Greening Summerfallow: On-Farm Evaluation of Legume Green Fallow Rotations.

Growing legumes as “green fallow” forages or green manures to replace summerfallow may be a sustainable way to intensify, diversify, and economize cropping systems in regions still using conventional wheat-fallow rotations in the northern Great Plains. Viability hinges on proper soil water management and the realization of biologically fixed-N benefits. To date, plot-scale research has shown that no-till management and first-flower stage termination of green fallow crops increases the agronomic viability of these practices by conserving critical soil water, and that legume N-fixation increases soil N fertility. Nonetheless, regional green fallow adoption has been negligible. Participatory on-farm assessments of legume green fallow vs. summerfallow – wheat rotations may help to better inform regionally appropriate adoption strategies. Soil water, nitrate levels (0-0.9 m), wheat yields, and producer adoption challenges are currently being assessed at six farmer-managed, field-scale sites in the wheat – fallow dominated region of north-central Montana. Results to date have shown that soil water conservation seems to be consistent with plot-scale results. Mean total soil water values under green fallow crops at termination were 82% of summerfallow values, with 94% of this soil water deficit concentrated in the 0-0.6 m depth. At one site, soil water equaled summerfallow values by September 2009, while at two other droughty sites, soil water recovered to 88% of fallow values by April 2010. Based on past research results, small soil water deficits from green fallow crops are unlikely to significantly affect wheat yields negatively. With equal soil water, N benefits from green fallow crops (analysis in process) should benefit subsequent wheat yields, and illustrate potential viability of green fallow rotations and legumes as an N source. Viable green fallow management across site and management variability will help illustrate adoption potentials, although qualitative assessments of adoption challenges suggest that seed cost may hinder adoption.