TL®, was sown on September 24 2012 and SYN 7B28 was sown on September 23 2013.
The level of fertilization in both experiments was 270 kg ha−1, the formulation used was NPK 5:30:15 kg that was distributed in the planting row, plus a top dressing of 150 kg ha−1 of nitrogen when the corn plants reached the stage of four-tosix leaves. The fertilizer recommendation was based on the chemical and physical characteristics of the soil, following the technical recommendations for corn (Brazilian Society of Weed Science 1995). Each experimental unit was 2.6 m wide and 5.0 m long (13.0 m2), comprising four lines of sowing. In order to determine the variables on the corn yield components, the two central lines of each experimental unit was considered, discarding even the 0.5 m front borders and end of the plots.
It is noteworthy that the mixture of atrazine + simazine is formulated by the industry, as the other mixtures were made in the spray tank, following what the corn producers of the Alto Uruguai region of the Rio Grande do Sul state do on their crops for the weed control. The weeds, the object of study, emerged from the soil seed bank in the area where the experiment was installed.
The herbicide application in both experiments was conducted by an accurate CO2 backpack sprayer, equipped with four spray nozzles (type range DG 110.02; Glendale Heights/Illinois/EUA), keeping a constant pressure of 2.0 kgf cm−2 and a travel speed of 3.6 km h−1, which gave a rate of 150 L ha−1 herbicidal mixture. The periods of application and weather conditions at the time of application are shown as follows. Experiment I:crop year, 2012/2013; application time pre-emergence: 100% sunlight; air temperature of 25°C; relative humidity of 75%; soil condition, moist; and wind speed of 4 km h−1. Postemergence: 98% sunlight; temperature of 27°C; relative humidity of 70%; soil condition, moist; and wind speed of 3 km h−1. Experiment II: crop year 2013/2014; application time pre-emergence: 96% sunlight; air temperature of 26°C; relative humidity of 74%; soil condition, moist; and wind speed of 4 km h−1. Postemergence: 100% sunlight; temperature of 24°C; relative humidity of 72%; soil condition, moist; and wind speed of 3 km h−1.
The applications of the herbicides were similar in both experiments: the pre-emergent being applied before the emergence of the weeds and cultivar and postemergence when corn reached the vegetative stage, V3 (three developed leaves). The weeds that were present in the experimental area for the two experiments were E. heterophylla (wild poinsettia), Ipomoea indivisa (morning glory) and U. plantaginea (Alexander grass or grass marmalade). At the time of the herbicide application postemergence for the two experiments, the wild poinsettia was at the stage of two-to-six leaves, morning glory at two-to-four leaves and Alexander grass at three leaves-to-two tillers, appearing in populations of 12, 19 and 236 plants per m2, respectively, for the experiment in the 2012/2013 season and 6, 11 and 98, respectively, for the experiment in the 2013/2014 season, as these plants originated from the soil seed bank. The variables that were evaluated in both experiments were the level of weed control at 7, 14, 21 and 28 days after treatment (DAT) and at the preharvest crop and the level of herbicide injury to the corn hybrids at 7, 14, 21 and 28 DAT. Both the control and phytotoxicity were evaluated visually by assigning notes of a percentage scale from zero to 100% by two evaluators. Zero (0%) corresponded to no weed control or injury to corn and 100 (100%) meant the complete death of the plants, according to the methodology proposed by the Brazilian Society of Weed Science (1995).
The yield component variables of the corn hybrids were determined in the preharvest or harvest. They were the ear length, number of kernel rows per ear, number of kernels per row, thousand-grain weight and grain yield. The ear length (cm) was measured with a measuring tape, the number of kernel rows per ear and number of kernels per row both were measured by counting. The thousand-grain weight (g) was determined by counting eight samples of 100 grains each and weighing them in an analytical balance. The grain yield (kg ha−1) was measured by picking up all the ears present in a field area of 3.9 m2 when the corn grain reached 18% humidity. The thousand-grain weight and productivity had the grain moisture adjusted to 13% and then extrapolated to kg ha−1.
The data were submitted to a variance analysis with theF-test; if significant, the treatment averages were submitted to the Tukey's honest significant difference test (P ≤ 0.05).
Results and Discussion
Experiment I
The results show that the tank mixtures of atrazine + simazine herbicides (applied postemergence at – 1250 + 1250 g ha−1), atrazine + simazine + nicosulfuron, atrazine + simazine + tembotrione, atrazine + simazine + mesotrione and tembotrione + mesotrione showed a morningglory control rate >80% in all periods (Table 2). According to Oliveira et al. (2009), 80% is the minimum control level that a particular herbicide must fulfill in order to be recommended for weed control in crops; thus, these mixtures, made by manufacturers or in the tank, can be considered to be effective for the control of morning glory. At 14 DAT, the efficiency of morning glory control for all treatments was observed, resulting in a control rate of >80%, being all the treatments that were different from the unweeded control (Table 2).
| Treatment | Dose | Control of morningglory (%) | ||||
|---|---|---|---|---|---|---|
| (g ha−1) | 7 DAT† | 14 DAT | 21 DAT | 28 DAT | Preharvest | |
| Atrazine + simazine‡ | 1250 + 1250 | 89.7abc§ | 93.0ab | 81.5d | 66.2c | 91.0abcd |
| Atrazine + simazine¶ | 625 + 625 | 87.5abc | 84.5bc | 81.0d | 72.5c | 82.8d |
| Atrazine + simazine¶ | 1250 + 1250 | 91.0abc | 92.7ab | 90.0bc | 91.0b | 88.2bcd |
| Nicosulfuron | 60 | 55.0d | 82.5c | 89.5bc | 93.7ab | 91.5abcd |
| Tembotrione | 24.2 | 78.2c | 93.5a | 89.5bc | 90.0b | 65.0e |
| Mesotrione | 76.8 | 79.2c | 94.0a | 100.0a | 98.7ab | 100.0a |
| Atrazine + simazine + nicosulfuron | 625 + 625 + 30 | 89.0abc | 98.0a | 95.7ab | 97.7ab | 81.5d |
| Atrazine + simazine + tembotrione | 625 + 625 + 12.1 | 95.5ab | 97.0a | 97.7a | 91.5ab | 86.2cd |
| Atrazine + simazine + mesotrione | 625 + 625 + 38.4 | 98.2a | 99.2a | 99.0a | 95.2ab | 97.2ab |
| Nicosulfuron + tembotrione | 30 + 12.1 | 61.2d | 93.0ab | 87.7cd | 92.5ab | 87.2bcd |
| Nicosulfuron + mesotrione | 30 + 38.4 | 55.0d | 96.5a | 97.7a | 98.2ab | 88.7bcd |
| Tembotrione + mesotrione | 12.1 + 38.4 | 83.0bc | 97.7a | 97.5a | 94.0ab | 94.0abc |
| Unweeded control | – | 0.0e | 0.0d | 0.0e | 0.0d | 0.0f |
| Weeded control | – | 100.0a | 100.0a | 100.0a | 100.0a | 100.0a |
| Coefficent of variation (%) | – | 6.9 | 3.9 | 3.1 | 4.2 | 4.9 |
- † Days after treatment.
