Seeding Rate Calculator Based on Kernel Weight
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Understanding Seeding Rate Calculation by Kernel Weight
Calculating seeding rates based on volume (bushels per acre) is an outdated practice that can lead to significant economic loss and inconsistent crop stands. Seed sizes vary dramatically between varieties and even within the same variety depending on growing conditions. By using the 1,000 Kernel Weight (TKW) method, farmers can ensure they are planting the exact number of seeds required to achieve their target plant population.
Why TKW Matters
Two lots of wheat seed might both weigh 60 lbs per bushel, but one might have small seeds (30g TKW) and the other large seeds (45g TKW). If you plant both at 120 lbs/acre:
- The small seed lot results in ~1.8 million seeds/acre (potentially too thick, leading to lodging).
- The large seed lot results in ~1.2 million seeds/acre (potentially too thin, reducing yield potential).
Calculating by weight ensures you plant a specific number of seeds, regardless of their individual size.
The Calculation Formula
To calculate the seeding rate in pounds per acre, the calculator uses the following logic:
Seeding Rate (lbs/ac) =
(Target Population × TKW) ÷ (Expected Survival % × 453.6)
Where:
- Target Population: The number of living plants you want per acre.
- TKW (1,000 Kernel Weight): The weight of 1,000 seeds in grams.
- Expected Survival: Calculated by multiplying Germination % by Emergence %. This accounts for seeds that don't sprout and seedlings that don't survive soil conditions.
- 453.6: The conversion factor from grams to pounds.
How to Determine 1,000 Kernel Weight (TKW)
If your seed tag does not list the TKW, you can calculate it yourself:
- Count out 1,000 seeds randomly from your seed lot.
- Weigh them in grams using a precise scale.
- Repeat this process 3 times and take the average.
Pro Tip: Always account for emergence loss. Even with 98% germination, field conditions (cold soil, crusting, depth issues) might reduce actual emergence to 85-90%. Inputting realistic emergence numbers prevents under-seeding.
Example Scenarios
Scenario 1: Spring Wheat
Target: 1,300,000 plants/acre
TKW: 34 grams
Germination: 95%, Emergence: 90% (Total Survival = 0.855)
Result: Approx 114 lbs/acre
Scenario 2: Soybeans
Target: 150,000 plants/acre
TKW: 160 grams
Germination: 90%, Emergence: 95% (Total Survival = 0.855)
Result: Approx 62 lbs/acre
function calculateSeedingRate() {
// 1. Get Input Values
var targetPop = parseFloat(document.getElementById('targetPop').value);
var tkw = parseFloat(document.getElementById('tkw').value);
var germination = parseFloat(document.getElementById('germination').value);
var emergence = parseFloat(document.getElementById('emergence').value);
var fieldSize = parseFloat(document.getElementById('fieldSize').value);
var seedCost = parseFloat(document.getElementById('seedCost').value);
// 2. Validate Inputs
if (isNaN(targetPop) || targetPop <= 0) {
alert("Please enter a valid target population.");
return;
}
if (isNaN(tkw) || tkw <= 0) {
alert("Please enter a valid 1,000 Kernel Weight.");
return;
}
if (isNaN(germination) || germination 100) {
alert("Please enter a valid germination percentage (1-100).");
return;
}
// Emergence is optional in some contexts, but we treat it as 100% if left empty or handle logic below
// However, the calculator implies it is required. Let's default to 100 if empty, or validate.
if (isNaN(emergence)) {
emergence = 100; // default if not provided
}
// 3. Calculation Logic
// Calculate Total Survival Rate (Decimal)
// Example: 95% Germination and 90% Emergence = 0.95 * 0.90 = 0.855 survival
var survivalRate = (germination / 100) * (emergence / 100);
// Calculate Seeds Needed Per Acre to achieve Target Population
// If we want 10 plants, and survival is 50%, we need to plant 20 seeds.
var seedsPerAcre = targetPop / survivalRate;
// Calculate Total Weight in Grams per Acre
// (Seeds Needed / 1000) * TKW
var gramsPerAcre = (seedsPerAcre / 1000) * tkw;
// Convert Grams to Pounds (1 lb = 453.59237 grams)
var lbsPerAcre = gramsPerAcre / 453.59237;
// Calculate Totals for Field
var totalSeedNeeded = 0;
var totalCost = 0;
if (!isNaN(fieldSize) && fieldSize > 0) {
totalSeedNeeded = lbsPerAcre * fieldSize;
if (!isNaN(seedCost) && seedCost > 0) {
totalCost = totalSeedNeeded * seedCost;
}
}
// 4. Update Display
document.getElementById('resultsArea').style.display = 'block';
// Formatting numbers
document.getElementById('resultRate').innerHTML = lbsPerAcre.toFixed(1) + " lbs/acre";
document.getElementById('resultSeedsPerAcre').innerHTML = Math.round(seedsPerAcre).toLocaleString();
if (totalSeedNeeded > 0) {
document.getElementById('resultTotalSeed').innerHTML = Math.ceil(totalSeedNeeded).toLocaleString() + " lbs";
} else {
document.getElementById('resultTotalSeed').innerHTML = "–";
}
if (totalCost > 0) {
document.getElementById('resultCost').innerHTML = "$" + totalCost.toLocaleString(undefined, {minimumFractionDigits: 2, maximumFractionDigits: 2});
} else {
document.getElementById('resultCost').innerHTML = "–";
}
}