GREEN FALLOW FOR THE CENTRAL GREAT-PLAINS

Use of fallow to store soil water is a common practice in semiarid reg ions. In the central Great Plains, the most common dryland cropping sy stem is winter wheat (Triticum aestivum L.)-fallow. Stubble mulching i nvolving-tillage is the predominant weed-control practice during the 1 4-mo fallow period. As a result of tillage, soil organic matter conten t has declined 40 to 70% since the early 1900s. This decline has calle d for development of cropping practices that control soil erosion and increase soil organic matter. Green fallow is the practice of growing a legume during the time period not devoted to crop production. Water is a major limiting factor for crop production in the central Plains, and water use by the legume could reduce grain yields. Field studies w ere conducted near Tribune, KS, from 1990 to 1994 to evaluate green fa llow in the central Great Plains. The objectives were to (i) evaluate the production potential of several dryland forage legumes, (ii) quant ify the water use of dryland legumes as a function of growth period, a nd (iii) measure the effects of legume growth on grain yield of subseq uent crops. Of 11 legume species evaluated, hairy vetch (Vicia villosa Roth) and yellow sweetclover (Melilotus officinalis Lam.) were the mo st promising in terms of biomass production and weed control. Hairy ve tch was planted in a green fallow system and allowed to grow for selec ted periods of time. In all cases, green fallow depleted soil water an d reduced grain yield of subsequent crops. Allowing hairy vetch growth throughout the fallow period reduced soil water by up to 178 mm and r educed grain yield by 42 to 83%. For every millimeter of soil,vater de pletion by vetch, grain fields decreased by 15 kg ha(-1). Although gre en fallow is too detrimental to subsequent crop yields to be recommend ed in the central Great Plains, dryland legumes may have potential as forage crops.