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The effect of irrigation regimes and N rates on corn (Zea mays L.) yield and redroot pigweed (Amaranthus retroflexus L.) growth in sandy soils is not completely understood. Therefore, field experiments were conducted in a semiarid region of Iran in 2010 and 2011 to determine the influence of N rates and irrigation regimes on corn and redroot pigweed leaf area index (LAI), dry matter and grain yield, N uptake, irrigation water productivity (IWP) and N leaching loss. The experiment was conducted using a randomized complete-block design with a split-factorial arrangement of treatments in four replicates. The main plots were subjected to either a low (L) or a full (F) irrigation regime. The subplots were arranged in a factorial scheme with four N rates (0, 150, 300, and 450 kg N ha-1) and two levels of redroot pigweed interference with corn (weed free, CW0; and weedy, CW1). The results indicated that the more N was applied, the higher were corn grain yield losses in both irrigation regimes due to weed interference. The difference in corn grain yield between CW0F and CW1F treatments was 400 and 1800 kg ha-1 at N0 and N450 levels, respectively. In contrast, the difference between CW0L and CW1L reached 300 and 800 kg grain ha-1 at N0 and N450 levels, respectively. Based on the information gained in this study, we do not recommend increasing inputs (water and N), especially in sandy soils, to diminish the redroot pigweed/corn competition. [ABSTRACT FROM AUTHOR]

Subsurface irrigation has been the focus of attention mainly because of its low evaporation rate and deep percolation losses. Uniformity of water applications and its stability, however, are still a matter of controversy and deserve more investigation. An experiment was conducted at the research station of Tabriz University to evaluate discharge variation among emitters and uniformity after 3 years of operation at different pressures, as well as to determine the optimum length for irrigation tapes. Four hydraulic pressures, 50, 90, 150, and 200 kPa, 4 irrigation tapes 34 m long (used tape) installed at a depth of 10 cm, and 5 new irrigation tapes (unused) 34, 50, 80, 100, and 120 m long comprised the experimental treatments. The used tapes were chosen from a subsurface irrigation system that operated for 3 years for onion irrigation. The new tapes (unused) were the same as the used ones and were laid on the soil surface during the experiment. Emitter discharge and pressure were measured every 2 m along both the used and unused tapes at the above mentioned operating pressures, and data were analyzed to compute several uniformity criteria using traditional and ASAE EP458 methods. In both used and unused tapes, emitter performance (Vpf) at 150 kPa was better than at 50, 90, and 200 kPa. The maximum uniformity coefficient (UC) values of the unused and used tapes 34 m long were 96.9% and 91.8%, respectively. Considering the variation in UC with tape length and an engineering approach toward the performance of the system for used tape, the 80-m length was determined as a suitable length for irrigation tape. The result indicated that both traditional and ASAE methods are suitable for the evaluation of subsurface drip irrigation systems. The ASAE method, however, showed slightly lower uniformity in both unused and used tapes (1.6% and 3.65%, respectively). [ABSTRACT FROM AUTHOR]

We examined the main and interactive effects of nitrogen (N) and deficit irrigation (DI) on the yield response factor (Ky), water use efficiency (WUE), and irrigation water use efficiency (IWUE) of silage maize from a semiarid region of Iran. Experiments were conducted in 2003 and 2004 that included three N fertigation rates (0, 150, and 200 kg N ha-1 N0, N150, and N200, respectively) and four irrigation levels (0.7, 0.85, 1.0, and 1.13 of soil water depletion, W1, W2, W3, and W4, respectively). The soil water content measurements showed that most of the water was extracted from the top 60 cm of the soil profile. DI increased WUE for all N fertilizer treatments with the maximum value being observed at the W2 level. The average of the IWUE for the two years of the study showed that the lowest IWUE was 1.38 kg m-3 for the N0W1 treatment, while the highest IWUE was 1.8 kg m-3 for the N200W3 treatment. A linear relationship was observed between evapotranspiration and the total biomass for all N fertilizer levels in 2003 and 2004. The minimum Ky to water was obtained from the N0 level as 0.64 in 2003 whereas the maximum Ky was recorded from the N200 level as 0.95 in 2004. This reveals that higher N rates application would enhance corn yield sensitivity to water stress. Overall, the sensitivity of the silage maize to water stress was affected by different planting date and nitrogen fertilizer levels. We also discuss emergent trends in water and nutrient management in light of the increased need for food security in the face of changing climate and growing populations. [ABSTRACT FROM AUTHOR]