L11 – Forage Value Lecture Notes

Forage Value Lecture Notes

Introduction

  • Speaker: Tiago Silva

  • Context: Lecture series integrating plant and animal components.

  • Duration: Lectures will be longer (2-hour slots).

  • Participation: Encouragement for students to ask questions throughout the lecture; breaks will be included.

  • Background: Tiago originates from Brazil and has a background in cattle farming, which influences his interest in the subject matter.

Grazing Systems vs. Cropping Systems

  • Primary Focus: Animal product production (milk, meat, wool) as opposed to grain harvest in cropping systems.

  • Key Differences:

    • Control: Animals have their own behaviors and can’t be controlled like crops.

    • Efficiency Steps: 1. Soil nutrients to plant growth

    1. Pasture consumed by animals

    2. Nutrient conversion to meat

  • Complexity: Grazing systems are complex due to multiple efficiency losses, unlike streamlined cropping systems.

Nutritional Value

  • Definition: Quality of feed is defined as the ability to provide nutrients fulfilling livestock's needs. Quality encompasses:

    • Nutritional value (nutrients available in feed)

    • Voluntary intake (amount actually consumed by the animal)

  • Importance: High nutritional value does not guarantee sufficient consumption to meet nutritional needs.

Parameters of Nutritional Value

  1. Dry Matter Digestibility (DMD):

    • Defined as the portion of the feed that is absorbed by the animal.

    • Measurement: Conducted in metabolic crates where feed and feces are measured to calculate digestibility.

  2. Metabolizable Energy (ME):

    • Total digestible energy minus energy lost in feces and urine.

    • Forms of Energy:

      • Gross Energy (measured using a bomb calorimeter)

      • Digestible Energy (gross energy minus fecal energy)

      • Metabolizable Energy (digestible energy minus urinary energy and methane losses)

      • Net Energy (NE): The energy remaining after all losses and available for maintenance, lactation, or growth.

  3. Crude Protein (CP): A key ingredient indicating protein levels in feed.

  4. Neutral Detergent Fiber (NDF) and Acid Detergent Fiber (ADF):

    • NDF represents the cell wall components and limits intake (higher NDF = lower intake efficiency).

    • ADF measures only cellulose and lignin, indicating indigestible fractions.

C3 and C4 Plants

  • C3 Plants:

    • Examples: Ryegrass, fescue (grown in temperate climates).

    • Photosynthesis: Uses Rubisco enzyme which can react with CO2 or O2 leading to photorespiration under stress conditions.

  • C4 Plants:

    • Examples: Buffel grass, elephant grass (tropical climates).

    • More efficient due to Pep Carboxylase enzyme that fixes CO2 without producing O2 byproducts.

    • Cranz anatomy protects Rubisco from photorespiration, making C4 plants better adapted to warm, dry climates.

Key Differences Between C3 and C4 Plants

  • Photosynthetic Efficiency:

    • C4 plants outperform C3 in hot and dry conditions.

    • Higher efficiency with less nitrogen requirement than C3 plants due to lower Rubisco concentrations.

  • Nutritional Value:

    • C3 plants often have higher crude protein due to more Rubisco (higher protein concentration).

Fiber and Its Role in Nutritional Value

  • Composition: Fiber is primarily from the cell wall, which includes cellulose, hemicellulose, and lignin.

    • NDF: Higher NDF indicates lower digestibility depending on the lignin concentration.

    • Analogy: Compare cell wall structure to a brick wall where:

    • Cellulose is the bricks and lignin acts as mortar; high lignin reduces digestibility.

  • Digestibility Relation: More lignin = less digestibility of the plant material.

Environmental Factors Affecting Nutritional Value

  • Temperature:

    • Affects growth, respiration rates, and overall plant development.

    • Optimal temperature for growth may differ from optimal for photosynthesis, impacting internal sugar concentrations.

  • Water Availability:

    • High water levels can lead to increased cell wall thickness and potentially lower digestibility.

  • Light Intensity:

    • Enhanced photosynthesis under higher light intensity increases nutritional value measured by carbohydrates.

  • Nitrogen Fertilization:

    • Critical nutrient acting as an accelerator for plant growth; enhances both old and new growth nutritional values.

Morphological Aspects Affecting Nutritional Value

  • Plant Structure:

    • Nutritional value varies between old and new leaves, and between leaves and stems.

    • Over-maturity leads to higher stem to leaf ratio, negatively affecting quality and animal preference.

  • Examples of Maturity Effects: High NDF and lignin content at later growth stages lowers protein content and digestibility.

General Observations

  • Nutritional Quality is a combination of nutritional value and intake; must consider voluntary intake as well.

  • Practical Implications for grazing management and forage evaluation are critical in animal production systems.

Conclusion

  • The next lecture will explore factors influencing animal intake and further examine morphological changes in tropical pastures.

  • Any questions are encouraged for clarification on these concepts.

Key Takeaways

  • Nutritional value = the combination of quality and voluntary intake.

  • The digestibility of feeds influences the overall metabolism in livestock. Understanding differences in plant types (C3 vs. C4) is essential.

  • Environmental factors play a significant role in altering the nutritional landscape of pastures.

  • Note: Students should review this material thoroughly in preparation for upcoming lectures, focusing on definitions, measurements, and the relationships between plant physiology and animal nutritional needs.