The critical period of weed control (CPWC) is a period in the crop growth cycle during which weeds must be controlled to prevent yield losses. In order to determine the critical period of weed control (CPWC) of CORN in Hamedan, an experiment was conducted in 2007 at Agricultural Research Station of Bu-Ali Sina University of Hamedan. The experiment was carried out as a randomized complete block design with three replications. The trial included twelve treatments consisted of six initial weed-free periods in which plots were kept free of weeds for 0, 15, 25, 35, 45 and 55 days after crop emergence (DAE), and then weeds were allowed to grow until harvest and six initial weed-infested periods in which, weeds were allowed to grow for 0, 15, 25, 35, 45 and 55 DAE, after which the plots were kept free of weeds until harvest. Each plat consisted of four rows (six meter per row) with a row spacing of 75 cm. . Weed sampling carried out in first series of treatments at the end of growth season and in second series of treatments at the4 end of infested periods. The results showed that beginning of CPWC ranged from 124 to 204 GDD, at 5% and 10% AYL, which equates to 8 and 16 days after crop emergence (3 and 5 leaf), respectively. The end of the CPWC varied from 395 to 289 GDD, at 5% and 10% AYL, which equates to 31 and 24 days after crop emergence (10 and 7 leaf), respectively. Total dry weight of weeds increased as the duration of weed infested period increased and it was decreased with increasing duration of the weed-free period. Grain yield of CORN increased with increasing duration of the weed-free period and decreased as the duration of weed infested period increased.

Mapping quantitative trait loci (QTL) not only provides important useful information on the number and location of loci that controls quantitative trait, but can also be used in marker assisted selection. In order to map QTLs controlling number of row per ear, number of grain per row, grain width, thickness and weight a F2:3 populations was assessed with cross K1264/1 and K47/2-2-1-3-3-1-1-1 inbred lines. The polymorphism of the population was assessed by 60 microsatellite primers and categorized in three affinity groups and covered 618.2 CM of CORN genome. Qualitative trait loci (QTL) analysis based on composite interval mapping for evaluated traits identified 59 QTLs. Five QTLs were identified for number of row per ear which explained 78.49% of the phenotypic variations. 19 QTLs were identified for grain width, that the first QTL with 3.49 CM distance from the phi104127 marker (on chromosome no.3) and ninth QTLs with 2 CM distance from the phi213984 (on chromosome no.4) could be used same as markers. For the number of grain per row, there was an identified one QTL which explained 17.9% of the phenotypic variations. 19 QTLs were identified for grain thickness and 15 QTLs for grain weight which explained 98.4% and 87.17% of the phenotypic variations, respectively. The identified QTLs in weight, width and grain thickness of grain and their repeated number showed the linkage in these traits. QTLs that were found in this study could be used for determining important gene locations in breeding programs.

Drought is one of the major problems affecting crops production, including CORN, in many parts of Iran. In order to detect drought tolerant grain CORN hybrids, an experiment with twenty CORN hybrids was conducted during 2006 in Qom Province, Iran, using a complete randomized block design with four replications, under optimal moisture and drought stress condition. Results showed diversity among hybrids in response to moisture conditions. BC504 and BC652 produced the highest yields and BC678 and NS504 produced the lowest yields under optimal and stress conditions, respectively.Assessing hybrids according to some selection indices lead to introduce BC504, BC652, BC404, KSC302, KSC320 and KSC647 as drought tolerant ones. It seems likely that Stress Tolerance Index, Geometric Mean Productivity, and Harmonic Mean indices, which showed the highest correlation with grain yield under both optimal and stress conditions, can be used as the best indices for maize breeding programs to introduce drought tolerant hybrids.