- ‡ Applied pre-emergence.
- § Averages followed by the same lower case letter in the column did not differ by Tukey's honest significant difference test at P ≤ 0.05.
- ¶ Applied postemergence.
It was observed that atrazine + simazine (1250 + 1250 g ha−1,applied both pre- and postemergence), atrazine + simazine + nicosulfuron, atrazine + simazine + tembotrione, atrazine + simazine + mesotrione and tembotrione + mesotrione showed a level of control of morning glory that was >81% at 7 and 21 DAT and also at the preharvesting of the crop (Table 2). The results showed that the tank mixtures that were composed of atrazine + simazine + nicosulfuron, atrazine + simazine + tembotrione and atrazine + simazine + mesotrione presented a control level of the morningglory that was statistically equal to the weeded control; that is, they were the best treatments in the tests that were conducted at 7, 14 and 21 DAT. The weeded control did not differ significantly from the mesotrione, nicosulfuron + mesotrione and tembotrione + mesotrione treatments to control morningglory at 14 and 21 DAT in corn. The lowest control level was observed at 28 DAT with the use of atrazine + simazine (applied pre-emergence) and also the use of the same mixture, but applied in the half dose (625 + 625 g ha−1) postemergence. At the preharvest, the tembotrione was the only one with a control level that was <80% of morningglory, when compared to the other treatments.
Among the isolated applied treatments, the mesotrione is highlighted as presenting a morningglory control equal to a weeded control from 14 DAT to preharvest, not differing significantly from the best treatments that were applied in a tank mix (Table 2).
For the control of Alexander grass, it was assessed at all times that the tank mixture composed of atrazine + simazine + tembotrione presented, at all times, a control rate that was statistically equal to the weeded control or with an average control of 96.5% in the five periods evaluated (Table 3). It was noticed also that atrazine + simazine + mesotrione demonstrated control of this weed, in all periods, equal to the weeded control, except at 28 DAT, which was statistically lower, but with a control rate of 91%, which still can be recommended, as explained previously. The results showed that the nicosulfuron and tembotrione herbicides were similar to the weeded control and were statistically greater or equal to the other treatments from 14 DAT, being stable until the preharvesting of the corn. It also was observed that nicosulfuron and tembotrione mixed together in the spray tank also showed an Alexander grass control rate equivalent to the weeded control from 14 DAT to preharvest.
| Treatment | Dose (g ha−1) | Control of Alexander grass (%) | ||||
|---|---|---|---|---|---|---|
| 7 DAT† | 14 DAT | 21 DAT | 28 DAT | Preharvest | ||
| Atrazine + simazine‡ | 1250 + 1250 | 63.7fg§ | 67.5d | 41.2d | 33.7e | 22.5e |
| Atrazine + simazine¶ | 625 + 625 | 50.0h | 81.0c | 72.5c | 72.00d | 52.5c |
| Atrazine + simazine¶ | 1250 + 1250 | 68.7ef | 87.2bc | 85.2b | 78.7cd | 75.0b |
| Nicosulfuron | 60 | 75.0de | 98.5a | 100.0a | 100.0a | 91.0a |
| Tembotrione | 24.2 | 82.5cd | 98.5a | 100.0a | 100.0a | 94.0a |
| Mesotrione | 76.8 | 84.5bc | 87.7b | 80.5b | 80.5c | 37.5d |
| Atrazine + simazine + nicosulfuron | 625 + 625 + 30 | 80.0cd | 96.7a | 94.5a | 94.7ab | 92.2a |
| Atrazine + simazine + tembotrione | 625 + 625 + 12.1 | 92.7ab | 99.7a | 99.5a | 100.0a | 90.7a |
| Atrazine + simazine + mesotrione | 625 + 625 + 38.4 | 94.7a | 98.2a | 95.5a | 91.0b | 94.0a |
| Nicosulfuron + tembotrione | 30 + 12.1 | 63.2fg | 98.7a | 100.0a | 100.0a | 96.7a |
| Nicosulfuron + mesotrione | 30 + 38.4 | 56.2gh | 95.2a | 95.0a | 96.7ab | 90.7a |
| Tembotrione + mesotrione | 12.1 + 38.4 | 82.2cd | 98.7a | 99.7a | 100.0a | 93.7a |
| Unweeded control | – | 0.0i | 0.0e | 0.0e | 0.0f | 0.0f |
| Weeded control | – | 100.0 a | 100.0a | 100.0a | 100.0a | 100.0a |
| Coefficent of variation (%) | – | 5.3 | 3.0 | 3.2 | 3.7 | 5.8 |
- † Days after treatment.
- ‡ Applied pre-emergence.
- § Averages followed by the same lower case letter in the column did not differ by Tukey's honest significant difference test at P ≤ 0.05.
- ¶ Applied postemergence.
At 7 DAT, the tank mixtures of atrazine + simazine + tembotrione and atrazine + simazine + mesotrione were the treatments that caused the greatest control of Alexander grass, with an average of 93.7%, statistically equivalent to the weeded control (Table 3). Zagonel et al. (2010) found efficient results for Alexander weed control of corn when applying mesotrione associated with atrazine, which corroborates with the data observed in this study.
In general, the treatments with atrazine + simazine, in pre and postemergence, showed an average control rate of <80% for all periods (Table 3). The treatment with lower efficiency in the control of Alexander grass was the commercial mixture composed of atrazine + simazine applied pre-emergence, which showed an average control rate of 45.7% for the five evaluated periods. Although they are considered as a tool in integrated pest management, the use of pre-emergent herbicides did not show efficiency in controlling Alexander grass, as observed by Galon et al. (2010), when no control of this weed was noted when using S-metolachlor pre-emergence.
