Grain sorghum production functions under different irrigation capacities

Water is the major factor limiting crop production in semi-arid regions of the southern and central US High Plains. The Ogallala Aquifer is the major source of irrigation water in the region. The water levels in the aquifer have been declining due to water withdrawals for irrigation exceeding mean annual recharge. As a result, some of the wells in the region are not able to meet the crop water requirements due to lower irrigation capacity (IC). Grain sorghum, one of the major crops grown in western Kansas is considered drought tolerant. There is limited literature on the effect of IC on grain sorghum yield under erratic rainfall patterns of the semi-arid High Plains. Study objectives were to: (1) evaluate the Decision Support System for Agrotechnology Transfer Cropping System Model (DSSAT-CSM) for assessing grain sorghum yield and water productivity; (2) develop grain sorghum water production functions under five different IC, two cultivars and three irrigation scheduling strategies; and (3) evaluate response of grain sorghum under various IC in western Kansas based on long term historical weather; 4) to determine the minimum irrigation capacity required for irrigating grain sorghum in western Kansas. A calibrated and validated late maturing cultivar in prior study was used, while measured yield and phenology data of an early maturing cultivar was used to evaluate an existing cultivar in DSSAT model. Results showed that the cultivar in DSSAT were representative of the cultivars in the field with Normalized Root Mean Square of Error (NW:SE) 18%, index of agreement (I) of 0.97 and percent of deviation (d) - 19 to 29%. The corresponding statistical goodness of fit values for days to flowering was 12%, 0.99 and 0-17%, respectively. Overall, the simulated yield and phenology values agreed with the measured indicating the satisfactory simulation performance of the model. Highest yield, crop water productivity and irrigation water use efficiencies were obtained from IC of 2.5-3.6 mm/day under the irrigation scheduling strategies from panicle initiation to grain filling. However, long term simulation showed that the minimum IC for growing grain sorghum varied depending on the minimum yield goal to be met based on long term perspective. In some wet years, lower IC up to 1.7 mm/day or even in some of the wettest years no irrigation was suitable. Our findings indicate that it might not be necessary to irrigate grain sorghum from planting to maturity but if producers are forced to do so by climatic conditions, an IC of 1.7-2.5 mm/day was found to produce a yield of at least 5 t/ha similar to that of 5 mm/day IC in 75% of the long-term growing seasons, If producers have to irrigate grain sorghum from panicle initiation to maturity, the 2.5 mm/day IC was found to be adequate to meet a minimum yield goal of 3 t/ha in 3 out of 4 years based on soils and long-term average climate around western Kansas. IC of 3.6 mm/day was found to be adequate if producers wish to irrigate only from panicle initiation to grain filling to meet a minimum yield goal of 6 t/ha in 75-90% of the long-term seasons.