In order to study effect yield and yield components of com (Zea Mays L.) in competition with Common Cocklebur (Xanthium strumarium), an experiment was conducted in agriculture research station of Islamic Azad University branch of Gonabad in 2005.The experiments were carried out as factorial in a Randomized complete block design with three replications. In the experiments, the factors included com densities at 3 levels (7.5, 8.5 & 9.5 p/m2) and Velvetleaf densities at 5 levels (0, 2, 4, 6 & 8 p/m2). The com variety in experiments was 714. The results indicated that with an increase in velvetleaf density, the ear length, ear diameter, grain number in row, row number, grain number in ear, grain yield and biological yield of com decrease. Also with an increase com density, the grain number in row, row number, grain yield and biological yield of com increase. The economic thresholds density of velvetleaf was 0.1 to 0.15 plants in meter of square in com different densities, and increase with com density increase.

In order to determine best density of silo CORN (zeamays L) for good competition with weeds and attainment of acceptable economical yield, an experiment was conducted during 2009 as second cultivation in research farm of Takestan. The experiment was carried out as RCBD in factorial with four replications. Treatments were included density factor (recommended density, 25 and 50% more than recommended one) and weed management (two time weeding, without weeding, nicosolforon herbicide in 2-4 leaved of CORN, 1 time cultivation in 20 cm height, two times cultivation in 35 cm of CORN). Results showed that CORN density and weed management had no significant effect on plant height, diameter of CORN with hide, the number of row in CORN, the number of seed in row, the number of seed in CORN, the grain yield and the weight of 1000 seeds. CORN recommended density together no weeding caused the most diameter of stem, the number of seed in row, the number of seed in CORN and CORN density 50% more than recommended cause the most biological yield. The density of CORN had a significant effect on density and dry weight of Amaranthus blit oides two months after treatments, while weed management had significantly effect in 1% probability level on density and the weeds dry weight in all sampling steps. CORN density by 50% more than recommended caused mostcontro of density and dry weight of Amaranthus blit oides and most decrease in the density and dry weight of Chenopodium album... The most control of dry weight of Amaranthus blit oides and Chenopodium album and most decrease density in Chenopodium album in all samplings most control of density of Amaranthus blitoides in last sampling in response to the treatments, two times weeding and one and two times cultivation was obtained.

In order to study the weed damage and to determinate the influence of weed species and multiple weed species competition in CORN (Zea mays L.), a field experiment was conducted based on interval mapping at the Agronomy Research Field of Ferdowsi University of Mashhad during 2009-2010. Treatments consisted of four density level of crop (5, 6, 7 and 9 plant.m-2) and four type of weed management (completely removed weed, broadleaf control, non control, and grass weeds control). Weed samples were taken at harvest time. Dry matters of CORN samples were measured. The Density and dry matter of weed samples were recorded separately for each species. Equations were fitted single plant weight ln of each weed species or crop as dependent variable to compare and evaluate the competition coefficient of inter and intra species. Results indicated that single plant weight ln and plant density per m-2 had high correlation to evaluate competition coefficient. Considering the coefficients of equation the function of single plant weight ln showed that crab grass and prostrate pigweed had the most stimulation effect on CORN. While redroot pigweed prostate common purslane field bindweed black night shade common lambsquars barnyardgrass purple grown nutsedge and Green foxtail had inhibition effect on CORN. The positive of the former group on CORN were resulted from the inhabitation effect on the latter group of weeds.

In order to study the different levels of irrigation on diversity, density and compound of weed species in CORN field, experiment was conducted at Agricultural Research Station, Ferdowsi University of Mashhad during growing season of 2008-2009. The experiment was with four levels of irrigation (6130, 7290, 8800 and 12330 m3) and four levels of weed control (complete control, broad-leaves control, grasses control and without control). Weed sampling was done at four stages including first, meddle and end of critical period weed control and harvest then all species counted species separate. The results showed that in different levels of irrigation the weed species were different. At the first time of critical period, the most diversity was in 8800 and 7290 m3 and the minimum diversity was in 12330 and 6130 m3 irrigation levels. At the harvest time, the most diversity was in 12330 and 6130 and 8800 m3 irrigation levels and least diversity within different indexes were in 7290, 8800 and 6130 m3 irrigation levels, respectively. Stability index changed at different levels of irrigation. In addition, maximum when the irrigation level was in minimum level, however, highest level of irrigation caused the maximum unstability of species but levels of irrigation had no significant on dominance index.