Supplementation in Grazing Systems

Overview of Supplementation in Grazing Systems

Supplementation is a series of strategic management decisions used to address the natural variability in pasture production throughout the year.
The primary objective of this lecture series is to understand the different types of supplements, the reasons for their use, and the biological and economic responses they elicit in grazing animals.
Future lectures in this series will focus specifically on: - Mineral supplementation (with a focus on phosphorus). - Concentrate supplementation.

Rationale for Supplementation: Managing Pasture Variability

Posture accumulation in subtropic grazing systems follows a distinct seasonal curve: - Summer: High production due to high water availability, favorable temperatures, and longer day lengths. - Winter/Spring: Production slows down significantly, creating a gap between forage supply and animal requirements.
Grazing Imbalances: - Under-grazing: Occurs when dry matter (DM) production is higher than the herd's intake requirements (typically in summer). - Over-grazing/Shortage: Occurs when the dry matter requirements of the herd exceed the pasture's production rate (typically in winter).
Consequences of Not Adjusting for Variability: - In periods of excess, swards become too tall, leading to lower nutritional value and lack of control over the sward. - In periods of shortage, swards become shorter than ideal, increasing weed competition and reducing overall plant productivity. - These fluctuations negatively impact animal dry matter intake and the survival/reproduction of the plants themselves.

Strategic Options for Addressing Feed Gaps

Option 1: No Change: Accepting the variation without intervention. This results in poor sward management and nutritional deficits during dry months.
Option 2: Conserving Excess Forage: Harvesting the surplus produced during summer months to fill the winter gap. - Hay: Grass is harvested, dehydrated in the field to reduce moisture concentrations, and baled. Low moisture restricts microorganisms from deteriorating the material. - Silage: Forage is harvested with higher moisture content and preserved through high fermentation of water-soluble carbohydrates, which reduces the $pH$ and prevents spoilage. - Stockpiling: Separating specific paddocks to be ungrazed during the summer, allowing them to grow for grazing during the winter or dry season. Alternatively, a lower overall stocking rate allows biomass to accumulate for winter.
Option 3: Intensive Summer Grazing + Supplementation: Maximizing pasture utilization during the summer with high stocking rates and then using concentrates to feed animals during the winter when they are either destocked or pasture is low.

Economic and Infrastructure Considerations of Supplementation

Relative Costs of Nutrition: - Grazed swards are always the cheapest source of nutrition in any grazing system, from northern to southern climates. - Conserved forages (silage and hay) harvested on-farm are more expensive than grazed grass but cheaper than outsourced supplements. - Grains and external commercial products are the most expensive sources per unit of metabolizable energy.
Infrastructure and Labor: - Transitioning from grazing to supplementation requires different infrastructure and increased labor. - Trough Space Requirements: - Mineral supplements: Require small troughs, approximately $1\,cm$ to $3\,cm$ per head. - Concentrate supplements ( $0.6\%$ to $1.0\%$ of body weight): Require significantly larger trough space to avoid competition, usually $15\,cm$ to $20\,cm$ per head. - Efficiency of high-cost supplements (like grains or silage) must not be compromised by poor infrastructure, as performance losses can negate financial investments.

Impact of Fertilization on Seasonal Feed Gaps

Fertilization (e.g., using nitrogen at levels from $195\,kg\,ha^{-1}$ up to $400\,kg\,ha^{-1}$ ) increases overall annual productivity.
However, fertilization does not decrease seasonal variability; it actually widens the gap between summer and winter production.
Because nitrogen response is highest when moisture and temperature are optimal (summer), the difference between peak production and winter dormancy becomes more extreme.
Therefore, more intensive fertilized systems require more robust supplementation strategies to manage the resulting larger feed gaps.

