Alfalfa Production with Subsurface Drip Irrigation in the Central Great Plains

Irrigated alfalfa production is gaining interest because of the growing number of dairies in the semi-arid U.S. Central Great Plains and its longstanding superior profitability compared to other alternative crops grown in the region. Irrigation requirements for alfalfa are great because of alfalfas long growing season, so it is important that irrigation be utilized efficiently in this region where water is pumped from the declining Ogallala aquifer. A three-year yield study (2005-2007) was conducted at the Kansas State University Northwest Research-Extension Center in Colby, Kansas, to evaluate production of subsurface drip irrigated alfalfa on a deep productive silt loam soil. Three irrigation levels (randomized complete block design of three replications) designed to replace 70%, 85%, or 100% of the calculated crop evapotranspiration minus precipitation were compared in terms of alfalfa yield, irrigation amount, plant-available soil water, crop water use, and crop water productivity. Alfalfa yield was also evaluated at three perpendicular horizontal distances from the 1.5 m spaced driplines (0, 0.38, and 0.76 m). Annual alfalfa yields were unaffected by irrigation level, averaging 20.8 Mg ha-1 while irrigation amounts averaged 396, 484, and 586 mm for the 70%, 80%, and 100% ET irrigation levels, respectively. Seasonal crop water use was significantly greater with increased irrigation, but only by an average difference of 137 mm among irrigation levels as compared to the 190 mm difference in irrigation amount. Crop water productivity was significantly greater with decreased levels of irrigation, with values of 23.5, 22.0, and 20.4 kg ha-1 mm-1 for the 70%, 85%, and 100% ET irrigation levels, respectively. No significant differences existed in alfalfa yield with respect to horizontal perpendicular distance from the dripline with the exception of the 0.76 m distance, which yielded less in the drier and warmer year 2006. Large and significant decreases in alfalfa yield and crop water productivity occurred with successive harvests, with the first harvest having 272% and 196% of the fifth harvest yield and crop water productivity, respectively. Plant-available soil water decreased as the season progressed for all irrigation levels, but to a much greater extent for the 70% ET level, averaging at the time of the fifth seasonal harvest only 41% and 54% of the soil water for the 85% and 100% ET irrigation levels, respectively. These large seasonal decreases in soil water for the 70% ET irrigation level would be anticipated to be of even greater concern in extended multiple-year drought periods. This fact coupled with the 102 mm reduction in seasonal irrigation amount for the 85% ET irrigation level compared to the 100% ET irrigation level leads to a recommendation of scheduling replacement of 85% of the ET minus precipitation when subsurface drip irrigating alfalfa in the Central Great Plains. This study was conducted in a region with deep silt loam soils where approximately 60% of the average annual precipitation occurs during the summer months. Additionally, 125 mm of fall irrigation was applied during the dormant fall season to reduce root intrusion and rodent damage. These combined facts need to be considered when applying the results of this study to other regions and when using different irrigation management strategies.