Plant Growth Concepts and Pasture Management

Plant Growth Concepts and Management Background

The primary goal of understanding plant growth concepts is to move from basic management decisions to a deeper understanding of what plants are actually doing.
Growth concepts allow managers to layer decisions on top of biological processes, specifically regarding pasture establishment, nutrition, and species interactions (e.g., competition between grasses and legumes).
Plant traits are categorized into two components:
- Above-ground traits: Primarily focused on light acquisition and sensing environmental cues such as temperature.
- Below-ground traits: Focused on roots foraging for moisture and nutrition.

Defining Pasture and Sward

Pasture: Refers to a grazing management unit that is enclosed or separated, dedicated specifically to forage production.
Pasture Sward: Refers to the population or community of plants existing within a pasture.
Management is often done for the "average" of a paddock (e.g., maintaining a Colwell P of $30$ for persistence), but swards contain significant variation.
Sward Variation: Variability is driven by environmental factors and management levels. A lack of stable management leads to variability in growth stages and plant populations/communities.
Common components of a pasture sward include grasses, legumes, and herbs. Interactions vary based on species requirements (e.g., two ryegrass plants have identical needs, whereas ryegrass and clover have different requirements).

Grass Growth Forms and Space Occupancy

Grass plants are the basis for most grazing systems, ranging from high-production dairy (solely grass and nitrogen) to rangelands.
Tufted/Tussocky Growth Forms: Grasses that grow in an upright fashion. Examples include ryegrass, fescue, Phalaris, Coxwood, Coxfoot, buffer grass, and Rhodes grass. These species occupy space through tillering.
Mat-forming Grasses: These occupy space through runners. Examples include Kiikuyu (or Ku) and couch.
- Rhizomes: Runners located below the ground.
- Stolons: Runners located above the ground.
Competition Strategy: Plants aim to "take over the world" by occupying space. More shoot material allows for more light capture, while larger root systems capture more nutrients and water.

Vegetative vs. Reproductive Growth

Vegetative Growth:
- Focused on leaf (green feed) production and biomass.
- The growing point is located at the base of the plant.
- Features a high ratio of leaf to stem.
Reproductive Growth:
- The growing point is carried above the leaves.
- Inflorescence (the seed head) is produced at the top of the stem.
- Results in stem elongation and increased structural components.
- Quality Trade-off: Stems are structurally sound but contain carbohydrates that are less digestible and of lower quality for livestock. Managers aim to limit stem production to maintain green leaf quality.

Primary Growth and Environmental Stimuli

Meristem: A localized area of cell division where primary growth originates.
Leaf Primordia: The initial part of the plant at the tip of the meristem that grows and subdivides into a leaf.
Meristems determine plant morphology. Grass meristems are at the base (for tillering), while legume meristems can be carried along the stem.
Meristems respond to reliable environmental cues to determine dormancy, growth, or reproduction:
1. Day Length: Consistent year-to-year. Temperate species respond to longer days (spring) for floral induction; tropical species respond to shorter days (autumn).
2. Light Quality: Determined by the ratio of red to far-red light at the base of the sward. This triggers tillering.
3. Temperature: Influences growth rates and specific requirements like Vernalization (cold requirement).
Soil moisture and fertility are less reliable cues because they vary too much to signal seasonal timing.

Germination and Emergence

Seed Size and Vigor: Larger seeds (wheat, barley, oats) have higher reserves and seedling vigor. Tropical grasses (Digit, Rhodes grass) have very small seeds and low initial vigor.
Germination: The initial stage where the seed imbibes water, swells, and the radical (little root) emerges.
Emergence: The stage where the seedling physically comes out of the ground.

Grass Architecture and Persistence

Apical Meristem: Located in the center-base of the plant; drives primary growth in an individual tiller.
Auxiliary Buds: Located to the side of the main stem; allow for the production of tillers.
Tillers: Independent units with their own roots and leaves; technically capable of living if separated from the mother plant.
Grazing Persistence: Lower growing points are less likely to be grazed. Phalaris has growing points below the soil surface, allowing for heavy grazing. Tropical species often lift their growing points higher during reproduction, making them susceptible to grazing damage.

