Summer legume 'green' nitrogen crops affect winter wheat forage in continuous rotations

Authors: Northup, Brian; RAO, SRINIVAS - Retired ARS Employee

Submitted to: Crops and Soils
Publication Type: Trade Journal
Publication Acceptance Date: 1/12/2016
Publication Date: 2/1/2016
Citation: Northup, B.K., Rao, S.C. 2016. Summer legume 'green' nitrogen crops affect winter wheat forage in continuous rotations. Crops and Soils Magazine. doi:10.2134/cs2016-49-1-10edv.

Interpretive Summary: Continuous winter wheat is important to agriculture in the U.S. southern Great Plains (SGP), and inorganic nitrogen (N) fertilizers play a major role in its production. Inorganic fertilizers allow producers to eliminate the time period required to capture N with green manures and plant wheat annually. However, the costs of inorganic fertilizers have increased, and they are one of the largest costs for wheat production. These costs have resulted in producer interest in growing annual legumes during summer fallow (June to September) to provide green N for following wheat crops. Lablab is a drought and heat-tolerant legume from the tropics and sub-tropics that could serve as a summer-grown green manure in the SGP. We undertook a study during 2008-2012 to describe whether the ‘Rio Verde’ cultivar of lablab that was developed in Texas could serve as a green manure to support forage production by continuous wheat systems in central Oklahoma. We compared the responses of wheat to lablab green manures to responses caused by the ‘Laredo’ forage cultivar of soybean and 3 different levels of inorganic N, under conventional and no-till systems of management. Precipitation during the study was 53-92% and 63-160% of the long-term averages for the wheat (27 inches) and legume (6.4 inches) growing seasons. Our results showed that neither legume was effective as N sources for forage production by wheat. The amounts of forage produced, and N in wheat forage, in response to the different N treatments varied among years. The responses of wheat to both lablab and soybean treatments did not exceed the responses that were recorded for no applied fertilizer. Therefore, producers in the SGP should be cautious about using these legumes as ‘green’ N sources to support continuous systems of forage production by winter wheat.

Technical Abstract: Costs for inorganic nitrogen (N) fertilizers in the southern Great Plains (SGP) have increased in recent years with the rise in oil prices. In response, producers have become interested in the potential merits of using annual legumes as N sources. This study described the influence of two summer forage legumes, used as ‘green’ N sources, on forage production by following crops of winter wheat. We tested ‘Laredo’ forage soybean and ‘Rio Verde’ lablab as summer-grown green manures in a small plot study on clay loam soils of bottomland landscapes in central Oklahoma during 2008 to 2012. The area of the plots was planted to unfertilized and conventionally tilled cereal cover crops for two years prior to the experiment. At the start of the study, 15-20 x 33 ft. plots were assigned to each of two tillage systems; no-till (glyphosate herbicide only) and conventional tillage (disking and roto-tilling). The ‘green’ N crops were planted after grain harvest of wheat (early-June) and allowed to grow through late-August. They were then shredded and tillage systems were applied; till to incorporate legume biomass, or leave legume biomass on soil surface and spray with glyphosate. Winter wheat was planted annually in early-September by assigned tillage systems. Nine additional tilled and no-till plots received one of a series of different levels of dry urea fertilizer (none, 40, and 80 lb. N/acre). Wheat was grown to maturity (early-June) and sampled at four times during growing seasons to describe harvestable forage available during December through May, and its quality. Results showed no statistical difference between effects of tillage system on production or crude protein (CP) content of wheat forage. The largest yield during growing seasons occurred at flowering (4,310 lb./acre). Amounts at elongation and grain fill stages were lower [3,475(±35) lb./acre]. The lowest forage yield occurred at the vegetative stage of development (2,295 lb./acre). A significant interaction occurred between N treatments and growing season in forage yield and CP content of winter wheat. Forage produced in response to control and inorganic treatments varied among years. The 80 lb. N/acre treatment produced the largest amount of forage, followed by the 40 lb. N/acre treatment and the unfertilized control. Wheat forage produced by legume treatments did not exceed levels produced by no fertilization, and declined with length of study. Crude protein in wheat forage increased incrementally with amount of applied inorganic fertilizer. Alternatively, CP content of wheat forage produced in response to legume treatments did not exceed levels produced by no fertilization during the first 3 growing seasons. All treatments showed significant declines in CP over the length of the study. In conclusion, the legumes were not effective N sources to support production of high quality wheat forage in continuous systems of production over 4 years. Supply of ‘green’ N to crops must be mineralized from soil organic matter by soil microbes, and increases in organic matter tend to require longer time periods (decades) in the SGP. ‘Green’ N crops will have to be applied for longer series of years before continuous systems of wheat production will receive fertilizer benefits. Though not valuable as green N sources in the short term, legumes could be effective for summer weed control or soil conservation. More importantly, legumes grown during summer fallow of continuous winter wheat in the SGP could serve as sources of additional income from hay, grazed forage, or grain crops during years when received precipitation is capable of supporting both winter wheat and summer legume.