Cotton-Grain Sorghum Rotation under Extreme Deficit Irrigation Conditions

There are over 1.0 million ha of cultivated land in the Southern High Plains of Texas, with about 40% in dryland production. Strategies that couple dryland production methods with very low levels of supplemental irrigation in contrast to current irrigated practices could stabilize crop output for an extended period at the expense of reduced agricultural productivity. An eight-year field experiment was conducted from 2001 to 2008 to develop cropping data for economic analysis. The principle goal was to compare a cotton (Gossypium hirsute L.) - sorghum [Sorghum bicolor (L.) Moench] (2:1) rotation system to a continuous cotton system, both efficiently irrigated at levels well below the evapotranspiration rate of these crops. Seasonal irrigation capacities were approximately 0%, 20%, and 40% of peak evapotranspiration rates. The rotation strategy from 2003 to 2005 allowed the movement of limited irrigation between crops based on relative water needs at critical growth periods, and from 2006 to 2008, based on water needs of the crop with the higher economic value. The results showed that in years of below average rain, cotton following sorghum resulted in 18% to 44% higher lint yields compared to continuous cotton. In years of average rain, cotton lint yields following grain sorghum were generally higher than those of continuous cotton, averaging 21%, with a portion of the increase due to higher irrigation. And, in years of above average rain, significant yield differences due to rotation were minimal, particularly at the highest irrigation capacity. Directing available irrigation to the crop of higher value (cotton) at the expense of the lower value crop (sorghum) appeared to be a major factor in elevating seasonal irrigation water use efficiency (0.448 to 0.513 kg lint m-3) compared to irrigating the most water stressed crop (0.200 to 0.35 kg lint m-3). Although cotton lint yield tended to be higher in the rotation than the continuous cotton treatments, the rotation resulted in significantly lower gross irrigation value than the continuous cotton cropping system in most years.