Grass pea as a nitrogen source for continuous no-till winter wheat

Authors: Rao, Srinivas; Northup, Brian

Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/25/2011
Publication Date: 6/1/2011
Citation: Rao, S.C., Northup, B.K. 2011. Grass pea as a nitrogen source for continuous no-till winter wheat. Crop Science. 51:1824-1831.

Interpretive Summary: Double cropping summer legumes after wheat harvest are common in parts of the US. However, double cropping is not common in the southern Great Plains (SGP), as the fallow period (June to August) is used to store moisture for the next wheat crop. Growing legumes during this period can reduce forage production during fall and wheat grain production in June. However, costs of inorganic nitrogen (N) fertilizers have increased in recent years and show no sign of dropping. In response, a search has begun for cheaper sources of N for continuous wheat, such as annual legumes, and ways to use them. 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.

Technical Abstract: Sources of organic nitrogen (N) for crop production in the southern Great Plains (SGP) - and methods of their use - need evaluation to find ways to counter the rising cost of N fertilizer. This study examined N availability from the cool-season pulse grass pea (GP) (Lathyrus sativus.L.) for continuous winter wheat (Triticum aestivum L.) under no-till. The study was conducted in Oklahoma (35º40’ N. 98º00’ W) during 2004 to 2008, on 3 replicate blocks of 4 experimental plots (6 by 20 m). Inoculated grass pea (cv. AC-greenfix) seeds were sown during late summer fallow (mid-August) in one randomly chosen plot per block (75 kg/ha in 60 cm rows; 75% germination). Three additional plots per block mimicked traditional summer fallow with no-, (control), 40, or 80 kg N ha-1 to support wheat production. All treatments were repeated on the same plots throughout the study. Aboveground (AG) biomass, amounts of N, and digestibility of grass pea were determined 50 days (~20 October) after planting. Grass pea plots were then shredded, the residues treated with herbicide, fertilizer treatments were applied, and wheat (cv. Jagger) sown (100 kg ha-1; 20 cm rows). Aboveground (AG) wheat biomass was collected at 3 growth stages (elongation, flowering, grain harvest) and analyzed for N content. Grass pea as a pre-plant N source for continuous no-till wheat was not effective. Aboveground biomass of GP contained enough N to meet the needs of wheat at planting (40 kg N ha-1) in only one production cycle. Total AG biomass and N accumulated in wheat under the GP treatments was intermediate between 0 and 40 kg N ha-1 application, as was yield and N content of wheat grain.