Stocking rate is a fundamental concept in livestock management and rangeland health. It refers to the number of animal units (AU) that a specific area of land can sustainably support over a given period without degrading the forage resources. A well-managed stocking rate is crucial for ensuring animal health and productivity while maintaining the long-term health and resilience of pastures and rangelands.
Calculating the correct stocking rate helps prevent overgrazing, which can lead to soil erosion, reduced plant diversity, and diminished forage production. Conversely, understocking can result in inefficient use of available forage and potentially lower economic returns.
Key Factors in Stocking Rate Calculation:
Pasture Area: The total size of the grazing land available.
Forage Yield: The amount of usable forage produced per unit area, typically measured in tons or pounds of dry matter per acre. This can vary significantly based on soil type, climate, rainfall, and pasture management practices.
Grazing Days: The length of time livestock will be allowed to graze on the pasture.
Animal Weight: The average weight of the livestock being grazed. Larger animals consume more forage.
Dry Matter Intake (DMI): The amount of forage an animal consumes daily, usually expressed as a percentage of its body weight. This is a critical factor for determining daily forage needs.
This calculator helps estimate the appropriate stocking rate by considering these key variables. It aims to provide a starting point for land managers to make informed decisions about how many animals to graze on a given pasture for a specified duration, considering the land's forage production capacity and the animals' nutritional requirements.
function calculateStockingRate() {
var pastureArea = parseFloat(document.getElementById("pastureArea").value);
var forageYield = parseFloat(document.getElementById("forageYield").value);
var grazingDays = parseFloat(document.getElementById("grazingDays").value);
var animalWeight = parseFloat(document.getElementById("animalWeight").value);
var dryMatterIntakePercent = parseFloat(document.getElementById("dryMatterIntake").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = ""; // Clear previous results
// Input validation
if (isNaN(pastureArea) || pastureArea <= 0 ||
isNaN(forageYield) || forageYield <= 0 ||
isNaN(grazingDays) || grazingDays <= 0 ||
isNaN(animalWeight) || animalWeight <= 0 ||
isNaN(dryMatterIntakePercent) || dryMatterIntakePercent <= 0) {
resultDiv.innerHTML = "Please enter valid positive numbers for all fields.";
return;
}
// Constants
var poundsPerTon = 2000;
var animalUnitWeight = 1000; // lbs, standard for Animal Unit (AU) calculation
// 1. Calculate Total Available Forage (lbs of Dry Matter)
var totalForageTons = pastureArea * forageYield;
var totalForageLbsDM = totalForageTons * poundsPerTon;
// 2. Calculate Average Daily Dry Matter Intake per Animal (lbs)
var averageDailyIntakeLbs = (animalWeight * (dryMatterIntakePercent / 100));
// 3. Calculate Total Forage Needed for One Animal Unit (AU) over the grazing period (lbs of Dry Matter)
// An Animal Unit (AU) is defined as a 1000 lb animal consuming 26 lbs of dry matter per day (NRA/LCI standard)
// We'll calculate based on the provided animal weight and intake, then normalize to AU if needed for a standard output,
// or present it as AUs based on the calculator's inputs.
// Let's calculate how many of *our* animals can be supported.
// Total forage consumed by one animal (as defined by inputs) over grazing days:
var totalForagePerAnimal = averageDailyIntakeLbs * grazingDays;
// 4. Calculate how many of 'our' animals can be supported.
var numberOfAnimalsSupported = totalForageLbsDM / totalForagePerAnimal;
// 5. Calculate Stocking Rate in Animal Units (AU) per Acre
// First, define what one AU consumes per day for comparison if we want AU/acre.
// Standard AU intake is ~26 lbs DM/day for a 1000 lb animal.
// Let's convert our animal's consumption to "AU equivalents" if their intake is different.
// If our animal is 1000lbs and eats 2.5% of body weight = 25 lbs/day. This is close to standard AU.
// If our animal is smaller/larger or eats more/less, we should account for that.
// An AU consumes approximately 26 lbs DM/day.
var dailyForagePerAU = 26; // lbs DM per day per 1000 lb animal
var totalForagePerAUOverPeriod = dailyForagePerAU * grazingDays;
// Number of Animal Units (AU) that can be supported:
var numberOfAUsSupported = totalForageLbsDM / totalForagePerAUOverPeriod;
// Stocking Rate in AUs per Acre:
var stockingRateAUPercAcre = numberOfAUsSupported / pastureArea;
// Output
var resultHTML = "
Calculation Results:
";
resultHTML += "Total Available Forage: " + totalForageLbsDM.toFixed(2) + " lbs of Dry Matter";
resultHTML += "Average Daily Dry Matter Intake per Animal: " + averageDailyIntakeLbs.toFixed(2) + " lbs";
resultHTML += "Total Forage Consumption per Animal over " + grazingDays + " days: " + totalForagePerAnimal.toFixed(2) + " lbs";
resultHTML += "——————————————–";
resultHTML += "Estimated Number of Animal Units (AU) Supported: " + numberOfAUsSupported.toFixed(2) + " AU";
resultHTML += "Estimated Stocking Rate: " + stockingRateAUPercAcre.toFixed(2) + " AU per Acre";
resultHTML += "(Assuming 1 AU = 1000 lb animal consuming 26 lbs DM/day)";
resultDiv.innerHTML = resultHTML;
}