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Week 4

Roughages

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Chemical Composition of Herbage

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  • Chemical composition and nutritional value of forage
    • Complex
    • Influenced by number of factors
      • Primarily plant species, environmental and soil conditions, and/or stage of maturity
  • Nitrogen component
    • Expressed as CP
      • Measures protein and nonprotein nitrogen compounds
    • Factors influencing CP
      • Plant species, environmental and soil conditions, and stage of maturity
    • Legumes higher than grasses
      • Legumes
        • Range: 15-30% (DM basis)
      • Cool-season grasses
        • Range: 10-20% (DM basis)
      • In specific situations, may be lower or higher
      • For grasses, CP content dependent on nitrogen fertilization
    • True protein is 75-85% of nitrogen
      • As matures, portion of nitrogen as true protein increases
    • Nonprotein nitrogen compounds
      • More than 200 identified in forages
      • Factors influencing NPN content
        • Soil and environmental conditions and plant species
      • Legumes higher than grasses
      • Nutritional value of nonprotein amino acids
        • For nonruminants, app. half value for protein amino acids
        • For ruminants, similar in nutritional value to amino acids in protein molecules
  • Carbohydrate content
    • Nonstructural carbohydrates
      • Readily available carbohydrates
      • Mono and disaccharides
        • Relatively low
          • In general, less than 8% (DM basis)
        • Primarily glucose, fructose, and sucrose
      • Polysaccharides
        • Primarily starch and fructosans
        • Stored energy for plant
      • Legumes
        • Sucrose and starch
      • Warm-season grasses
        • Starch
      • Cool-season grasses
        • Fructosans
      • Cool-season grasses and temperate legumes harvested at similar maturities, similar TNC content
      • Factors influencing TNC content
        • Plant species, environmental conditions, and soil fertility
      • Correction to text, nonstructural carbohydrate content highest at end of the day
        • To increase NSC content at harvest, harvest at end of daylight hours
    • Structural carbohydrates
      • Polysaccharides
      • Three classes
        • Pectic substances
        • Noncellulosic polymers
          • Hemicellulose
          • Digestible only by microbial enzymes
        • Cellulose
          • Digestible only by microbial enzymes
      • Factors influencing structural carbohydrate content
        • Species and environmental conditions
      • As plants mature
        • As legumes mature
          • Increase in stem cellulose
        • As grasses mature
          • Increase in stem cellulose and hemicellulose
  • Lipids
    • In leaf tissue
      • Content is 3-10%
      • Decreases with maturity
      • Primarily galactolipids and phospholipids
      • Linolenic acid is 60-75% of fatty acids
      • Upon disruption of surface, unsaturated lipids susceptible to oxidative processes
  • Organic acids
    • In general, not reported specifically
    • In general, included with other components
      • NFE portion of proximate analysis
      • Cell contents or solubles of detergent fiber system
    • Most nonvolatile
    • Range: 2-8% (DM basis)
    • Factors influencing organic acid content
      • Plant species, maturity, and fertilization
    • Palatability of feedstuff
  • Minerals
    • Table 7-7 on page 126 of text – Range and typical mineral concentration for grasses and alfalfa
      • Does not include all macro and microminerals in forages
    • General
      • Grasses and legumes high in K
      • Legumes higher contents of Ca, Mg, S, and Cu than grasses
      • Grasses higher contents of Mn and Zn
      • Grasses and legumes moderate to low in P
    • Variable
    • Factors influencing mineral content
      • Plant species and soil and environmental conditions

 

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