Forage Legume Nitrogen Fixation

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Forage Legume Nitrogen Fixation

• Gerald R. Smith
• TAES Overton

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Topics

• Advantages of Legumes
• Biological Nitrogen Fixation
• Management Summaries

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Advantages of Legumes

• High quality forage
• Extend the grazing season
• comparison to warm season grasses
• common use is overseeding
• Capable of biological nitrogen fixation
• amount of N fixed depends on many factors
• Many annual legumes can be managed for reseeding

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Biological Nitrogen Fixation

• Legume plants and Rhizobium bacteria team up to
  remove N from the air
• Nodules form on legume roots when this system is
  working

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Legume Rhizobium Team

• Legume Plant
• forms a nodule in response to Rhizobium
• provides energy and protection for the bacteria
  in the nodule
• converts fixed N to organic N and produces high
  protein forage

• Rhizobium bacteria
• infects plant
• provides genetic information that allows N
  fixation
• uses the plant energy and nodule environment to
  accomplish N fixation

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Biological Nitrogen Fixation Concerns

• Legume species must be matched with correct
  Rhizobium species or strain
• Fixed N is first available to the legume plant
  and through recycling available to other plants
• N fixation is regulated by the plant based on
  need. High soil N will reduce N fixation.

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Biological Nitrogen Fixation Concerns

• Legume species must be matched with correct
  Rhizobium species or strain
• inoculation is the process of applying a
  commercial culture of Rhizobium to legume seed
  before planting.
• correct inoculant must be used for each forage
  legume.
• wrong Rhizobium clover no N fixation.

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Clovers Groups Based on Response to Rhizobium
Strain

• 12 or more groups
• Group A berseem, crimson, and persian
• Group B ball, white, and red
• Group I arrowleaf
• One general clover inoculant does NOT exist!!!!!

Burton, J.C. 1985
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Burton, J. C. 1985
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What if I cant find nodules or if my clover
plants are small and light green??? 1. No
inoculation 2. Wrong inoculum (Rhizobium
strain) 3. Hot, dry conditions after planting 4.
Very acid soils (pH less than 5.0)
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What about mixing inoculated clover seed with
fertilizer and applying both at the same
time?? 1. Not the best idea. 2. Would be fine
if you need to add seed to and existing stand.
Dont bother with inoculum. 3. The inoculum
cannot survive very long exposed to the chemical
salts in the fertilizer.
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Preinoculated Seed?? 1. Generally refers to
seed preinoculated and then coated with lime or
an inert clay. Fungicides can also be added. 2.
No problems after two years of trials at
Overton. 3. Research in progress.
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Inoculating Forage Legume Seed

• Be sure to use the correct inoculant
• Follow package instructions
• Inoculants that include an adhesive are best
• Inoculation is very important the first time you
  establish a new forage legume
• Remember that the inoculant is carrying live
  bacteria. Protect from heat and sun.

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Biological Nitrogen Fixation Concerns

• Fixed N is first available to the legume plant
  and through recycling available to other plants
• clover will not directly provide N for the
  ryegrass in a clover-ryegrass mixture
• clover will provide N through recycling to warm
  season grass pastures

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Clover and Ryegrass Response to Nitrogen
Fertilization
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Biological Nitrogen Fixation Concerns

• Nitrogen fixation is regulated by the plant,
  based on need.
• high soil N will reduce N fixation.
• fertilizer N applied to clover-ryegrass mixtures
• will cause rapid yield response in ryegrass
• will cause only moderate yield response in total
  mixture (clover ryegrass)

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Clover and Ryegrass Response to Nitrogen
Fertilization
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Response of Clover-Ryegrass Mixture to Applied
Nitrogen
Total Season Yield (lb dry forage / acre)
Lb N / acre
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Early (Feb 21) Forage Yield of Clover - Ryegrass
Mixture
60 lb N/ac applied in early winter (Dec. or Jan.)
gives highest efficiency on early forage yields
of mixtures (Evers and Haby, 1995)
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Biological Nitrogen Fixation Summary

• Inoculate to match legume with Rhizobium
• Avoid nitrogen fertilization of pure stands of
  forage legumes
• Use reduced rates of N fertilizer on
  legume-grass mixtures
• Plan for efficient use of legume forage and
  recycled N

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Vegetation Areas of Texas
Blackland Prairies
Post Oak Savannah
High Plains
Rolling Plains
Cross Timbers and Prairies
Pineywoods
Trans-Pecos
Edwards Plateau
South Texas Plains
Gulf Prairies and Marshes
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Pineywoods

• Crimson Clover
• Ball Clover
• White Clover
• Hairy Vetch
• Rose Clover

• Arrowleaf Clover
• disease problems
• Red Clover
• no reseeding
• late production
• Alfalfa

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Seasonal distribution of forage yield at Overton,
TX. Six year averages.
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Arrowleaf Clover, May 25 2000, Rusk Co. TX
Yuchi
Overton Experimental
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Overton Experimental
Yuchi
May 25. 2000, Rusk Co. TX
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Blacklands and Cross Timbers

• Medics
• Bur Clover
• Button Clover
• Hairy Vetch
• Sweetclover
• new breeding effort

• Rose Clover
• no wet soils
• Red Clover
• no reseeding
• Berseem Clover
• poor cold tolerance
• reseeding ?

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Gulf Prairies and Marshes

• White Clover
• Berseem Clover
• reseeding ?
• Bur Clover
• Sub Clover

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La S-1 White Clover
Ladino White Clover
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Cost Comparisons
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Legume PasturePrinciples

• Soil Test
• follow recommendations
• Select forage legume
• consider forage program objectives
• use best adapted forage legume for site
• soil drainage and type, potential cold damage
• Buy high quality seed and inoculum
• Inoculate

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Legume PasturePrinciples -- page 2

• Plan legume acreage needs carefully
• consider seed costs
• Prepare site
• Use best seeding method available
• Develop a plan for late spring growth
• graze
• hay ???
• reseeding

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Manage for Reseeding?

• Can you afford to defer grazing
• do you have more forage than your cattle can use
  in the late spring?
• Can you accept a slow start on warm season grass
• Risk level (high risk no reseeding)
• ball, white and rose low
• arrowleaf low if you allow seed to mature
• crimson medium (high if summer is wet)
• red and berseem high