Regarding wild poinsettia control (Table 4), at 7 DAT it can be seen that atrazine + simazine applied postemergence at two doses (625 + 625 and 1250 + 1250 g ha−1) and the mixtures in the tank of atrazine + simazine + tembotrione and atrazine + simazine + mesotrione did not differ from the weeded control. From 14 DAT to preharvest, it was found that the use of nicosulfuron, atrazine + simazine + nicosulfuron, nicosulfuron + tembotrione and nicosulfuron + mesotrione did not differ from the weeded control, emphasizing the positive effect of nicosulfuron in controlling the wild poinsettia weed on corn, both applied alone or in tank-mixing.
| Treatment | Dose (g ha−1) | Control of wild poinsettia (%) | ||||
|---|---|---|---|---|---|---|
| 7 DAT† | 14 DAT | 21 DAT | 28 DAT | Preharvest | ||
| Atrazine + simazine‡ | 1250 + 1250 | 87.0bc§ | 85.5d | 71.2f | 62.2e | 88.5ab |
| Atrazine + simazine¶ | 625 + 625 | 92.5ab | 86.3cd | 89.8bcde | 80.00d | 79.25bc |
| Atrazine + simazine¶ | 1250 + 1250 | 92.5ab | 94.2abcd | 86.5de | 86.2cd | 85.7ab |
| Nicosulfuron | 60 | 73.2de | 97.5a | 100.0a | 100.0a | 95.0a |
| Tembotrione | 24.2 | 79.0cd | 94.0abcd | 89.0cde | 87.2cd | 60.0d |
| Mesotrione | 76.8 | 77.0cd | 88.2bcd | 83.0e | 68.7e | 72.5c |
| Atrazine + simazine + nicosulfuron | 625 + 625 + 30 | 86.7bc | 98.0a | 99.2a | 96.7ab | 92.5a |
| Atrazine + simazine + tembotrione | 625 + 625 + 12.1 | 95.7ab | 94.0abcd | 92.2abcd | 87.5cd | 52.5d |
| Atrazine + simazine + mesotrione | 625 + 625 + 38.4 | 97.0ab | 97.2ab | 92.0abcd | 93.7abc | 56.7d |
| Nicosulfuron + tembotrione | 30 + 12.1 | 65.0e | 94.5abcd | 95.7abc | 97.5ab | 93.0a |
| Nicosulfuron + mesotrione | 30 + 38.4 | 47.5f | 95.2abc | 97.5ab | 100.0a | 89.5ab |
| Tembotrione + mesotrione | 12.1 + 38.4 | 79.5cd | 92.7abcd | 88.5cde | 88.5bcd | 85.0ab |
| Unweeded control | – | 0.0g | 0.0e | 0.0g | 0.0f | 0.0e |
| Weeded control | – | 100.0a | 100.0a | 100.0a | 100.0a | 100.0a |
| Coefficent of variation (%) | – | 5.5 | 4.2 | 4.0 | 4.5 | 6.2 |
- † Days after treatment.
- ‡ Applied pre-emergence.
- § Averages followed by the same lower case letter in the column did not differ by Tukey's honest significant difference test at P≤ 0.05.
- ¶ Applied postemergence.
The lowest results for wild poinsettia control were observed when applying nicosulfuron + tembotrione and nicosulfuron + mesotrione at 7 DAT, atrazine + simazine in pre-emergence at 21 and 28 DAT and mesotrione at 28 DAT (Table 4). In the preharvest evaluations, the treatments with tembotrione, mesotrione, atrazine + simazine + tembotrione and atrazine + simazine + mesotrione showed a rate of control of the wild poinsettia that was <73%, which is therefore not recommended for this purpose.
The phytotoxicity of the herbicides, both alone or in tank-mixing, to the corn hybrid, Formula TL®, was low for all treatments, with values of <4.0% up to 14 DAT (Table 5). After 14 DAT, no more symptoms of injury on the corn hybrid were observed, when the plants were recovering from the damage that was caused by the herbicides. The results corroborate with those observed by Adegas et al. (2011), who tested atrazine, mesotrione, nicosulfuron and tembotrione in isolation and in mixture, which showed a level of phytotoxicity of <7.0% up to 14 DAT and the evaluation on 28 DAT found no phytotoxic effect. Freitas et al. (2009) observed in their work, among the herbicides tested, that the tembotrione herbicide showed less phytotoxicity to the tested popcorn cultivars, not observing symptoms from 12 DAT.
| Treatment | Dose (g ha−1) | Phytotoxicity (%) | |||
|---|---|---|---|---|---|
| 7 DAT† | 14 DAT | 21 DAT | 28 DAT | ||
| Atrazine + simazine‡ | 1250 + 1250 | 0.0c§ | 0.0c | 0.0a | 0.0a |
| Atrazine + simazine¶ | 625 + 625 | 2.5ab | 2.8ab | 0.0a | 0.0a |
| Atrazine + simazine¶ | 1250 + 1250 | 2.2b | 2.7ab | 0.0a | 0.0a |
| Nicosulfuron | 60 | 2.7ab | 2.7ab | 0.0a | 0.0a |
| Tembotrione | 24.2 | 2.7ab | 3.2ab | 0.0a | 0.0a |
| Mesotrione | 76.8 | 2.5ab | 2.5ab | 0.0a | 0.0a |
| Atrazine + simazine + nicosulfuron | 625 + 625 + 30 | 3.5ab | 3.5ab | 0.0a | 0.0a |
| Atrazine + simazine + tembotrione | 625 + 625 + 12.1 | 3.5ab | 2.2b | 0.0a | 0.0a |
| Atrazine + simazine + mesotrione | 625 + 625 + 38.4 | 3.7a | 2.7ab | 0.0a | 0.0a |
| Nicosulfuron + tembotrione | 30 + 12.1 | 3.0ab | 3.2ab | 0.0a | 0.0a |
| Nicosulfuron + mesotrione | 30 + 38.4 | 2.5ab | 3.7a | 0.0a | 0.0a |
| Tembotrione + mesotrione | 12.1 + 38.4 | 3.0ab | 3.2ab | 0.0a | 0.0a |
| Weeded control | – | 0.0c | 0.0c | 0.0a | 0.0a |
| Unweeded control | – | 0.0c | 0.0c | 0.0a | 0.0a |
| Coefficent of variation (%) | – | 25.3 | 18.3 | 1.0 | 1.0 |
- † Days after treatment.
- ‡ Applied pre-emergence.
- § Averages followed by the same lower case letter in the column did not differ by Tukey's honest significant difference test at P ≤ 0.05.
- ¶ Applied postemergence.