The Programmatic Three-Leaf Stage (Ryegrass Example)

Plants produce leaves in a specific sequence. For ryegrass, the plant maintains exactly $3$ live leaves.
As the $4^{\text{th}}$ leaf emerges, the $1^{\text{st}}$ leaf starts to senesce (die off).
Species-specific live leaf counts:
- Phalaris and Tall Fescue: $4$ live leaves.
- Coxfoot: $4$ to $5$ live leaves.
- Ku: More than $5$ live leaves.
- Certain tropical species: $7$ to $10$ or higher.
Management Trigger: Grazing at the $2.5$ to $3$ leaf stage for ryegrass balances biomass accumulation with high feed quality before senescence begins.

Carbohydrate Reserves and Recovery

Plants rely on internal carbohydrate reserves to regrow after defoliation.
The Regrowth Cycle:
1. After grazing, reserves drop as they are used to push out the first leaf.
2. Once enough photosynthetic material exists, the plant generates energy and recovers reserves.
3. Reserves must be fully recovered before the next grazing to ensure plant survival.
Constant grazing at the $1.5$ to $2$ leaf stage prevents reserve recovery and leads to plant death.

Clover Morphology and Growth

White Clover: Stolenniferous with a horizontal development form. It branches away from the initial growing point using stolons creeping across the soil surface.
Persistence: White clover relies on the burial and seasonal replacement of stolons. This is different from Red Clover.
Light Capture: Clover has flat leaves and a lower Critical Leaf Area Index than upright grasses.
Reproduction Forms:
- Determinate: The terminal meristem becomes reproductive and growth for that segment ends (common in grasses).
- Indeterminate: Lateral meristems become reproductive, allowing the shoot to continue growing (e.g., erredella). This allows the plant to respond to late-season rainfall by producing more seed.

Growth vs. Development

Growth: The rate at which the plant produces biomass/leaf material.
Development: The rate at which the plant progresses through maturity stages (e.g., vegetative to reproductive).
Temperature influences both. Development is determined by the accumulation of temperature over time.

Above-Ground Biomass and Light Capture

Growth is strongly correlated with the interception of Photosynthetically Active Radiation (PAR).
Light capture is inefficient ( $1\%$ ) and radiation cannot be stored (it must be used, reflected, or lost).
Leaf Area Index (LAI): The ratio of leaf surface area to soil surface area.
- LAI of $0$ : Bare soil.
- LAI of $1$ : Leaf area equals soil area; only captures ~ $50\%$ of light due to leaf orientation.
Critical Leaf Area Index: The amount of leaf required to achieve $95\%$ light interception.
- Perennial ryegrass: Requires LAI of ~ $7$ .
- White clover: Requires LAI of ~ $3.5$ due to horizontal leaves.
Closed canopies prevent weed emergence and maximize competition.

Temperature and Thermal Time

Leaf Emergence Rates (example from Victoria):
- April: $10$ days.
- Winter: $20-23$ days (growth slows due to low temperature).
Thermal Time (Growing Degree Days - GDD): A measure of temperature accumulation.
- $GDD$ Equation: $\text{GDD} = \frac{T_{\text{max}} + T_{\text{min}}}{2} - T_{\text{base}}$
Base Temperature: The temperature below which no development occurs.
- Perennial Ryegrass: $1.4$ to $2.4^{\circ}\text{C}$ .
- Coxfoot: $0^{\circ}\text{C}$ .
Emergence GDD requirements:
- Ryegrass: $70-78$ GDD.
- Coxfoot: $208-319$ GDD.
Species with lower GDD requirements for emergence establish faster and can become dominant.

Sward Compensation and Light Ratios

Size-Density Trade-off:
- Tall, ungrazed swards ( $20-25\,cm$ ) have a low density of large shoots.
- Highly grazed, short swards ( $10-12\,cm$ ) have a high density of small tillers.
Light Ratios: Grass leaves are transparent to far-red (infrared) light but absorb red light.
- In a tall sward, only far-red light reaches the base, signaling the plant to stop tillering.
- Opening the canopy with grazing allows red light to reach the base, changing the red to far-red ratio and triggering tillering.

The Sigmoidal Growth Pattern

Phase 1: Slow growth, low biomass, high quality. Occurs until critical leaf area is achieved.
Phase 2: Rapid growth, rebuilding carbohydrates, optimal light interception, high to moderate quality.
Phase 3: Slow growth, senescence, reproductive phase (stem elongation), low quality.

Management Goal: Maintain consistency. Grazing at the end of Phase 2 maximizes biomass while ensuring quality and carbohydrate recovery for persistence.

