Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 3, 2011
Publication Date: April 1, 2011
Citation: Rao, S.C., Northup, B.K. 2011. Grass pea (Lathyrus Sativus L.) as a pre-plant N source for continuous conventionally tilled winter wheat. Crop Science. 51:1-9. Interpretive Summary: Double-cropping summer legumes after wheat harvest are common in parts of the US, but are not common 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 (none, 40, and 80 lb N/acre). Both forage and grain production by wheat, and amounts of N in wheat biomass was determined. Grass pea was not effective as a pre-plant source of N for following wheat crops. 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: Sources of organic nitrogen (N) for the southern Great Plains (SGP) - and methods of their use – need testing to find ways to counter the rising cost of N fertilizer. This study examined the function of the cool-season pulse grass pea (GP) as a pre-plant N source for continuous, conventionally tilled winter wheat (Triticum aestivum). It was conducted in Oklahoma (35º40’ N. 98º00’ W) during 2004 to 2008, on 3 replicate blocks of 4 experimental plots (6 by 10 m). Inoculated grass pea (cv. AC-Greenfix) seed was 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 inorganic N ha-1. All treatments were repeated on the same plots throughout the study. Harvest of grass pea was undertaken at flowering over a range of 41-59 d in different years, and was analyzed for N, and digestibility. The above ground (AG) biomass was shredded with flail mower and incorporated before applying fertilizer treatments, and sowing wheat (cv. Jagger) in 20 cm rows at a rate of 100 kg ha-1. Aboveground wheat biomass was collected at 3 growth stages (elongation, flowering, grain harvest) and analyzed for N. Biomass of GP contained enough N to meet the needs of wheat at planting (40 kg N ha-1) in only 2 years. Total AG biomass and N content of wheat under GP was similar to the control at elongation, and intermediate between no and 40 kg N ha-1 at grain fill in 2 years. Grain yield and N content of wheat under GP were intermediate between no and 40 kg N ha-1 in the first 2 years, and similar to the control thereafter. Grass pea as pre-plant N for wheat was only marginally effective for a limited period, and should therefore be considered a short-term tactical tool in the SGP.