Pastoral Livestock Production Systems and Pasture Management

Fundamentals of Pastoral Livestock Production

• Pastoral livestock production systems vary in intensity, ranging from intensive to extensive configurations.

• Pasture is considered a "cheap" and renewable feed source for grazing livestock.

• The New Zealand environment and climate are characterized as temperate.

• Herbage quality and quantity represent the primary constraints to animal production within pasture-based systems.

The Biological Cycle of Grass Growth

• The grass growth rate follows a distinct physiological progression:

  • Stage 1: Initial slow growth occurs as the grass plant builds root reserves necessary to support a larger structure.

  • Stage 2: Rapid vegetative growth phase.

  • Stage 3: Grass reaches maturity, leading to seed formation. Growth begins to slow during this phase.

  • Stage 4: Growth stops completely. The plant produces a dry seedhead and eventually dies.

• Nutrient value declines sharply as the plant transitions from vegetative growth to seedhead production and death.

Pasture Quality: Digestibility and Energy Correlations

• The nutritional value of pasture is determined by its digestibility, energy content, and growth stage, which directly impact animal production outcomes:

  • High Production (Active growth, green):

  • Digestibility: $75-80\%$

  • Energy: $10.8-11.6\,MJ\,ME/kg\,DM$

  • High Production (Late vegetative, green):

  • Digestibility: $70-75\%$

  • Energy: $9.9-10.8\,MJ\,ME/kg\,DM$

  • Moderate Production (Early flowering):

  • Digestibility: $65-70\%$

  • Energy: $9.1-9.9\,MJ\,ME/kg\,DM$

  • Maintenance of Dry Stock (Late flowering, in head):

  • Digestibility: $60-65\%$

  • Energy: $8.2-9.1\,MJ\,ME/kg\,DM$

  • Maintenance of Dry Stock (Dry grass and leaf):

  • Digestibility: $55-60\%$

  • Energy: $7.4-8.2\,MJ\,ME/kg\,DM$

  • Weight Loss of Dry Stock (Dry stalks):

  • Digestibility: $50-55\%$

  • Energy: $6.5-7.4\,MJ\,ME/kg\,DM$

  • Weight Loss of Dry Stock (Late stage dry stalks):

  • Digestibility: $45-50\%$

  • Energy: $5.7-6.5\,MJ\,ME/kg\,DM$

Dynamics of Pasture Supply and Demand

• Managing a pasture-based system is centered on the balance between Supply (In) and Demand (Out).

• Factors Influencing Animal/Pasture Demand (Nutrient/Energy Requirements):

  • Animal activity and production levels.

  • Physiological state of the animal including maintenance, growth, lactation, and pregnancy.

  • Environmental temperature.

• Seasonal Synchronization:

  • Pasture supply is driven by the season, with peak supply occurring in the Spring.

  • Peak nutrient and energy demands coincide with lactation.

  • Management goals aim to align animal birth and lactation with peak spring pasture supply to maximize efficiency.

Biological Breeding Patterns in Livestock

• Seasonal Breeders (Photoperiod sensitive):

  • Sheep: Characterized as short-day breeders.

  • Goats: Seasonal breeders.

  • Deer: Seasonal breeders.

  • Horses: Characterized as long-day breeders.

• Non-Seasonal Breeders (Polyestrus/Continuous):

  • Cattle: Are polyestrus and can breed throughout the year, though New Zealand systems often manage them seasonally.

  • Pigs: Continuous breeders.

  • Poultry: Continuous breeders.

• Gestation Lengths:

  • Sheep: Approximately $147\,\text{days}$.

  • Cattle: Approximately $9\,\text{months}$.

Seasonal Production Activities: Sheep Systems

• The seasonal sheep cycle in New Zealand follows a generalized pattern to match pasture availability:

  • Spring: Lambing and lactation occur. This period aligns with peak pasture supply and often results in a pasture surplus.

  • Summer: Weaning takes place. Producers must focus on feeding growing lambs and maintaining ewe condition for breeding. Summer dry conditions often lead to a pasture deficit, necessitating the use of supplementary feeds or specialist crops.

  • Autumn: Mating (breeding) season.

  • Winter: Pregnancy period. Pasture growth is slow, creating a deficit relative to animal requirements. Winter forage crops (planted in spring/early summer and grown through summer/autumn) are grazed to bridge this gap.

Seasonal Production Activities: Dairy Systems

• While cows are not naturally seasonal breeders, they are typically farmed seasonally in New Zealand to optimize pasture use:

  • Spring: Calving (starting late winter), early lactation, and breeding.

  • Summer: Mid-lactation period.

  • Autumn: Late lactation period.

  • Winter: Cows are "dried off" (lactation ceases) in preparation for the next calving season. Supplementation or specialized crops are used during this period.

Management Metrics: Pasture Mass and Intake

• Successful production relies on producers accurately assessing several variables:

  • Animal nutrient demands.

  • Current pasture mass available.

  • Forecasted pasture growth rates.

  • Future requirements and necessary supplementary feed levels.

• Key Definitions:

  • Pre-grazing pasture mass/cover: The amount of pasture (expressed in $kg\,DM/ha$) present before livestock begin grazing.

  • Post-grazing pasture mass/cover: The amount of pasture (expressed in $kg\,DM/ha$) remaining after livestock have finished grazing.

  • Pasture Utilisation: The amount of pasture eaten relative to the amount of pasture grown. Maximizing utilization helps reduce feed wastage and increase efficiency.

• Intake Calculations:

  • Estimated Intake per Hectare is calculated as: $\text{Intake} = \text{Pre-grazing cover} - \text{Post-grazing cover}$.

  • Example Calculation: If a paddock has a pre-grazing cover of $2000\,kg\,DM/ha$ and a post-grazing cover of $1500\,kg\,DM/ha$, the total intake is $500\,kg\,DM/ha$.

  • Stocking Rate Application: In a $1\,\text{hectare}$ paddock with $30\,\text{cows}$, the individual intake per animal would be approximately $16-17\,kg\,DM$ (calculated as $\frac{500}{30}$).

Grazing Targets and Management Rationale

• Targets are established to optimize intake and avoid both restriction and excess feeding.

• General Pre- and Post-grazing Targets for Sheep:

  • Pre-grazing cover: $1100\text{ to }1500\,kg\,DM/ha$

  • Post-grazing cover: $800\text{ to }1000\,kg\,DM/ha$

• General Pre- and Post-grazing Targets for Dairy Cows:

  • Pre-grazing cover: $1900\text{ to }2100\,kg\,DM/ha$

  • Post-grazing cover: $1500\text{ to }1600\,kg\,DM/ha$

• Functional Relationships: Intake is a function of sward height, pasture mass, and pasture allowance.

• Pasture Management Risks:

  • If grass is allowed to grow too long, its nutritional quality (digestibility and energy) declines significantly.

  • If grass is grazed too short, it may exhaust root reserves, slowing subsequent regrowth and reducing total pasture supply.