Managing Grazing in Northern Australia Rangelands

Overview of Northern Australian Grazing Systems

• The management of grazing in Northern Australia involves applying fundamental principles to an extreme context characterized by a distinct wet-dry season cycle and tropical C4 grasses.

• This topic is critical because the vast majority of Australia's beef production originates from Northern Australia, primarily from rangeland systems.

• The study will be divided across two or three lectures, focusing on the differences between tropical and temperate systems, soil types, and management sustainability.

Key Differences Between Tropical and Temperate Systems

• Photosynthetic Pathways:

  • Tropical systems are dominated by C4 grasses.

  • Temperate systems are dominated by C3 grasses.

• Seasonality:

  • The primary limiting factor in the tropics is the wet-dry season cycle.

  • In the far north of Australia, summer is the wet season, while the dry season is extremely dry with negligible rainfall.

  • High temperatures lead to very high evaporation rates, which significantly impacts the biological health of both plants and livestock.

Nutrient Dynamics and Animal Growth Patterns

• Soil and Plant Quality:

  • Northern soils are often marginal and highly deficient in phosphorus ($P$).

  • Growth occurs in a non-linear pattern: rapid growth and high nutrient uptake (protein and phosphorus) during the wet season, followed by a sharp decline in quality as pastures dry off.

• Livestock Weight Trends:

  • Livestock gain weight during the wet season but often experience negative weight change (weight loss) during the dry season.

  • Compensatory Growth: Once nutrition becomes available in the early wet season, animals experience rapid live weight increases. During the dry season, an animal's skeleton continues to grow, but muscle mass and body weight stall or decline due to lack of nutrition. These losses are recoverd quickly when green grass returns.

  • The Saw-tooth Pattern: A term used to describe the cyclic nature of weight gains and losses in tropical systems.

• Management Implications:

  • Stocking rates are generally low, requiring large areas of land (high capital investment in land and fencing) for the same production output as more intensive systems.

  • Return on investment (ROI) in these extensive systems can be quite low.

Defining and Characterizing Rangelands

• Global and Australian Diversity:

  • Rangelands vary from wet tropics to the steppes of Mongolia.

  • Australian rangelands include northern forests (common in North Queensland), spinifex systems (Eastern Northern Territory), and acacia-grass-forb mixes (Western Queensland and Southern Northern Territory).

• Geographic Scope:

  • What is not a rangeland: Subtropical and temperate systems around southern WA, southeast Australia, and the eastern side of the Great Dividing Range (up to Central Queensland).

  • Rangelands: Grasslands in Western New South Wales, Western Queensland, deserts, and the tropical zones of the far north.

• Defining Features:

  • Dominated by natural ecological processes.

  • Extensively managed: Systems are distant from human centers with very little human input or control.

  • Characterized by low or highly constrained seasonal rainfall.

  • Highly variable climates (intra-annual, inter-annual, and spatial).

Physical and Biological Constraints of Tropical Grasses

• C4 Grass Limitations:

  • Higher total biomass yield compared to temperate swards, but significantly lower quality (digestibility, protein, energy, and minerals).

  • Animal intake is limited by rumen fill; even if biomass is abundant, low digestibility prevents animals from consuming enough to meet high production needs.

• Stem-to-Leaf Ratios:

  • Tropical grasses have a much higher stem-to-leaf ratio than temperate grasses (e.g., Ryegrass or Fescue).

  • Stems are structurally complex, containing more cellulose and less starch/sugars, which makes them less edible.

  • As plants age (e.g., past $12$ months), species like Rhodes grass or Buffel grass develop hard, stick-like green stems.

• Digestibility Statistics:

  • Leaf digestibility of a C4 grass may reach $60\%$.

  • Green stem digestibility is often well below $40\%$.

  • A digestibility of $40\%$ implies that $60\%$ of consumed dry matter is excreted.

Management Challenges: Variability and Budgeting

• Spatial Heterogeneity:

  • Rangelands are not monocultures; they contain thousands of plant species. Unlike improved pastures (which might be planted with $12$ species and whittle down to $4$ or $5$), rangeland diversity is inherently high.

• Intra-annual (Wet/Dry) Variability:

  • Growth is impossible during the dry season.

  • Feed budgeting must occur at the end of the wet season/start of the dry season. The amount of feed on the ground at that moment is the total budget to last the next $6-8$ months.