For the length of the corn ear, it was found that the weeded control showed the best performance, not differing from nicosulfuron, tembotrione, atrazine + simazine + nicosulfuron, atrazine + simazine + tembotrione and tembotrione + mesotrione (Table 6). This is explained by the fact that these herbicides are selective for corn, not interfering in its development (Freitas et al. 2009; Karam & Oliveira 2007). Also, they have presented a good weed control level for morningglory, Alexander grass and wild poinsettia, nullifying the interference of these species on corn.
| Treatment | Dose (g ha−1) | Ear length (cm) | No. of rows per ear | No. of kernels per row | Thousand-grain weight (g) | Grain yield (t ha−1) |
|---|---|---|---|---|---|---|
| Atrazine + simazine† | 1250 + 1250 | 16.90cd‡ | 17.83bc | 39.20bc | 306.44ab | 9.047h |
| Atrazine + simazine§ | 625 + 625 | 16.93cd | 17.35de | 39.30bc | 323.48ab | 9.196h |
| Atrazine + simazine§ | 1250 + 1250 | 16.23d | 17.90bc | 38.30c | 324.43a | 10.206d |
| Nicosulfuron | 60 | 17.80ab | 17.90bc | 39.80bc | 301.40bc | 10.361c |
| Tembotrione | 24.2 | 17.73ab | 17.65cd | 38.50c | 316.79ab | 10.748b |
| Mesotrione | 76.8 | 16.95c | 17.95bc | 33.05e | 306.61ab | 9.929e |
| Atrazine + simazine + nicosulfuron | 625 + 625 + 30 | 17.95ab | 17.60cd | 40.85ab | 311.64ab | 10.476c |
| Atrazine + simazine + tembotrione | 625 + 625 + 12.1 | 17.70ab | 18.45ab | 39.40bc | 319.34ab | 9.561g |
| Atrazine + simazine + mesotrione | 625 + 625 + 38.4 | 17.23bc | 18.65a | 38.50c | 296.91c | 10.430c |
| Nicosulfuron + tembotrione | 30 + 12.1 | 17.23bc | 17.40d | 39.10c | 305.14ab | 10.965a |
| Nicosulfuron + mesotrione | 30 + 38.4 | 16.75cd | 16.75e | 36.55d | 323.02ab | 9.501g |
| Tembotrione + mesotrione | 12.1 + 38.4 | 17.68ab | 18.20ab | 38.50c | 306.65ab | 9.767f |
| Unweeded control | – | 16.58cd | 17.55cd | 33.70e | 301.85ab | 7.845i |
| Weeded control | – | 18.15a | 18.50ab | 41.75a | 307.40ab | 10.188d |
| Coefficent of variation (%) | – | 2.59 | 2.41 | 2.79 | 4.30 | 1.09 |
- † Applied pre-emergence.
- ‡ Averages followed by the same lower case letter in the column did not differ by Tukey's honest significant difference test at P≤ 0.05.
- § Applied postemergence.
The application of atrazine + simazine pre and postemergence (1250 + 1250 g ha−1 postemergence), atrazine + simazine (625 + 625 g ha−1 postemergence), nicosulfuron + mesotrione and the infested control showed ear lengths of 6.89, 10.57, 6.72, 7.71 and 8.65% lower than the weeded control, respectively (Table 6). Different results were obtained by López-Ovejero et al. (2003) when working with atrazine + simazine and the dose of 1250 + 1250 g ha−1 postemergence, when the treatment did not differ from the weeded control. The shorter length of the ears that was observed in the treatments consisting of atrazine + simazine (1250 + 1250 g ha−1 postemergence) and atrazine + simazine pre- and postemergence (625 + 625 g ha−1) might be associated with inefficient weed control of morningglory, Alexander grass and wild poinsettia (Tables 3-5), as the presence of these species can affect all components that are related to the yield of corn grain, especially Alexander grass, to present high competition, which also was observed by Galon et al. (2010).
The largest number of rows per ear (Table 6) statistically matched with the weeded control when applying the herbicides of tembotrione + mesotrione, atrazine + simazine + tembotrione, atrazine + simazine + mesotrione, mesotrione, nicosulfuron, atrazine + simazine (1250 + 1250 g ha−1) pre- and postemergence. As mentioned before, this is related in part to the good rate of control of all the weeds that were present in the experimental area, except for wild poinsettia at preharvest, preventing them to compete with the corn in the early stages of development that expresses the cultivar's productivity components. According to Fornasieri Filho (2007), the expression of the number of rows per ear and number of kernels per row (size of the spike) occur in the V8 and V12 stages (8–12 fully developed leaves), respectively, comprising the initial developmental stages of the corn crop and this cannot pass by abiotic or biotic stresses in order to be able to express all their productive potential.
Both mesotrione and tembotrione presented good results when applied isolated in corn (Freitas et al. 2009; Dan et al. 2011). It can be inferred therefore that the tembotrione + mesotrione, atrazine + simazine + mesotrione and atrazine + simazine + tembotrione mixtures have a positive interactive effect on corn, causing a phytotoxic rate of <4.0% to the cultivar during the study period (Table 5) and showing satisfactory weed control, with the exception of preharvest (Tables 2-4), which did not cause interference in the expression of the components of the grain yield of the corn hybrid, Formula TL®.
There was the largest number of kernels per row in the weeded control, not differing statistically from the treatments involving atrazine + simazine + nicosulfuron (Table 6). The atrazine + simazine + nicosulfuron also presented control of morningglory (Table 2), Alexander grass (Table 3) and wild poinsettia (Table 4) that was statistically equal to the weeded control from 14–28 DAT, showing an efficient control of weeds that positively influenced the measured variable because this variable is defined 1 week before the corn-flowering stage in the V17 (17 fully expanded leaves) stage. In stage V3, the establishment of the number of grains, or the definition of potential output are defined (Magalhães & Durães 2006), and therefore there is the need for corn to be free of weed infestation or other biotic or abiotic factors that might interfere with this variable.
The weeded control and the tank mixture, atrazine + simazine + nicosulfuron, showed that the interaction of these herbicides has positive results on weed control and, consequently, on the number of kernels per row, which was 17.5% higher than the infested control. It is noteworthy that the mixture of atrazine + simazine is highly selective to maize and that nicosulfuron is a herbicide that is rapidly detoxified by the cultivar (Cavalieri et al. 2008), thus causing low phytotoxicity to corn (Table 5), which will consequently express a higher number of kernels per row.
