Evaluating the Competitive Ability of Maize (Zea mays L.) Genotypes against Weeds under Different Nitrogen Rates

Maize is among the world's three most important cereal crops. Weeds are one of the most important factors that reduce maize production. They cause important yield losses worldwide with an average of 12.8% despite weed control applications and 29.2% in the case of no weed control. Weeds compete with crops when they remove a portion of a resource from a shared resource pool, leaving the crop with less of the resource than is needed for optimum growth. Competition may occur for water, creating or exacerbating water stress. It may occur for nutrients such as nitrogen, leading to chlorosis, leaf senescence and yield loss. Increasing the ability of a crop to compete with weeds has been considered an approach to improving weed management in recent years. The importance of weed competition in maize depends on the maize genotype, weed species, crop growth stage, the weed density, the level of water, and nutrient stress. Some crops can significantly suppress weeds by their ability to grow faster than weeds or to endure greater sowing densities. The plant competitive ability can be expressed in the following two modes: First, the crop weed suppressive ability (WSA) that results in the reduction of weed biomass and second, the crop tolerance ability (CTA) to tolerate weed effects and to have high yields at the same time. Crop tolerance to weed interference aims to improve stability of yield and quality in weedy fields, whereas weed-suppressive ability targets the long-term management of weed populations by reducing seedbank size. Several traits, including the growth intensity in the initial developmental stages, rapid development of a high leaf structure, the plant height, maximum LAI, rate of canopy closure, height of LAI, and leaf architecture, may improve weed-suppressive ability and dent maize tolerance to weed interference. Therefore, the objective of this experiment was to evaluating the competitive ability of maize (Zea mays L.) genotypes against weeds under different nitrogen rates.
Experiments were carried out on clay loam soil with low organic matter content (less than 1%) and a pH of 2.9 at south west Iran in 2011-2012 growing season. The experiment site had a hot climate with a moderate winter and dry and hot summer. Treatments consisted of a split-plot with randomized complete blocks design and three replicates. Nitrogen rates (0, 90, 180 and 260 kgNha-1) were the main plots and four dent maize hybrids Sc.704, DKC6589, Mobeen and Sc.640 were planted in sub-plots. DKC6589, Mobeen and Sc.640 are an early-season hybrids, whereas Sc.704 is relatively late-season hybrid. Plots consisted of 6 maize rows (distance between and within the rows 75 and 18 cm, respectively) and 6 m long. Maize hybrids were sown on 1st Jun. Fields received 100 kgPha-1 and 150 kg K ha-1. The P and K were supplied in the form of single superphosphate and potassium sulfate, respectively. Nitrogen treatments were applied in two equal splits as urea (46% N) at two and six weeks after planting by side placement. Each plot was divided into two parts hypothetical. One part was maintained weed-free and another weedy for the all maize duration growth stages. In weed-free parts, weed removal was started immediately after crop emergence and the plots were kept weed-free for different growth stage durations until physiological maturity of maize. The experiment was properly monitored and irrigated whenever water was needed. Weed plants were harvested at the R1 (silking) growth stage of maize using 0.5×0.5m quadrate, and weed biomass obtained by drying tissues at 68 C to constant mass. Weed density and species also, determined. Interference of maize and weed was quantified by calculating weed index (WI), weed interference tolerance index (WITI) and competition index (CI(. Statistical analysis was made using the SAS statistical software. Differences between traits means were assessed using Duncan's Multiple Range Test.
The percent loss of grain yield, 100 grain weight, biological yield and grain number per rows under weed interference conditions were 21.7, 5.3, 24 and 10%, respectively. Grain yield reduction in weedy plots was resulted from decreasing grain number per rows and biological yield. Negative impact of weeds on maize yield was enhanced under higher rates of nitrogen. When maize competed with weeds, the reduction of grain yield at 260 kg N ha-1 treatment was more compared to 180 kg N ha-1maize. Maize genotypes were different in response to weed competition. The highest weed tolerance interference index was obtained in DKC6589 line due to higher grain yield in both weedy and weed-free plots. Competition index was higher in Sc. 704 and DKC6589 than other genotypes. Although Mobeen and Sc.640 hybrids had lower grain yield reduction trend under weed competition conditions, but DKC6589 and Sc.704 showed a high grain yield potential and high ability to withstand weed competition.