AGSY102 - Lecture 8: Pastures

Pasture Types & Distribution

• Pastures cover ≈ $70\%$ (≈ $400\,\text{M ha}$) of Australia; crops ≈ $24\,\text{M ha}$.

• Key zones: rangelands, high-rainfall, wheat-sheep.

• Three main pasture types
  • Shrublands (e.g. saltbush, bluebush – semi-arid)
  • Native grasslands (≈ $1000$ spp.; sclerophyll grasses such as Mitchell, black spear)
  • Sown/improved pastures
  – Tropical: signal, buffel, stylo, twining legumes
  – Temperate: phalaris, ryegrass, clovers (high rainfall, ley systems, cool-temperate dairy)

Historical Context

• Pre-European: cultural/mosaic burning → biodiversity, nutrient cycling, bushfire risk ↓.

• Post-European: broad vegetation clearing → soil degradation, biodiversity loss.

• $1880$s onward: exotic species + fertiliser use.

• Today: Australia leads pasture agronomy, yet holds only ≈ $4\%$ of world animal units on vast but low-quality pastures.

Climate Adaptation

• C3 vs C4 natives: C3 = cool-season, higher quality; C4 = warm-season, higher production & drought tolerance.

• Tropical legumes: Desmanthus, Leucaena, Stylosanthes (drought-persistent, high feed quality).

• Temperate species: perennial ryegrass, phalaris, subterranean & white clover (winter-active; environmental benefits: weed suppression, erosion & salinity control, soil C↑).

Pasture Growth Phases

1. Phase $1$ – Establishment: high quality, low biomass.

2. Phase $2$ – Vegetative: rapid growth; optimal quality + biomass → ideal grazing window ★.

3. Phase $3$ – Reproductive: quality ↓, biomass ↑.

4. Phase $4$ – Senescence: low quality & biomass.

Northern vs Southern Systems

• Northern: extensive (>$1000\,\text{ha}$ paddocks), low stocking (≈ $1$ beast/ $5{-}20\,\text{ha}$), mostly summer C4 natives, lower infrastructure.

• Southern: intensive (<$50\,\text{ha}$), higher stocking (≈ $10{-}20$ DSE/ha), improved pastures, higher infrastructure.

• Core task: manage seasonal feed gaps via species mix (e.g. digit grass, native perennials, oats, lucerne).

Pasture Selection & Establishment

Selection criteria

• Climate: rainfall pattern, extremes, growing season.

• Soil: pH, drainage, fertility, salinity.

• Plant traits: lifecycle (annual/biennial/perennial), persistence, pest/disease & herbicide tolerance, end use.
  Establishment steps

1. Paddock selection

2. Soil testing/correction (pH, $N,P,K$, salinity, structure)

3. Weed/pest/disease control

4. Seed-bed preparation (firm, weed-free)

5. Sowing (timing, method)

6. First grazing (quick, at $10{-}15\,\text{cm}$ height to promote tillering)

Production Challenges

• Nutrition management (soil & livestock).

• Weeds, pests, diseases; resistance issues.

• Land degradation: acidity, salinity, erosion.

• Financial constraints: seed & ongoing management costs.

• Climate variability/global warming.

• Emerging: carbon farming, biosecurity, labour, tech adoption.

Sustainability Benefits of Mixed Pastures

• Biological $N_2$ fixation by legumes.

• Soil protection & organic matter ↑; carbon sequestration ≈ $0.5{-}1.0\,\text{t C ha}^{-1}\text{ yr}^{-1}$.

• Improved water infiltration/storage.

• Biodiversity ↑ (root exudate diversity, pollinators).

• Disease breaks.

• Key practices: perennial inclusion, ground cover maintenance, rest periods, integrated pest management.

Future Outlook

Challenges: climate change, market & sustainability demands, tech integration, efficiency.
Opportunities: remote sensing, climate-smart cultivars, carbon markets.
Research priorities: drought resilience, nutrient efficiency, disease resistance.