For the thousand-grain weight, it was found that there were differences only from the application of the tank mix of atrazine + simazine + mesotrione to the other treatments, except for the nicosulfuron applied alone, which did not differ between them, and showed the lowest results for this variable, compared with the other treatments (Table 6). Regarding nicosulfuron, López-Ovejero et al. (2003) found the opposite results. The authors found that the application of this herbicide at different developmental stages did not statistically reduce the weight of a thousand grains of corn, when compared to the weeded control.
The explanation for the mixture, atrazine + simazine + mesotrione, reducing the thousand-grain weight could be that the treatment had shown shorter corn ears in relation to the weeded control, it reduced the physical space of the grains and increased the partition of assimilates, resulting in more, but smaller and low-weight, grains. Cruz-Aguado et al. (2001) reported that assimilates partitioned in organs of a plant can be changed during the grain-filling stage due to the greater amount of drains present in the plant, which explains the results of this work.
The good performance of nicosulfuron + tembotrione is related to its low phytotoxicity to the cultivar, the effectiveness of weed control and different mechanisms of action that enable better weed control. Works involving nicosulfuron demonstrated high selectivity to corn (Portugal 2013) and also a wide spectrum of action against the infesting weeds of corn (Moraes et al. 2013), thus causing a high grain yield.
The results show a higher productivity of grains in the corn hybrid, Formula TL®, when applying the nicosulfuron + tembotrione tank mixture (Table 6). Among the herbicides applied in isolation, there is the tembotrione, which showed a higher level of productivity than the weeded control but lower than the mixture of nicosulfuron + tembotrione. The lowest grain yield, except the unweeded control, was observed in the pre- and postemergence application of atrazine + simazine (625 + 625 g ha−1), a result that is associated with the low level of weed control in corn.
It is noteworthy that the weeded control, despite being 23% higher than the infested corn, was not among the best results, a fact that is probably related to the damage caused by the hoeing to the roots, and with the regrowth of the weeds, thus might have had less nutrients. In addition, the use of the mechanical method of control (weeding) in the corn crops is costly, inefficient and takes a lot of labor, which generates high costs, compared to the chemical control method. Results similar to the present study were found by Galon et al. (2012) when studying the use of different methods of weed management in sugar cane.
Experiment II
The treatments in the morning glory control that were observed in experiment II showed similarities to the first experiment, and likewise, it was found that only the tank mixture of atrazine + simazine + mesotrione controlled the weed, not differing from the weeded control in all periods (Table 7). Also, the effect of atrazine + simazine + tembotrione being assessed from 7–28 DAT and the treatments with the isolated application of mesotrione and the tank mixture of tembotrione + mesotrione did not differ from the weeded control from 14 DAT to preharvesting, with the morningglory control rate >90%.
| Treatment | Dose (g ha−1) | Control of morningglory (%) | ||||
|---|---|---|---|---|---|---|
| 7 DAT† | 14 DAT | 21 DAT | 28 DAT | Preharvest | ||
| Atrazine + simazine‡ | 1250 + 1250 | 88.0bc§ | 68.7e | 61.2e | 41.2h | 55.0ghi |
| Atrazine + simazine¶ | 625 + 625 | 83.8c | 86.5cd | 84.5d | 61.3g | 45.0hi |
| Atrazine + simazine¶ | 1250 + 1250 | 93.5ab | 98.5a | 90.2bcd | 84.0cdef | 78.7bc |
| Nicosulfuron | 60 | 52.5e | 78.0d | 83.7d | 81.5def | 44.5i |
| Tembotrione | 24.2 | 82.5c | 88.0bc | 87.7cd | 72.2fg | 59.7efg |
| Mesotrione | 76.8 | 67.5d | 96.5ab | 100.0a | 97.5ab | 97.0a |
| Atrazine + simazine + nicosulfuron | 625 + 625 + 30 | 82.7c | 93.7abc | 90.0bcd | 88.0bcde | 56.2fgh |
| Atrazine + simazine + tembotrione | 625 + 625 + 12.1 | 94.5ab | 99.5a | 96.2ab | 89.5abcd | 73.7cd |
| Atrazine + simazine + mesotrione | 625 + 625 + 38.4 | 94.5ab | 99.0a | 99.5a | 98.7ab | 86.2ab |
| Nicosulfuron + tembotrione | 30 + 12.1 | 57.5e | 88.0bc | 87.5cd | 76.7ef | 67.0def |
| Nicosulfuron + mesotrione | 30 + 38.4 | 57.5e | 97.2a | 97.7a | 95.5abc | 71.2cde |
| Tembotrione + mesotrione | 12.1 + 38.4 | 82.7c | 97.7a | 93.7abc | 96.0ab | 94.0a |
| Unweeded control | – | 0.0f | 0.0f | 0.0f | 0.0i | 0.0j |
| Weeded control | – | 100.0a | 100.0a | 100.0a | 100.0a | 100.0a |
| Coefficent of variation (%) | – | 5.1 | 4.2 | 3.5 | 6.1 | 7.2 |
- † Days after treatment.
- ‡ Applied pre-emergence.
- § Averages followed by the same lower case letter in the column did not differ by Tukey's honest significant difference test at P ≤ 0.05.
- ¶ Applied postemergence.
There was also the effect on the control of morningglory of the mixture of nicosulfuron + mesotrione, not differing from the weeded control at 14, 21 and 28 DAT, with an average control rate of 96.8% (Table 7). It was found that the treatments involved with mesotrione, applied alone or in a tank mix, showed high percentages of control at almost all the evaluated dates, a fact that also was observed in the first experiment to control the morning glory weed.
The treatments consisting of atrazine + simazine applied postemergence (1250 + 1250 g ha−1) and the tank mixture of atrazine + simazine + nicosulfuron demonstrated an average control rate of morning glory >80%, except for the preharvest period, with an average of 78.7 and 56.2%, respectively (Table 7). The 80% control rate is used as a reference. As explained in the first experiment, the authors Oliveira et al. (2009) and Maciel et al. (2013) reported that control (>80%) can now be recommended for the application of a particular herbicide. Timossi and Freitas (2011) found that the highest I. triloba control rates were obtained by using a bigger dose of nicosulfuron (60 g ha−1), as well as when it was associated with atrazine.
The treatment with less control of morningglory, besides the unweeded control, was the atrazine + simazine treatment (1250 + 1250 g ha−1) applied pre-emergence, with a control average of 62.8% in all the evaluated periods (Table 7).
