فهرست مطالب مریم آریان مهر

In each farm field and over time due to the long-term use of inputs and varied tillage practices as well as different management operations, the factors affecting plant yield would be complex (Liu et al., 2013). The impacts of these factors are not usually taken into consideration in farm management. For example, the herbicides are usually applied uniformly based on the mean weed pressure in the field. Uniform management in situations where there is spatial variation, not only will not be economically efficient but also will have detrimental environmental consequences. Cardina and Doohan (2008) reported that although weeds are often dense in some parts of the field, their density is not independent of each other. In other words, their density depends on the sampling distance of species. Identifying weed distribution on the farm is a necessary step before site-specific management. Site-specific management is in the direction of sustainable agricultural purposes. This thereby highlights the importance of the study of the spatial distribution of weeds. The aim of this study was to investigate the spatial distribution of pigweed and nightshade under different weed control methods.

This experiment was conducted at the experimental station of Ferdowsi University of Mashhad during the growing season of 2016. A plot of field measuring 56.25 × 21 m was selected and maize S.C 704 was planted in it. Each control method including chemical control, integrated control (mechanical+chemical) and weedy control was applied to one-third of the field. Data collected from sampling at 270 points based on a 1.87×2.5 m grid at two stages at the first and the last of corn growth season (30 and 90 days after corn planting). Amaranthus retroflexus and Solanum nigrum species had more than 96% density of total weeds.The evaluating of the spatial distribution of weed species was done by a geostatistical analysis of the species counts. The principal tool of geostatistics is the variogram (Goudy et al., 2001). The function showed in equation (1) links the expected squared difference of a variable between any two places:equation (1)Where z(x) and z(x + h) are random variables at positions x and x + h. h is the distance of pair of points. Validation of the variogram model was determined by (equation 2) that calculating the root mean square error (RMSE):
equation (2)

According to the percentage of RMSE error, The fitted variogram models to density pigweed and nightshade were most often in accordance with the spherical model in integrated control, chemical control and weedy control at the beginning and end of the growing season. This result, in addition to indicating a patchy distribution of species, also showed that this structure in species has been preserved after mechanical and chemical control. In the integrated control, the range of pigweed and nightshade, decreased about 15.5 m (48%) and 2.8 m (30%), at the end of the growing season respectively, while in the weedy control, the range of these species increased about 0.5 m (6%) and 2 m (17%) at the end of the growing season, respectively. In the chemical control the range of pigweed decreased about 0.68 m, and the range of nightshade, increased about 0.6 m at the end of the growing season of corn.

The results of many studies indicate a patchy distribution of weeds. In this study, the patchy structure of weeds remained even after mechanical and chemical control of weeds. It can be said mechanical and chemical control of weeds destroy around of patches of weeds, but the center of the weed patches remains. Also in this study, the range of species was affected by weed control methods so that integrated control of weeds decreased the range of pigweed and nightshade and chemical control of weeds prevented expanding the range of pigweed.

Two important factors in producing forage maize (Zea mays L.) are weed control and nitrogen fertilizer management. Low maize yields are typically attributed to low corn growth rate at the beginning of the growing season, wide spacing of plant rows and rapid establishment of weeds. Weeds are a major challenge to row crops and research has usually emphasized on the integrated weed management using chemicals and mechanical control (Chauhan, 2013). In modern agriculture, weed control is primarily done with herbicides due to the effectiveness and ease of use. However, environmental concerns about pollution of groundwater and surface water, have led researchers to focus on integrated weed management and reduced herbicide consumption. On the other, selection of nitrogen fertilizer source may be an important component of weed management. Some studies have investigated nitrogen fertilizer management in order to integrated weed management. The present study was carried out to investigate the effect of chemical control and mechanical + chemical control of weeds and different sources of nitrogen fertilizer on weeds control and quantitative and qualitative characteristics of forage maize.

This experiment was conducted with a factorial arrangement based on randomized complete block design with three replications at the experimental station of Ferdowsi University of Mashhad during the growing season of 2015 -2016. The treatments were control methods of weeds including chemical control, integrated control (mechanical+chemical) and weedy control, and the sources of nitrogen fertilizers including urea (U), ammonium sulfate (AS), calcium nitrate (CN), ammonium nitrate phosphate (ANP) and no fertilizer (NF). In this study, Kjeldahl method was used to measure nitrogen content of the forage. Crude protein (CP) of forage maize was calculated using the following equation (1). Measurements of acid detergent fiber (ADF) and neutral detergent fiber (NDF) was done based on Van Soest method (1967). The equation (2), proposed by Oddy et al (1983) was used to determine digestible dry matter (DMD). Equation 1: 6.25 × N (%) = CP (%) Equation 2: DMD (%) =83.58 – 0.82 ×ADF(%) + 2.262 × N(%).

The integrated control of weeds reduced dry matter of weeds compared to chemical control by 68% and 57% in the first and second year of the experiment, respectively (50 days after corn planting), leading to increased dry matter yield of maize by 17% and 14%, respectively. Furthermore, in the integrated control, herbicide consumption decreased 25% compared to chemical control. However chemical control of weeds reduced dry matter of weed compared to weedy control by 71% and 79% in the first and second year, respectively. The greatest dry matter yield (DM-Y), dry matter digestible yield (DMD-Y) and crude protein yield (CP-Y) were observed with the integrated control and ANP or AS application (there were no significant differences between ANP and AS fertilizers), while the lowest DM-Y, DMD-Y and CP-Y were obtained with weedy control (there were no significant differences between fertilizers treatment).

In the integrated control with proper implementation to reduce weed dry matter, not only did increased quantitative and qualitative yield of forage but also herbicide consumption decreased 25% compared to chemical control. Reducing herbicide consumption through integrated weed management will preserve the environment, improve human health and increase product quality and quantity. On the other hand, it seems that (ANP and AS) as nitrogen fertilizer sources are more effective in terms of increasing maize production compared to other nitrogen sources. It can be suggested, using ANP fertilizer that contains both nitrogen forms (nitrate and ammonium) leads to increasing maize yield. Also, assimilation of ammonium requires the low energy and occurs faster than nitrate in plants. This energy savings may lead to greater dry weight production for plants with an ammonium diet. In addition, the sulfur in AS fertilizer as an essential element for plants may improved plant growth and metabolism.