Evaluation of Amount and Time of Nitrogen Application, and Herbicide Usage on Spatial Distribution of Prostrate Pigweed (Amaranthus blitoides). Narges Poortoosi 1, Elmira Mohammadvand1, Mohammad Hassan Rashed Mohassel1; 1Ferdowsi University of Mashhad, Mashhad, Khorasan Razavi, Iran To evaluate the effect of different management on composition, density and distribution patterns of prostrate pigweed and redroot pigweed seed banks, an experiment was conducted in a grain corn field at the Agricultural Research Station, Ferdowsi University of Mashhad 2006 growing season.The treatments consisted of application of: 25 kg/ha nitrogen fertilizer at the time of corn planting with 2,4-D+MCPA herbicide (533 g. ai./ha 2.4.D+467 g. ai./ha MCPA), 25 kg/ha nitrogen fertilizer at the time of corn planting without 2,4-D+MCPA, 25 kg/ha nitrogen fertilizer at the time of corn planting and six-leaf stage with 2,4-D+MCPA, 25 kg/ha nitrogen fertilizer at the time of corn planting and six-leaf stage without 2,4-D+MCPA, 120 kg/ha nitrogen fertilizer at the time of corn planting with 2,4-D+MCPA, 120 kg/ha nitrogen fertilizer at the time of corn planting without 2,4- D+MCPA, 120 kg/ha nitrogen fertilizer at the time of corn planting and six-leaf stage with 2,4-D+MCPA and 120 kg/ha nitrogen fertilizer at the time of corn planting and six-leaf stage without 2,4-D+MCPA.The percentage of Amaranthus blitoides seed free spaces was zero throughout the season. Application of herbicide was useful in reducing A. blitoides seed bank. Semivariograms of seeds were fitted with spherical and exponential models.Semivariogram analysis in the levels of treatments indicated a range of influence of 1.17 m to 50.21 m for prostrate pigweed and 1.15 m to 44.98 m for redroot pigweed . The highest spatial correlation for prostrate pigweed was 1.91% and the lowest one was 32.37%. Weed seed bank patches was obvious in two species maps.The nitrogen fertilizer did not have a strong effect on reducing the amount of seed bank but the application of herbicide was a useful factor to reducing weed seed bank.

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