For the control of Alexander grass (Table 8), a similar behavior to what happened in the first experiment was observed: the tank mixture of atrazine + simazine + tembotrione was the only one that did not differ from the weeded control for all the evaluated dates. At 7 DAT, the tank mixtures involving atrazine + simazine + tembotrione and atrazine + simazine + mesotrione were the treatments that did not differ from the weeded control, with an average control rate of >90%. For this same date, the worst treatments, besides the infested control, were the atrazine + simazine (625 + 625 g ha−1 pre and postemergence), nicosulfuron, nicosulfuron + tembotrione and nicosulfuron + mesotrione, which did not differ, with an average control rate of 59.9% of Alexander grass (Table 8).
| Treatment | Dose (g ha−1) | Control of Alexander grass (%) | ||||
|---|---|---|---|---|---|---|
| 7 DAT† | 14 DAT | 21 DAT | 28 DAT | Preharvest | ||
| Atrazine + simazine‡ | 1250 + 1250 | 62.5gh§ | 70.0d | 57.5c | 35.0f | 50.0b |
| Atrazine + simazine¶ | 625 + 625 | 57.5h | 89.8bc | 81.3b | 78.3c | 41.3bc |
| Atrazine + simazine¶ | 1250 + 1250 | 70.2efg | 87.5c | 75.0b | 61.2d | 35.0c |
| Nicosulfuron | 60 | 55.0h | 99.7a | 100.0a | 100.0a | 93.5a |
| Tembotrione | 24.2 | 81.2bcd | 100.0a | 99.7a | 97.7ab | 93.2a |
| Mesotrione | 76.8 | 76.7de | 85.0c | 83.2b | 78.2c | 34.5c |
| Atrazine + simazine + nicosulfuron | 625 + 625 + 30 | 74.5def | 98.5a | 96.0a | 98.5ab | 94.5a |
| Atrazine + simazine + tembotrione | 625 + 625 + 12.1 | 90.7abc | 99.7a | 96.7a | 97.0ab | 92.2a |
| Atrazine + simazine + mesotrione | 625 + 625 + 38.4 | 91.5ab | 100.0a | 96.0a | 92.2b | 90.7a |
| Nicosulfuron + tembotrione | 30 + 12.1 | 64.5fgh | 98.0a | 97.5a | 98.2ab | 93.0a |
| Nicosulfuron + mesotrione | 30 + 38.4 | 60.0gh | 94.2ab | 94.5a | 91.5b | 90.0a |
| Tembotrione + mesotrione | 12.1 + 38.4 | 80.0cde | 100.0a | 98.7a | 98.0ab | 95.5a |
| Unweeded control | – | 0.0i | 0.0e | 0.0d | 0.0g | 0.0d |
| Weeded control | – | 100.0a | 100.0a | 100.0a | 100.0a | 86.7a |
| Coefficent of variation (%) | – | 6.2 | 2.7 | 3.9 | 3.9 | 5.1 |
- † Days after treatment.
- ‡ Applied pre-emergence.
- § Averages followed by the same lower case letter in the column did not differ by Tukey's honest significant difference test at P ≤ 0.05.
- ¶ Applied postemergence.
There was <80% control of Alexander grass only when applying atrazine + simazine (1250 + 1250 g ha−1 pre-emergence) for all the assessment periods and atrazine + simazine (1250 + 1250 g ha−1 postemergence) for all assessments periods, except for at 14 DAT (Table 8). This did not differ from the weeded control or the treatments involving nicosulfuron, tembotrione, atrazine + simazine + nicosulfuron, atrazine + simazine + tembotrione, nicosulfuron + tembotrione and tembotrione + mesotrione at 28 DAT, with an average control rate of 97.2% of Alexander grass. In preharvesting, the only treatments that showed Alexander grass control of <80%, besides the infested control, were the atrazine + simazine applied at doses of 1250 + 1250 g ha−1 pre and postemergence and 625 + 625 g ha−1 postemergence and mesotrione that demonstrated an average control rate of 40.2%.
Regarding the control of the wild poinsettia, it was observed that, at 7 DAT, the treatments with atrazine + simazine, when applied postemergence (1250 + 1250 g ha−1), atrazine + simazine + tembotrione and atrazine + simazine + mesotrione did not differ from the weeded control, with an average control rate of 95.1% (Table 9). These treatments maintained control rates >90% at 14 and 21 DAT, with the exception of atrazine + simazine (1250 + 1250 g ha−1 postemergence) that showed a control rate of 85% at 21 DAT.
| Treatment | Dose (g ha−1) | Control of wild poinsettia (%) | ||||
|---|---|---|---|---|---|---|
| 7 DAT† | 14 DAT | 21 DAT | 28 DAT | Preharvest | ||
| Atrazine + simazine‡ | 1250 + 1250 | 89.5bc§ | 74.0c | 55.0d | 35.0f | 62.5def |
| Atrazine + simazine¶ | 625 + 625 | 86.3c | 92.5ab | 86.8bc | 58.8e | 66.3cde |
| Atrazine + simazine¶ | 1250 + 1250 | 94.7ab | 91.0ab | 85.0bc | 76.2cd | 47.5g |
| Nicosulfuron | 60 | 52.5f | 93.7ab | 99.2a | 99.7a | 63.2def |
| Tembotrione | 24.2 | 82.5c | 95.5a | 94.2ab | 77.5cd | 56.2efg |
| Mesotrione | 76.8 | 70.0d | 85.7b | 81.2c | 81.7bc | 71.2bcd |
| Atrazine + simazine + nicosulfuron | 625 + 625 + 30 | 89.7bc | 98.7a | 100.0a | 98.2a | 67.0cde |
| Atrazine + simazine + tembotrione | 625 + 625 + 12.1 | 95.5ab | 95.5a | 92.5ab | 87.5b | 77.5bc |
| Atrazine + simazine + mesotrione | 625 + 625 + 38.4 | 95.0ab | 94.7ab | 92.0ab | 87.0b | 58.7ef |
| Nicosulfuron + tembotrione | 30 + 12.1 | 61.2e | 100.0a | 98.5a | 100.0a | 78.2b |
| Nicosulfuron + mesotrione | 30 + 38.4 | 63.7de | 96.5a | 99.5a | 96.5a | 76.7bc |
| Tembotrione + mesotrione | 12.1 + 38.4 | 82.5c | 93.0ab | 87.2bc | 70.0d | 54.2fg |
| Unweeded control | – | 0.0g | 0.0d | 0.0e | 0.0g | 0.0h |
| Weeded control | – | 100.0a | 100.0a | 100.0a | 100.0a | 100.0a |
| Coefficent of variation (%) | – | 4.3 | 4.4 | 4.8 | 4.6 | 8.1 |
- † Days after treatment.
