Maize is one of the world's major cereals, and alexandergrass is the most damaging weed in the crop. The objective of this work was to evaluate the relative competitive ability of maize hybrids in the presence of alexandergrass through replacement series experiments. The experiments were set up in a complete randomized block design, with four replications. Firstly, for the maize hybrids as well as for the alexandergrass, the plant population was determined in which the final production becomes constant. were composed of plastic pots with a capacity of 8 dm3, and in each experimental unit was placed in the proportion corresponding to each treatment (100: 00, 75:25, 50:50, 25:75 and 0: 100) in each experimental unit, the species competing or not between them. For each hybrid, a separate experiment was considered with the objective of evaluating intra and interspecific competition. The analysis of the competitiveness of the species was carried out through diagrams applied to the replacement experiments and by the relative competitiveness indexes. At 50 days after the emergence of the maize the leaf area (LA) and the dry mass of the aerial part (DM) of the plants were checked. The relative competition was occured among the weed and each maize hybrid, being negatively affected for both of species, independently of the proportion of weed causing reductions in LA and DM of the maize. Interspecific competition causes less damage to LA and DM of species than intraspecific competition. There is basically competition for the same environment resources (water, CO2, sun light and nutrients) between maize and alexandergrass, being the crop more competitive than the weed.

Nowadays, the maize weevil Sitophilus zeamais—Motsch 1855 is the most relevant pest found during storage of maize grains. Aiming at protecting the agricultural crops, high amounts of synthetic pesticides are used around the world. In this sense, this work aimed at evaluating the insecticidal and repellent effect of essential oil of Ocotea odorifera (Vellozo) Rohwer (Lauraceae) (O. odorifera (sassafras)) leaves against the maize weevil in maize grains. The essential oil's chemical composition showed that sassafras contains camphor (43%) and safrole (42%) as major compounds. Our results show that sassafras oil was insecticidal and repellent to maize weevil. About 100% mortality was achieved using a concentration of 0.32 μl cm−2, after 24 h of exposure. The LD50 value was 14.1 μl or 0.09 μl cm−2 and the study of the effect on insect mortality at the lethal doses (LD50) showed that after 72 h mortality was 100%. The values of the preference index varied from −0.39 (0.065 μl cm−2) to −0.63 (0.659 μl cm−2), demonstrating that the plant oil presents repellent activity. The repellency bioassay simulating small bins showed the repellent effect level from 0.64 (0.36 μl cm−3) to 0.94 (2.9 μl cm−3). The essential oil of sassafras presented insecticidal and repellency against maize weevil. Although from an economical point of view synthetic chemicals are still more frequently used as repellents, natural products (essential oils) have the potential to provide efficient and safer repellents for humans and the environment.

ABSTRACT: The objective of this work was to evaluate the interference and to determine the level of economic damage (NDE) of the weed when infesting the corn crop. Treatments were composed by corn densities (2.60, 3.10, 3.65, 4.00 and 4.80 plants m-1) and 10 Alexandergrass populations for each crop seeding density. The population of plants, leaf area, soil cover and shoot dry mass of Alexandergrass were evaluated as indicator of infestation. Shoot dry mass of Alexandergrass presents better adjustment to the model of rectangular hyperbole, and losses of grain yield due to interference of the weed were satisfactorily estimated by this model. Corn sowing densities of 2.60; 3.10 and 3.65 plants m-1 in average, were more competitive of all evaluated variables in the presence of Alexandergrass. Corn densities of 2.60; 3.10 and 3.65 plants m-1 increase the level of economic damage, justifying the adoption of control measures of Alexandergrass when in higher populations. NDE values ranged from 1.58 to 9.37 plants m-2 at the densities of 4.00 and 4.80 maize plants m-1, which were less competitive with Alexandergrass.

The seeds physiological quality is a fundamental characteristic to obtain competitive and productive plants, where seeds treatment is an important tool to keep seeds physiological high-quality. The present study aimed to evaluate chemical treatments and storage period influence on seeds physiological quality. It was used a factorial (4 x12) randomized complete block design, with eight replications. In factor A, the chemicals (thiamethoxam, metalaxyl + fludioxonil + thiabendazole and pyraclostrobin + thiophanate-methyl + fipronil) and control. In the factor B, the treated seeds storage periods (0, 3, 7, 14, 21, 28, 35, 42, 49, 56, 63 and 90 days). The variables evaluated were vigor and germination, and the tests were performed at p≤0.05. The results indicate that the maize seeds vigor is maintained with the fungicide application, regardless of the vigor test used. As the treated seeds storage period increases, their vigor was reduced by the first count test. However, maize seeds germination was not influenced by seed treatment or storage period. Highlighted Conclusions 1) The maize seeds vigor is maintained applying fungicides, regardless of the vigor test used. 2) The first count test indicates that the longer the untreated or treated maize seeds with thiamethoxam are stored the lower the seed vigor. 3) Maize seeds germination is not influenced by the thiamethoxam, metalaxyl + fludioxonil + thiabendazole and pyraclostrobin + thiophanate-methyl + fipronil chemicals, not even by the storage period up to 90 days of treated seeds.