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Title: Green manures in continuous wheat systems affect grain yield and nitrogen content

item Northup, Brian
item Rao, Srinivas

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/26/2015
Publication Date: 6/1/2015
Citation: Northup, B.K., Rao, S.C. 2015. Green manures in continuous wheat systems affect grain yield and nitrogen content. Agronomy Journal. 107(5):1666-1672.

Interpretive Summary: Double-cropping summer legumes after winter cereals are common in parts of the US. However, this technique is not used in the southern Great Plains (SGP) due to a shortage of available soil moisture. The fallow period (June to August) of continuous wheat – when the legumes would be grown - is used to store moisture for the next wheat crop. Growing legumes during this period can reduce the production of wheat. However, the cost of inorganic nitrogen (N) fertilizers has increased dramatically, causing farmers to reconsider growing legumes during summer fallow to supply ‘Green N’. We examined the potential for using the tropical legume lablab (cv. Rio Verde) as a pre-plant N source for conventionally tilled and no-till wheat. We planted lablab and soybeans (a legume commonly planted in southern Plains) in a series of plots that were managed by either till or no-till methods. The legumes were allowed to grow until early-September. The legumes were then shredded and incorporated in tilled plots, or left on the soil surface in no-till plots. Wheat was sown into all plots and allowed to grow until grain fill. Grain yields and N contents in grain in response to the legumes were compared to grain produced under 3 rates of inorganic N (none, 35 and 70 lbs/acre). We found some single year improvements in yields in response to lablab, but no identifiable trends over the entire study. The legumes also reduced N contents of wheat grain compared to no fertilizer. Both yield and N content of wheat grain in response to the legumes was lower during the drier years. Grain yield of wheat during the study was related more to precipitation received than the effects of legume N. Results indicate that using legumes as green sources of N in continuous wheat systems may not be effective long-term tools for the SGP.

Technical Abstract: Continuous winter wheat (Triticum aestivum L. em Thell.) is the foundation for most U.S. southern Great Plains (SGP) agriculture. Inorganic nitrogen (N) fertilizers are important to wheat production, but increasing N prices have caused farmers to reconsider growing legumes during summer fallow for ‘Green N’. This study was conducted during 2008-2012 to determine the potential for using lablab [Lablab purpureus (L.) Sweet cv. Rio Verde] to support wheat under conventional and no-till management, compared to soybean [Glycine max (L.) Merr. cv. Laredo], and 3 inorganic fertilizer treatments (none, 40 and 80 kg ha-1). Legume seeds were inoculated and sown after wheat harvest each year, grown from June-August, and terminated in early-September. Wheat was then sown with or without pre-plant tillage and grown to maturity. Grain yield, N concentrations, and N accumulation in grain were determined, and analyzed to define N treatment, tillage, and year effects. The amount and distribution of precipitation during 2008-2012 varied from 53-92% and 63-160% of the long-term averages for wheat (688 mm) and legume (162 mm) phases. Tillage effects were not significant, but N treatment x year interactions were significant for grain yield, N concentration, and N accumulated in grain (P<0.01). Lablab green manures resulted in some single year increases in grain yield, but the overall grain response was inconsistent. The legumes reduced N levels in wheat grain compared to the control. Based on these results, we conclude that neither legume was an effective N source for continuous wheat production in the SGP.