Below-Ground Root Traits

Root Types:
- Monocots (Grasses): Fibrous root systems; effective at spatial foraging.
- Dicots (Legumes): Taproot systems; often deeper but more constrained in spatial foraging.
Soil Profile Factors: Nutrition is primarily in the topsoil (top $5-10\,cm$ ) due to bioaccumulation from decaying material, animal excreta, and broadcast fertilizer.
Phosphorus ( $P$ ) is immobile and requires active root exploration. Nitrogen ( $N$ ) and Sulfur ( $S$ ) are mobile and move with water.
Optimal Strategy: Roots near the surface for nutrients; deep roots for water acquisition.

Nutrient and Moisture Acquisition

Nutrient Efficiency: Enhanced by branching, root hairs (increased surface area), and finer roots (less carbon investment per unit of length).
Mycorrhizal Fungi: Symbiotic fungi that extend the root network to acquire $P$ and Zinc ( $Zn$ ) in exchange for carbohydrates.
Moisture Bucket: Ryegrass is shallow-rooted. Coxfoot, fescue, and Lucerne are deep-rooted, allowing them to tap into a larger "bucket" of water during dry periods.
Digit Grass vs. Desmanthus: Grasses (Digit) produce more root material and hairs more quickly in the first $4-5$ weeks than legumes (Desmanthus or Desmanhus).

Nitrogen Fixation

Legumes fix atmospheric nitrogen through a symbiotic relationship with Rhizobia bacteria.
The Process: Root hairs release chemicals to signal Rhizobia, which form an infection zone and eventually a Nodule.
Nodule Assessment:
- Ideal color: Pink/red (like the color under a fingernail).
- Ineffective/dead: White or black.
N-Fixation Ratios: On average, for every $1\,ton$ of dry matter produced by a legume, $22$ to $25\,kg$ of nitrogen per hectare is fixed.
Efficiency Factors:
- Lazy Legumes: If soil nitrogen is high, plants will use that rather than fixing their own (as fixation costs carbon/energy).
- Requirements: Rhizobia presence (can be added as seed coating), moisture, temperature, and nutrition ( $P$ , $S$ , Molybdenum ( $Mo$ ), Nickel ( $Ni$ )).
Nitrogen Availability: Nitrogen is bound in shoot material and only becomes available to grasses as that material decays or passes through an animal (excreta).

Additional Considerations

Organic Exudates: Some species exude organic acids to dissolve and acquire phosphorus in low-P soils.
Aerenchyma: Specialized tissue for gas exchange in roots.
Grazing Impact on Roots: Overgrazing reduces root surface area and foraging ability, slowing overall growth.
Application Strategies: Banded application of fertilizer (placing it in concentrated rows deeper in the profile) can improve nutrient use efficiency compared to surface broadcasting.

Questions & Discussion

Question: Can I replace a deep-rooted species with a different pasture species? Response: Yes, it is possible. However, keep in mind that a deep-rooted species like Lucerne may have dried out the profile significantly, which might impact the success of the species that follows. Management should always prioritize the species with the most critical requirements currently being met in the sward.

The primary goal of understanding plant growth concepts is to move from basic management decisions to a deeper understanding of what plants are actually doing.
Growth concepts allow managers to layer decisions on top of biological processes, specifically regarding pasture establishment, nutrition, and species interactions (e.g., competition between grasses and legumes).
Plant traits are categorized into two components:
- Above-ground traits: Focused on light acquisition and sensing environmental cues such as temperature.
- Below-ground traits: Focused on roots foraging for moisture and nutrition.
Pasture Definition: Refers to a grazing management unit dedicated specifically to forage production.
Sward Variation: Variability in plant communities driven by environmental factors and management levels.
Grass Growth Forms: Including tufted/tussocky and mat-forming species, with different space occupancy strategies.
Vegetative vs. Reproductive Growth:
- Vegetative growth focuses on biomass and leaf production.
- Reproductive growth involves stem elongation and seed head production with different quality characteristics.
Meristem Function: Localized areas of cell division determining plant morphology, responding to environmental stimuli.
Germination and Emergence: Seed size and vigor significantly affect the rates of these processes.
Clover Morphology: Differences in growth forms among clovers impact their persistence and light capture.
Growth vs. Development: Growth is the biomass production rate, while development measures progression through maturity stages.
Nutrient Acquisition: Efficient root systems and symbiotic relationships like those with mycorrhizal fungi enhance nutrient uptake.
Nitrogen Fixation: Legumes fix atmospheric nitrogen through a symbiotic relationship with bacteria, which is critical for soil fertility.
Management Strategies: Effective grazing practices, nutrient applications, and understanding of plant responses to environmental cues are essential for successful pasture management.