Water use in a modified summer fallow system on semiarid northern Great Plains

Wheat (Triticum aestivum L.) is the major crop on semiarid northern Great Plains of the USA. Attempts to introduce alternate crops have had limited success. Alternate fallow-spring wheat rotation is the most common cultural practice. Our objective was to investigate water use and water use efficiency and suitability of alternative crops in semiarid northern Great Plains agricultural environment. The study was on glacial till Williams loam (fine-loamy mixed, Typic Argiboroll) 11 km north of Culbertson, MT. Plots, replicated four times in randomized blocks, were 12 m × 15 m. Rotations were: (1) fallow, sunflower (Helianthus annuus L.), barley (Hordeum vulgare L), winter wheat; (2) fallow, safflower (Carthamus tinctorious L.), barley, winter wheat; (3) fallow, buckwheat (Fagopyrum esculentum Moench.), annual legume/grain forage crop, spring wheat; (4) fallow, buckwheat, annual legume/grain forage crop, winter wheat; (5) fallow, spring wheat; (6) continuous spring wheat. Soil water to 1.8 m depth was determined near time of seeding and of harvest by neutron attenuation. The soil reached an upper drained limit of 0.20–0.25 m3 m−3 water in a 1.8 m profile, equating to no more than 450 m water. Safflower and sunflower used ca. 500 mm water, more water than any of the other crops used. The greatest growing season water use efficiency was captured by the annual forage crop. Except following safflower and sunflower, soil water every spring was near the upper drained limit. Deep rooted crops can have a place in rotations on the semiarid northern Great Plains. But one must be prepared for variable yields and potential reduced yields following deep rooted crops, and for an occasional crop failure. Crop and soil management for alternative crops differ from that of small grain management, requiring some adaptation of management practices.