Effects of legume green manures on forage produced in continuous wheat systems

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

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/4/2015
Publication Date: 10/16/2015
Citation: Northup, B.K., Rao, S. 2015. Effects of legume green manures on forage produced in continuous wheat systems. Agronomy Journal. 108(1):101-108.

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: Inorganic nitrogen (N) fertilizers are important to continuous systems of winter wheat (Triticum aestivum L. em Thell.) in the U.S. southern Great Plains (SGP). Costs have increased in recent years and resulted in producers considering legumes grown during summer fallow as N sources. This study (2008-2012) defined the potential of lablab [Lablab purpureus (L.) Sweet cv. Rio Verde] to support biomass production by wheat under conventional and no-till management, compared to soybean [Glycine max (L.) Merr. cv. Laredo], and 3 inorganic fertilizer treatments (0, 40, and 80 kg N/ha). Legume seeds were inoculated and sown annually after wheat harvest, grown during June-August, and terminated in early-September. Wheat was then sown, grown (early-September to early-June) to maturity, and samples collected at 4 growth stages. Wheat (non-grain) biomass (kg/ha), N concentrations (g/kg) and N accumulated (kg N/ha) in biomass were analyzed to define N treatment, tillage system, growth stage, and year effects. Precipitation varied from 53-92% of the long-term average (688 mm) for the growing period (October-May) of winter wheat. Nitrogen treatment x year interactions were significant for biomass, N concentration, and N accumulated in biomass (P<0.01). Biomass produced in response to N treatments varied among years; responses to legume treatments did not exceed responses to 0 kg applied N. Responses of N concentrations and N accumulated in wheat biomass produced under legume treatments largely did not exceed responses to 0 kg applied N; responses to all treatments declined with length of study. Neither legume appeared effective as N sources for continuous systems of biomass production by wheat in the SGP.