- ‡ Applied pre-emergence.
- § Averages followed by the same lower case letter in the column did not differ by Tukey's honest significant difference test at P ≤ 0.05.
- ¶ Applied postemergence.
In the assessments that were made at 14 and 21 DAT, only the treatment atrazine + simazine applied pre-emergence presented wild poinsettia control of <80%, as well as what occurred in the infested control (Table 9). In the evaluation at 28 DAT, the treatments involving nicosulfuron, atrazine + simazine + nicosulfuron, nicosulfuron + tembotrione and nicosulfuron + mesotrione did not differ from the weeded control, noting that a high level of control of wild poinsettia was found whenever nicosulfuron was present, which also was found in experiment I (Table 4). It is noteworthy that the tank mixture of nicosulfuron with atrazine has been widely used to increase the spectrum of action to control mono and dicotyledon weeds (Petter et al. 2011). Still, at 28 DAT, the worst wild poinsettia control was observed with the treatment using atrazine + simazine pre-emergence (35.0%), followed by atrazine + simazine post-emergence (at both doses), tembotrione and mesotrione + tembotrione, with a control of <76.2%.
The evaluation that was carried out in the corn pre-harvest showed that none of the tested treatments kept good control of wild poinsettia, being all <80% (Table 9). This fact is associated with the alternating light that occurred in the field; that is, in the early stages, the corn hybrid, SYN 7B28, shaded the ground by its higher leaf area index, and at the end of the cycle, there was a greater passage of light to the ground, which did not occur with the hybrid, Formula TL®. This meant that there were new wild poinsettia emergency flows, as this species has its germination favored when there is alternating light in the tillage (Kissmann & Groth 1999).
Just as occurred in the first experiment, when evaluating the effect of the herbicides on the hybrid, Formula TL®, the phytotoxicity of the treatments that were tested in experiment II on the hybrid, SYN 7B28, also proved to be low, observing rates of <6% to 14 DAT, and from 21 DAT, no phytotoxicity to the crop was observed (Table 10); that is, the symptoms that were caused by the herbicides disappeared. Adegas et al. (2011) tested atrazine, mesotrione, nicosulfuron and tembotrione in isolated and/or mixing and Freitas et al. (2009) worked with tembotrione and they observed phytotoxicity rates of <8.75% for the different evaluated hybrids. Low phytotoxicity is related to the fact that all the tested herbicides present selectivity to corn.
| Treatment | Dose (g ha−1) | Phytotoxicity (%) | |||
|---|---|---|---|---|---|
| 7 DAT† | 14 DAT | 21 DAT | 28 DAT | ||
| Atrazine + simazine‡ | 1250 + 1250 | 0.0b§ | 2.7c | 0.0a | 0.0a |
| Atrazine + simazine (2.5 L)¶ | 625 + 625 | 3.3a | 5.0ab | 0.0a | 0.0a |
| Atrazine + simazine (5.0 L)¶ | 1250 + 1250 | 3.5a | 4.0bc | 0.0a | 0.0a |
| Nicosulfuron | 60 | 4.0a | 5.5ab | 0.0a | 0.0a |
| Tembotrione | 24.2 | 3.5a | 4.7ab | 0.0a | 0.0a |
| Mesotrione | 76.8 | 4.0a | 6.0a | 0.0a | 0.0a |
| Atrazine + simazine + nicosulfuron | 625 + 625 + 30 | 3.2a | 4.5ab | 0.0a | 0.0a |
| Atrazine + simazine + tembotrione | 625 + 625 + 12.1 | 3.5a | 4.7ab | 0.0a | 0.0a |
| Atrazine + simazine + mesotrione | 625 + 625 + 38.4 | 4.5a | 5.0ab | 0.0a | 0.0a |
| Nicosulfuron + tembotrione | 30 + 12.1 | 3.2a | 5.5ab | 0.0a | 0.0a |
| Nicosulfuron + mesotrione | 30 + 38.4 | 3.5a | 5.2ab | 0.0a | 0.0a |
| Tembotrione + mesotrione | 12.1 + 38.4 | 3.5a | 4.5ab | 0.0a | 0.0a |
| Weeded control | – | 0.0b | 0.0d | 0.0a | 0.0a |
| Unweeded control | – | 0.0b | 0.0d | 0.0a | 0.0a |
| Coefficent of variation (%) | – | 24.5 | 14.5 | 1.0 | 1.0 |
- † Days after treatment.
- ‡ Applied pre-emergence.
- § Averages followed by the same lower case letter in the column did not differ by Tukey's honest significant difference test at P ≤ 0.05.
- ¶ Applied postemergence.
The worst treatment for corn ear length was the mixture of atrazine + simazine when applied pre-emergence (Table 11). This can be explained by the fact that the treatment does not control the weeds until the end of the corn cycle. Contrary results were found by López-Ovejero et al. (2003), who found that the application of atrazine at the beginning of the corn cycle did not provide a difference for the ear length variable.
| Treatment | Dose (g ha−1) | Ear length (cm) | No. of rows per ear | No. of kernels per row | Thousand-grain weight (g) | Grain yield (t ha−1) |
|---|---|---|---|---|---|---|
| Atrazine + simazine† | 1250 + 1250 | 15.08d‡ | 16.05f | 28.15ab | 286.04e | 13.563h |
| Atrazine + simazine§ | 625 + 625 | 16.98ab | 16.90ab | 29.55a | 315.57bc | 12.804i |
| Atrazine + simazine§ | 1250 + 1250 | 16.98ab | 17.05ab | 29.70a | 309.34cd | 13.906f |
| Nicosulfuron | 60 | 17.05a | 17.30ab | 29.05a | 316.91bc | 13.723g |
| Tembotrione | 24.2 | 16.28bc | 17.10ab | 27.10b | 317.61bc | 13.730g |
| Mesotrione | 76.8 | 17.13a | 16.50de | 28.30ab | 339.24a | 11.194j |
| Atrazine + simazine + nicosulfuron | 625 + 625 + 30 | 17.14a | 16.90ab | 29.70a | 301.58cd | 14.577d |
| Atrazine + simazine + tembotrione | 625 + 625 + 12.1 | 16.28bc | 17.55a | 28.35ab | 315.24bc | 16.206a |
| Atrazine + simazine + mesotrione | 625 + 625 + 38.4 | 17.43a | 16.30ef | 28.45ab | 331.75ab | 14.792c |
| Nicosulfuron + tembotrione | 30 + 12.1 | 16.18c | 17.20ab | 29.00a | 309.00cd | 13.452h |
| Nicosulfuron + mesotrione | 30 + 38.4 | 16.79ab | 16.80bc | 28.83a | 289.51de | 15.969b |
| Tembotrione + mesotrione | 12.1 + 38.4 | 16.95ab | 16.60cd | 28.43ab | 311.97bc | 14.414e |
| Unweeded control | – | 16.95ab | 16.25ef | 29.40a | 295.66cd | 10.422l |
| Weeded control | – | 17.38a | 17.35ab | 29.63a | 310.51bc | 14.755c |
| Coefficent of variation (%) | – | 2.67 | 2.54 | 3.63 | 4.3 | 0.61 |
- † Applied pre-emergence.
