Mineral Supplementation: Focus on Phosphorus

Introduction and Learning Objectives

This lecture focuses on mineral supplementation, specifically focusing on Phosphorus ( $P$ ) supplementation for grazing ruminants.
The primary learning objectives include: * Understanding the hierarchy of nutrient importance for grazing ruminants. * Identifying why Phosphorus is the most relevant mineral for grazing ruminants in Australia, particularly in Northern Australia. * Analyzing the impact of Phosphorus on growing animals. * Evaluating the impact of Phosphorus on breeder performance. * Reviewing specific diagnostic tools and supplementation strategies.

The Law of the Minimum (Liebig's Law)

Concept Definition: The "Law of the Minimum" states that the growth or production of an organism (plant or animal) is limited by the nutrient that is in the shortest supply relative to its demand.
The Barrel Analogy: * Production (e.g., live weight gain) is represented by the amount of water a barrel can hold. * Each stave of the barrel represents a specific nutrient (e.g., Nitrogen, Phosphorus, Potassium, energy, protein). * The level of production is limited by the shortest stave; increasing other nutrients will not increase production if the limiting nutrient is not addressed.
Application to Animals: * If production is limited by crude protein, adding more energy or minerals will not improve performance. * In grazing systems, the limiting factor can also be environmental, such as temperature or soil moisture. * Producers must identify the most limiting nutrient for their specific production system to ensure efficient supplementation.

Hierarchy of Nutrient Importance

In most grazing ruminant systems, the hierarchical order of limiting factors is typically: 1. Metabolizable Energy ( $ME$ ) 2. Crude Protein ( $CP$ ) 3. Phosphorus ( $P$ )
Exceptions to the hierarchy: * While Potassium ( $K$ ) and Calcium ( $Ca$ ) often have higher raw requirements than Phosphorus, they are rarely limited in forage. * A notable exception is dairy cows in peak lactation, where Calcium requirements may exceed Phosphorus and become the primary mineral limitation. * In the beef and sheep industries, especially in Northern Australia, Phosphorus is frequently the most limiting mineral nutrient.

Physiological Roles and Metabolism of Phosphorus

Biological Importance: * Phosphorus is a component of DNA and RNA. * It is essential for biochemical reactions related to energy metabolism, specifically as a part of Adenosine Triphosphate ( $ATP$ ). * It facilitates all energy transactions within the physiological responses of the animal.
Absorption and Distribution: * Ruminants obtain phosphorus solely through feed (pasture, supplements, mineral mixes). * Phosphorus enters the rumen but is not absorbed there; absorption occurs in the small intestine. * From the small intestine, it enters the bloodstream (plasma) and is allocated to different body parts based on demand.
The "Bone Bank" Concept: * The skeleton serves as a metabolic reservoir for Phosphorus and Calcium. * Surplus Stage: When intake of energy, protein, Phosphorus, and Calcium exceeds immediate demand, the body stores these minerals in the bone. * Deficit Stage: When dietary supply is lower than physiological demand, the body mobilizes (consumes) minerals from the bone to supply vital organs and functions. * Key demand centers include soft tissues (muscles) and, most critically, milk production in lactating cows.

Rumen Microorganism Requirements

Phosphorus is required not only for the animal's body but also for the microorganisms within the rumen.
The required concentration for microbial activity is approximately $50$ to $80\,\text{mg/L}$ of rumen fluid.
If concentrations fall below this threshold: * Microbial growth slows down. * Digestibility of forage may be negatively affected. * Note: While microbial activity is affected, the most significant impact of low Phosphorus is the reduction in the animal’s voluntary feed intake (appetite).

Quantitative Requirements for Phosphorus

Requirements increase proportionally with the performance level of the animal.
For a $210\,\text{kg}$ non-castrated male Bos indicus: * At a gain of $0.3\,\text{kg/d}$ , Phosphorus requirements are lower than at a gain of $1.3\,\text{kg/d}$ .
Mobilization Capacity: * A breeder weighing $400$ to $500\,\text{kg}$ has a skeleton containing approximately $3\,\text{kg}$ of bone mineral. * Within this skeleton, roughly $0.9\,\text{kg}$ of Phosphorus is available for mobilization during periods of high requirement.
Lactation Demands: * Lactation significantly spikes Phosphorus needs. * A beef breeder producing $5\,\text{L}$ of milk per day has a much higher requirement than a dry animal. * Estimation formula: $1.6\,\text{g}$ of Phosphorus is required per Liter of milk produced ( $1.6\,\text{g P/L}$ ). * Milk production peaks around $30$ to $40$ days post-calving; Phosphorus and Calcium demand is highest during this peak.

