Transactions of the ASABE Vol. 55(4): 1203-1212 © 2012 American Society of Agricultural and Biological Engineers ISSN 2151-0032 1203 ALFALFA PRODUCTION WITH SUBSURFACE DRIP IRRIGATION IN THE CENTRAL GREAT PLAINS F. R. Lamm, K. R. Harmoney, A. A. Aboukheira, S. K. Johnson ABSTRACT. 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 alfalfa’s 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 (ran- domized complete block design of three replications) designed to replace 70%, 85%, or 100% of the calculated crop evap- otranspiration 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 hori- zontal 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 succes- sive harvests, with the first harvest having 272% and 196% of the fifth harvest yield and crop water productivity, respec- tively. 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 irrigat- ing alfalfa in the Central Great Plains. This study was conducted in a region with deep silt loam soils where approximate- ly 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 con- sidered when applying the results of this study to other regions and when using different irrigation management strategies. Keywords. Forage, Irrigation management, Microirrigation, Subsurface drip irrigation, Water productivity. lfalfa (Medicago sativa L.), a forage crop, has relatively large crop water needs, and thus high- ly efficient irrigation systems, such as subsur- face drip irrigation (SDI), might substantially reduce total irrigation water demand. In some regions, the water allocation for irrigation is limited by hydrogeological or institutional constraints, so SDI can effectively increase alfalfa production by increasing the crop transpiration while reducing or eliminating irrigation runoff, deep perco- lation, and soil water evaporation. Annual alfalfa dry matter yields are typically linearly related to crop evapotranspira- tion (ET), and within-season yields often decrease with successive harvests as the crop experiences more stressful drier and hotter climatic conditions (Bauder et al., 1978; Metochis, 1980; Grimes et al., 1992; Saeed and El-Nadi 1997). Since alfalfa is such a large water user and has a very long growing season, irrigation labor requirements can be reduced with SDI relative to less efficient alternative ir- Submitted for review in December 2011 as manuscript number SW 9544; approved for publication by the Soil & Water Division of ASABE in May 2012. Contribution No. 12-221-J from the Kansas Agricultural Experiment Station, Kansas State University, Manhattan, Kansas. Mention of trade names is for informational purposes and does not constitute endorsement of the product by the authors or Kansas State University. The authors are Freddie Ray Lamm, ASABE Member, Professor and Research Irrigation Engineer, Kansas State University Northwest Research- Extension Center, Colby, Kansas; Keith R. Harmoney, Professor and Range and Forage Scientist, Kansas State University Agricultural Research Center, Hays, Kansas; Abdrabbo A. Aboukheira, Research Irrigation Engineer and Visiting Scientist, Kansas State University Northwest Research-Extension Center, Colby, Kansas; and Sandy K. Johnson, Associate Professor and Extension Livestock Specialist, Kansas State University Northwest Area Office, Colby, Kansas. Corresponding author: Freddie Ray Lamm, Kansas State University Northwest Research-Extension Center, P.O. Box 505, Colby, KS 67701; phone 785-462-6281; e-mail: flamm@ksu.edu. A