  • Managing these systems is difficult because they are often breeding systems (pregnant cows with calves at foot) rather than trading systems, giving managers low flexibility to move stock off.

• Accessibility:

  • During the wet season, access to animals and pastures is often restricted due to environmental conditions, making it hard for graziers to be responsive to seasonal breaks.

Nutritional Supplementation and Production Goals

• Nitrogen Supplementation:

  • Providing Urea or other protein sources during the dry season is a recommended best practice.

  • Supplemental nitrogen can limit live weight losses to approximately $10\%$ of the animal's weight at the end of the wet season.

• Growth Rate Comparisons:

  • In tropical rangelands, cattle might gain $700\,g/day$ during the wet season.

  • In high-performance temperate pastures (e.g., New Zealand, with fertilization), cattle can gain $1.4\,kg/day$.

Soil Types and Paddock Management

• Red Soils:

  • Higher sand content.

  • Respond quickly to small rainfall events.

  • Low organic matter; they dry out quickly, leading to short flushes of growth.

• Black Soils:

  • Higher clay and organic matter content.

  • Slower to respond; require larger rainfall events for substantial growth.

  • Higher water-holding capacity permits longer periods of biomass production.

• Paddock Composition Recommendation:

  • While intensive systems aim for homogeneity (separating soil types), rangelands benefit from including both red and black soils in a single paddock.

  • This provides "security": Red soils respond to small rains, while black soils support long-term growth. However, this diversity can lead to patch grazing and localized degradation.

Infrastructure, Labor, and Trees

• Extensive Scale:

  • Paddocks can be $15,000\,hectares$ with only $2-3$ watering points.

  • Stocking rates are low, often expressed as hectares per animal (e.g., $8\,hectares/AE$).

  • Intensive rotational grazing is rarely economically viable due to the cost of infrastructure relative to the low number of animals.

• Tree-Grass Interactions:

  • Trees (e.g., Eucalypt savannahs, Acacia) compete with grass for water and nutrients. Grass yield usually decreases near tree stems.

  • Animals prefer open spaces between trees, leading to increased soil compaction in those areas.

  • Australian soils are ancient and fragile; evidence suggests they do not tolerate cattle as well as African ecosystems tolerate herds of wildebeests.

  • Managing woody cover (using fire) is necessary to prevent woody weeds from dominating and reducing grass yield.

Land Condition and Carrying Capacity

• ABCD Condition Framework:

  • A (Good): High cover of 3P perennial grasses (Palatable, Perennial, Productive), low bare ground, no erosion.

  • B (Fair): Slight decline in 3P species, some increase in weeds/bare ground.

  • C (Poor): Significant decline in 3P species (below $30\%$), increased erosion risk.

  • D (Very Poor): Lack of perennial grasses; system has degraded to annuals; high risk of scalding and erosion (below $5\%$ 3P species).

• Carrying Capacity Case Study (Northern Territory):

  • At $1.9\,AE/km^2$, land condition was maintained over $20$ years.

  • At $2.4\,AE/km^2$, the system degraded significantly.

  • At $2.6\,AE/km^2$, degradation was dramatic.

  • Conclusion: The margin of error is extremely small; differences of less than one animal per square kilometer can cause long-term ecological collapse.

• Long-Term Carrying Capacity (LTCC): Defined as the maximum stocking rate sustainable over a $10+$ year period without degradation. Safe utilization rates for biomass are generally between $15-30\%$.

The Jones and Sandland Model in Rangelands

• Conventional Model: Suggests productivity per hectare increases to a peak as stocking rate increases, then declines as competition lowers individual animal growth.

• Rangeland Failure: The model often doesn't apply because the landscape degrades faster than livestock production can respond.

  • Managers may see high profit per hectare in the short term by overstocking.

  • By year $3$ or $8$, landscape degradation (e.g., woody weed ingress) reduces the potential for growth per head and enterprise profit.

  • The economic optimum (taking money now) is often far from the ecological optimum (preserving the landscape for the future).

Definitions Recap

• 3P Species: Grasses that are Palatable, Perennial, and Productive.

• LTCC: Long-Term Carrying Capacity.

• AE: Animal Equivalent ($1.0\,AE$ is approximately one dry cow).