- ‡ Applied postemergence.
- § Averages followed by the same lower case letter in the column did not differ by Tukey's honest significant difference test at P ≤ 0.05.
For the variable number of kernel rows per ear, the best results were obtained with the tank mixtures of atrazine + simazine + tembotrione, atrazine + simazine + nicosulfuron, nicosulfuron + tembotrione and nicosulfuron + mesotrione and the isolated applications from two doses of atrazine + simazine postemergence. Nicosulfuron and tembotrione did not differ from the weeded control (Table 11). The mesotrione herbicide did not differ statistically from the unweeded control in the number of grain rows per ear. Generally, corn hybrids are tolerant to the herbicide, mesotrione, due to their ability to rapidly metabolize the product, producing metabolites without toxic activity (Karam & Cruz 2004); however, in some cases, it might happen that there are phytotoxic effects, which visually cannot be detected, but when evaluating the performance of the yield components, it has a negative influence. This fact also has been observed by other researchers of corn (Furtado 2004), wheat (Galon et al. 2015), rice (Pinto et al. 2008) and sugar cane (Tironi et al. 2012) crops.
For the variable number of kernels per row, it was found that the treatment with tembotrione had fewer grains per row, except when this herbicide was associated with nicosulfuron, which were statistically equal to the weeded control (Table 11). This combination with nicosulfuron might have caused an antagonistic effect on the cultivar; that is, when mixing two herbicides of different mechanisms of action, one may influence the herbicidal action of the other, resulting in no phytotoxic effect. It is common to have an antagonistic effect when mixing herbicides, a fact also confirmed by Ratier et al. (2015) when mixing atrazine with glyphosate.
The phytotoxicity of the SYN 7B28 hybrid did not exceed 6% in the 7 and 14 DAT evaluations, regardless of the applied herbicide. The phytotoxicity was considered to be low and in the evaluations of 21 and 28 DAT, no injury was observed, demonstrating the selectivity of the herbicides used (Table 10).
The best results for the thousand-grain weight were obtained when applying in isolation the mesotrione herbicide and the tank mixture of atrazine + simazine + mesotrione (Table 11), which can be explained because these treatments showed a low number of rows per ear and high ear length. Thus, there is more space and photosynthate supply to the grains from this treatment, resulting in heavier grains. These results corroborate with those that were observed by López-Ovejero et al. (2003), who reported that the highest thousand-grain weight was related to the corn ear having lower numbers of rows and fewer kernels per row.
Just as seen in the first experiment, the hybrid, SYN 7B28, also presented a productivity of corn grain that was influenced by the tested treatments. The highest productivity was observed with the tank mixture of atrazine + simazine + tembotrione (Table 11). Oliveira (2013) evaluated the effect of herbicide applications and found that the higher corn yields were found when applying mesotrione and nicosulfuron in isolation. Although these treatments had not presented the highest yields in this study, they showed intermediate results, when compared to the other treatments.
The productivity of the mixture of atrazine + simazine + tembotrione was 55.5% higher than the unweeded control. This is explained by the efficient control of weeds, until 28 DAT, and the low level of phytotoxicity of this mixture to the cultivar (Table 11). The critical period of prevention of weed interference in the maize crop is at the beginning of its cycle and can be understood, generally, in the initial periods of the crop because it is in these stages that weeds increase their dry mass, requiring a greater supply of nutrients, water and light from the environment (Alford et al. 2005; Kozlowski et al. 2009). Although the atrazine + simazine + mesotrione mixture showed a low visual phytotoxicity and high control, it did not obtain satisfactory productivity because, in particular, the mesotrione herbicide might have caused injuries that were not visible, compromising some of the grain yield components, and consequently, productivity.
As in the first experiment, despite the weeded control proffering 29.87% higher productivity than the infested control (Table 6), it is not among the best results, which can be attributed to the disruption and cutting of the roots during the weeding process or even the regrowth of the weeds after weeding, as explained in experiment I.
None of the herbicides that was tested in mixture or in isolation showed a satisfactory control level up to the pre-harvest for morning glory, wild poinsettia and Alexander grass, but in the initial period comprising the evaluations of 7–28 DAT, the herbicides that best excelled in the two experiments for the control of morning glory, wild poinsettia and Alexander grass were atrazine + simazine + tembotrione and atrazine + simazine + mesotrione for morningglory, wild poinsettia and Alexander grass.
The use of herbicide mixtures can reduce the problems of herbicides that are resistant weeds due to the use of more than one active ingredient.
Conclusion
There was low phytotoxicity to the corn hybrids, Formula TL® and SYN 7B28, regarding all the herbicides, tank-mixed or in isolated form. The best control of morningglory, Alexander grass and wild poinsettia and the low corn phytotoxicity in both experiments were, in general, when using the tank mixtures of atrazine + simazine + tembotrione and atrazine + simazine + mesotrione under the conditions in which the experiments were conducted.
The application of atrazine + simazine + tembotrione in the SYN 7B28 hybrid provided, except for the thousand-grain weight, the best expression of grain yield components, combining a low phytotoxicity level and best control of weeds. The Formula TL® hybrid presented the highest mass of a thousand grains and yield with the application of nicosulfuron + tembotrione.
The difference between the yield average of the herbicide treatments and the unweeded control showed an increase of 34.59 and 27.66% in the yield of grains of corn hybrids, Formula TL® and SYN 7B28, respectively.
Acknowledgements
The authors thank the National Scientific and Technological Development Council (CNPq) and the Research Support Foundation of the Rio Grande do Sul State (FAPERGS), Brazil, for the financial support to the research (Process no. 482144/2012-2/CNPq and 12/2265-3/FAPERGS, respectively) and for the research grant concessions.
Disclosure Statement
The authors declare no conflict of interest.
