effects of deficit irrigation on quantity and quality of soybean crop yield in kermanshah region

introduction: expansion of the area of oilseed crops such as soybean is one of the policies of iranian agricultural policy makers as iran is one of the major oilseed importers in the world. however, the area of this crop in kermanshah province is negligible, but it could be cultivated in most parts of this province. the quantity and quality of the produced grain could be affected by environmental factors such as weather parameters and water availability. the aim of the current study was to investigate the effects of levels of deficit irrigation on the quantity and quality of soybean crop yield in kermanshah, iran. materials and methods: for this purpose, a field study was conducted as randomized complete block design with four replications and four irrigation treatments at the research farm of razi university, kermanshah in 2012. the size of each plot was 4 * 4 m. irrigation treatments consisted of four irrigation levels: 20% over irrigation (t4), full irrigation (t3 as control), 20% less irrigation (t2) and 40% less irrigation (t1). the reason to choose t4 treatment was the lack of confidence in estimated crop evapotranspiration as there was no local calibration of crop coefficient (kc) for this crop. the required water for t3 treatment was calculated based on daily weather data using fao-penman-montith equation. daily weather data was recorded in a weather station which was located in the research farm and is available in the www.fieldclimate.com. as there was no rainfall during the crop season, all of the required water was supplied through irrigation. the required water for treatments of t1, t2 and t4 was considered as 60%, 80% and 120% of t3 treatment. the required water was applied using a hose connected to a volumetric flow meter with a liter precision. total amount of applied water during the crop season was 4399, 5865, 7331 and 8797 m3.ha-1 in the treatments. fertilizers were applied based on the recommendations of soil fertility experts. weeds were controlled manually. finally, the area of two square meters in the middle of each plot was harvested in order to determine crop yield in terms of grain, biomass, stem, pod, seed protein content and fat percentage and also water productivity index. dry weights of the samples were measured after drying samples in the oven for48hours at 70° c. the percentage of fat and protein in the grains are also measured in the laboratory. water productivity index was calculated for each treatment by dividing crop yield (in terms of grain, biomass, protein and fat) over seasonal water use. statistical analysis of the results is also done using mstatc software. results and discussion: the highest and lowest crop yields were measured respectively in the treatments t4 and t1.the mean value of grain yield was 1084, 1367, 1716 and 1940 kg.ha-1,respectively in the treatments t1, t2, t3 and t4. these results showed a 36% decrease in the grain yield by decreasing 40% in the amount of supplied water. however, biological yield was decreasedby the level of irrigation, but the rate of reduction was lower than that of grain yield. by reducing irrigation application, thepercentage of grain protein content increased while the percentage of fat in the grain decreased. considering simultaneous reduction in grain yield and fat content in the grain, severe reductions in fat yield (oil content) were observed under water stress conditions. crop yield in terms of fat was reduced by 26.2 and 50.1 %, respectively in treatments t2 and t1 in comparison with t3 (control treatment). the maximum and minimum percentages of protein in the treatments were 31% and 27%, respectively in the treatments t1 and t4. maximum water productivity in terms of grain, biomass and protein was achieved in t1 treatment respectively with the amounts of 0.24, 0.81 and 0.077 kg.m-3. maximum and minimum fat percentage was 0.052 and 0.040 kg.m-3, respectively in the t4 and t1 treatments. in addition,the results indicated that water productivity index in terms of grain, biomass and protein increased while they decreased in terms of fat yield.the results of statistical analysis indicated that water productivity index in all terms except protein had significant differences (at 5%) with t3 treatment. conclusion: crop yield and water productivity (except in terms of fat) was increased by increasing applied water. considering all indices of treatment t2 (20% deficit irrigation), itwas suggested as the best treatment.