Submitted to: Natural Resources Research Update (NRRU)
Publication Type: Research Technical Update
Publication Acceptance Date: 10/23/2011
Publication Date: N/A
Interpretive Summary: Double-cropping summer legumes after wheat harvest is common in parts of the US, but not in the southern Great Plains (SGP). The fallow period (June to August) of continuous wheat production in the SGP is used to store moisture for the next wheat crop. Growing legumes during this period can reduce forage production during fall and grain production in June. However, costs of inorganic nitrogen (N) fertilizer have increased in recent years and show no sign of dropping. In response, a search has begun for cheaper sources of N fertilizer for continuous wheat, such as annual legumes, and ways to use them. We examined how grass pea (GP), a cool-season pulse crop, might function as a pre-plant N source for continuous, conventionally tilled winter wheat. We planted grass pea (65 lb/acre, 24 inch row spacing) into 3 experimental plots during mid-August of 2005 through 2008 and allowed growth until mid-October, when the plots were shredded and grass pea tissues tilled into the soil. After tillage, winter wheat was planted into these and 9 additional tilled plots that received one of 3 levels of fertilizer (40, and 80 lb N/acre). Forage and grain production by wheat, and amounts of N in wheat biomass were determined. Grass pea accumulated enough N to meet the needs of wheat at planting (40 lb N/acre) in only 2 years. Total biomass and N content of wheat under GP was similar to applying no fertilizer at elongation, and intermediate between no and 40 lb N/acre at grain fill in 2 years. Grain yield and N content of wheat under GP did not exceed 40 lb N/acre in the first two years, and was similar to no fertilization thereafter. Therefore, GP planted during late summer was only marginally effective as a pre-plant N source for wheat for a short period.
Technical Abstract: We examined how grass pea (GP), a cool-season pulse, might function as a pre-plant N source for continuous, no-till winter wheat. We planted grass pea into 3 experimental plots during mid-August 2005 through 2008 and allowed it to grow until mid-October when the plots were shredded, sprayed with herbicide to prevent grass pea re-growth, and planted to wheat. Nine additional wheat plots that received one of 3 fertilizer levels were included in the study. Both forage and grain production by wheat, and amounts of N in wheat were determined. Results indicated that grass pea was not effective as a pre-plant source of N for wheat in a continuous no-till system. It contained enough N to meet the needs of wheat at planting in only 1 year. Forage production and N accumulated in wheat under the GP treatments was similar to no fertilizer, while yield and N content of wheat grain was between no and 40 lb N/acre. Grass pea was therefore only marginally effective as a pre-plant source of N for wheat under continuous no-till. Results also point to areas for future research to more fully describe GP as an organic fertilizer, including N distribution in different tissues, and N turnover rates.