Managing an annual legume green manure crop for fallow replacement in southwestern Saskatchewan
Some scientists have suggested that in the Brown soil zone an annual legume green manure crop (GM) could be used as a partial-fallow replacement to protect the soil against erosion and increase its N fertility, particularly when combined with a snow trapping technique to replenish soil water used by the legume. We assessed this possibility by comparing yields, N economy, water use efficiency, and economic returns of hard red spring wheat (W) grown in rotation with Indianhead black lentil (i.e., GM-W-W) vs. that obtained in a F-W-W system. Further, we assessed whether a change in management of the GM crop (i.e., moving to earlier seeding and earlier turn-down) was advantageous to the overall performance of this practice. The study was conducted over 12 years (1988-99) on a loam soil at Swift Current, SK. (wheat stubble was left tall to trap snow, tillage was kept to a minimum, and the wheat was fertilized based on soil tests). When examined after 6 years, the results suggested that by waiting for full bloom of the legume (usually late July or early August) to maximize N2 fixation, soil water was being depleted to the detriment of yields of the following wheat crop. However, the change in management of the GM crop since 1994 has resulted in wheat yields following GM equalling those after fallow. It also produced a significant increase (after one rotation cycle) in grain protein and N yields of aboveground parts of wheat in the GM-W-W compared to the F-W-W system, and lead to a gradual decrease in fertilizer N requirements of wheat in the GM system in the last 6 years. These savings in N fertilizer, together with savings in tillage and herbicide costs for weed control on partial-fallow vs conventional-fallow areas, and higher revenues from the enhanced grain protein, more than offset the added costs for seed and management of the GM crop. Thus, our results imply that, with proper management and given sufficient time, an annual legume GMcereal rotation is a viable option for area producers.
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