Bone Dynamics: Formation and Resorption

Bone Turnover: The continuous combination of bone formation and bone reabsorption.
Osteoblasts: Cells responsible for producing new bone and storing minerals (bone formation).
Osteoclasts: Cells responsible for the resorption (breakdown) of bone matrix to release nutrients into the bloodstream.
Biochemical Markers: * When osteoclasts degrade the bone matrix, they release a component called $CTX1$ . * Elevated concentrations of $CTX1$ in the blood indicate high levels of bone turnover/resorption.
Bone Density Assessment: * Bone biopsies (e.g., from the Tuber coxae) allow researchers to measure bone elongation at the growth plate and density in the trabecular bone. * Trabecular bone density increases with mineral surplus and decreases during mobilization.

Impact of Phosphorus on Growing Animals: Trial Data

Trial Parameters: * Subjects: Bos indicus steers ( $227\,\text{kg}$ live weight). * Basal Diet: $110\,\text{g}$ Crude Protein ( $11\%\,CP$ ), $65\%$ dry matter digestibility (representative of wet season pasture). * Variable: Increasing levels of Phosphorus vs. No Phosphorus. * Duration: $172$ days.
Observation Results: * Dry Matter Intake (DMI): Animals on low Phosphorus diets showed a significant reduction in DMI over time. High Phosphorus treatments had vastly higher intake. * Bone Elongation: The highest Phosphorus treatment group grew significantly taller (measured via hip height) than all other groups. * Correlation: There was a very high linear correlation between Phosphorus intake and DMI ( $R^2$ values were notably high), and between Phosphorus intake and live weight gain.

Diagnostic Methods for Phosphorus Deficiency

Plasma Inorganic Phosphorus (PIP): * Measures the concentration of Phosphorus in the bloodstream. * Highly correlated with actual Phosphorus intake; considered one of the best diagnostic methods.
Fecal Phosphorus: * Used as an alternative, though it is less precise than PIP ( $R^2 = 0.69$ ).
Urine Phosphorus: * The relationship between intake and urinary excretion is weak, making it an unreliable diagnostic tool.
Current Recommendation Protocols: * Test a minimum of $25$ animals. * Combine PIP measurements with an assessment of dietary dry matter digestibility. * Colwell Phosphorus (Soil Test): If Colwell P is below $6$ to $8\,\text{mg/kg}$ , Phosphorus deficiency in the herd is likely.

Impact of Phosphorus on Breeders: Pen Study Results

Trial Parameters: * Subjects: First-calf pregnant Droughtmaster heifers ( $370\,\text{kg}$ ). * Treatments: Combinations of High P/Low P during pregnancy ( $14$ weeks) and High P/Low P during lactation.
Pregnancy Phase Findings: * Low P cows mobilized bone to meet the Phosphorus requirements of the fetus. * Low P cows showed higher $CTX1$ concentrations (indicating bone resorption) and thinner cortical bone. * Birth weights of calves were generally unaffected by the cow's P status during pregnancy.
Lactation Phase Findings: * Phosphorus requirement during lactation is approximately $22.6\,\text{g/d}$ . * Low P cows throughout the whole trial lost massive amounts of body weight and had calves with much lower live weight gains. * Cows with High P during pregnancy but Low P during lactation maintained better milk production by mobilizing "stored" bone minerals (demineralization visible in biopsies as blue sections around the bone).

Long-term Impact in Grazing Environments: Northern Territory Trial

Context: $5$ -year study in the Northern Territory on Bos indicus heifers in a low-Phosphorus soil environment.
Experimental Groups: Phosphorus supplementation (year-round) vs. No Phosphorus.
Performance Outcomes: * Weight: Supplemented cows were $90\,\text{kg}$ heavier. * Height: Supplemented cows were significantly taller. * Condition: Non-supplemented cows showed visible ribs, low body condition scores, and dull coats. * Pregnancy Rates: Supplemented cows had pregnancy rates up to $63\%$ units higher ( $80\%$ vs. $20\%$ in some years). * Weaning Weight: Calves from supplemented cows were $33\,\text{kg}$ heavier. * Mortality: Mortality was $12\%$ higher in the non-supplemented group.
Economic Return: Each dollar invested in Phosphorus supplement returned $7.8$ dollars in production ( $1:7.8$ ROI).

Summary and Strategic Recommendations

Prevalence: Phosphorus is the third most limiting factor in Australian ruminant systems (after energy and protein).
Performance Impact: Deficiency directly reduces DMI and live weight gain across all animal classes.
Breeders: Lactating breeders have the highest requirements; continuous reliance on bone reserves without replenishment leads to increased mortality and reproductive failure.
Supplementation Timing: * Ideally, supplement year-round in Phosphorus-deficient country. * Wet Season: Essential for building bone reserves when energy and protein are high. * Dry Season: Must associate Phosphorus with protein (Non-Protein Nitrogen/Urea) specifically to address the Law of the Minimum (as protein becomes the primary limitation).
Economic Caution: While Phosphorus supplementation is highly profitable in deficient areas, feeding it where it is not required will negatively affect profitability.