Nutritional Challenges in Northern Australian Systems

Climate and Pasture Quality: Rainfall is often concentrated in a short period, leading to a spike in dietary crude protein at the start of the season followed by a sharp decline as plants mature.
Protein Threshold: Rumen function generally requires a minimum of $6\%$ dietary crude protein (CP).
Observations on Native Grasses: - Buffel Grass: Represented as white circles in data, often drops below the $6\%$ threshold during the dry season. - Mitchell Grass: Represented as black dots, also shows significant portions of the year where animals are restricted by CP content.
Mineral Deficiencies: Large areas of Northern Australia (identifiable on phosphorus maps in red) have acute soil phosphorus deficiencies. In these regions, animals are highly likely to show positive responses to phosphorus supplementation.

Classification of Supplements

Conserved Forages: - Hay (preservation by dehydration). - Silage (preservation by fermentation/low $pH$ ). - Stockpiled pasture (standing hay/accumulated biomass).
Concentrates: - Grains (corn, wheat, barley). - Byproducts (copra meal, cottonseed meal, molasses). - Commercial pellets (pre-made combinations of grains and byproducts). - Urea (non-protein nitrogen source).
Minerals: - Loose licks. - Mineral blocks. - Rumen bullets. - Injections.

Animal Intake Responses to Supplementation

There are three distinct relationships between supplement intake ( $X$ -axis) and total intake ( $Y$ -axis, comprising herbage + supplement):

Supplementation Response: - Total intake increases as supplement intake increases. - This occurs when pasture intake is already severely restricted by low forage availability (e.g., drought feeding).
Substitution Response: - As supplement intake increases, forage (pasture) intake decreases, but total intake still increases. - This is the most common response when feeding grains or protein meals to animals with high forage availability.
Complementation Response: - As supplement intake increases, pasture intake also increases, leading to a significant jump in total intake. - This occurs when the supplement fixes a primary nutritional limitation, such as phosphorus or urea deficiency in dry, low-protein pastures.
Toxicity and Limits: Supplementation does not increase intake indefinitely. For source like urea, excessive intake becomes toxic, leading to a sharp decrease in animal intake.

Interaction Between Sward Height and Supplementation

Research indicates that sward structure significantly impacts the efficiency of supplementation: - At low sward heights (e.g., $2\,cm$ or $3\,cm$ ), total dry matter intake increases as concentrate (e.g., $0.5\,kg$ to $1.0\,kg$ ) is added. - However, if forage availability is high, the marginal gain from supplementation decreases because the animal is already near its intake limit.
Stocking Rate and Gain per Area: - High stocking rates on short swards typically reduce individual average daily gain (ADG). - Supplementation allows managers to increase stocking rates while maintaining target ADG, thereby increasing the total meat production per hectare ( $kg/ha$ ). - In research trials, animals on short swards ( $15\,cm$ ) with high supplementation ( $0.6\%$ BW) achieved similar ADG to animals on tall swards ( $35\,cm$ ) with no supplement, but produced much higher weight gain per area due to the higher animal density.

Consequences of Withholding Supplements and Compensatory Growth

Growth Paths: Animals without supplementation during dry seasons take much longer to reach mature market weight.
Compensatory Growth: - Animals restricted in the dry season often exhibit faster growth rates during the following wet season compared to previously supplemented animals. - However, this compensation is rarely complete; the animals may "catch up" slightly but usually remain lighter than those that were never restricted. - Dependency on Pasture Quality: Compensatory growth only happens if the animal has access to high-quality and high-quantity pasture during the recovery phase. If the sward structure is poor at the start of the wet season, the animal will follow a growth path parallel to supplemented animals rather than a steeper "catch up" path.

Economic Analysis of Supplementation Strategies

The decision to supplement depends on the Cost-Benefit relationship, often analyzed through gross margin modeling.
Key Variables: - Live weight price of cattle (e.g., market value in Darwin for live export). - Price of the supplement per ton. - Biological response (how much extra weight the animal puts on per units of supplement).
Example Scenarios (Modeling from Northern Territory): - If cattle sell at $\$3.00/kg$ , a farmer can afford to pay up to $\$600/ton$ for supplement to break even. If the supplement costs more, they lose money. - If cattle sell at $\$4.50/kg$ , the farmer can afford to pay nearly $\$900/ton$ for the supplement and still remain profitable.
Summary: The higher the value of the final product, the more a producer can justify investing in supplements to increase productivity and shorten the path to market.