Why is seed size variation a concern when setting a seed treatment rate?

• A. Seed color affects coating uniformity and should drive rate calculations.
• B. Seed size has no impact on coating uniformity.
• C. Different seed sizes affect surface area and coverage; rates per weight may not provide uniform coating unless calibrated.
• D. Larger seeds automatically receive more coating without calibration.

Answer: (C)

Coating uniformity depends on surface area, and seed sizes change how much surface area you have per unit of seed mass. When you set a coating rate by weight, larger seeds have more mass but relatively less surface area per gram than smaller seeds. That means the same weight-based rate will produce a thinner coating on bigger seeds and a thicker coating on smaller seeds. If seeds in a batch vary in size, a single rate won’t give every seed the same film thickness, which can reduce protection or cause issues with handling and emergence. To achieve consistent coverage, you either calibrate the rate to the average seed size or adjust by size so the coating thickness is similar across the lot.

Some might think seed color or size doesn’t matter, or that bigger seeds automatically get more coating, but those ideas ignore the geometry of coating. The key is that surface area—not just weight—drives how much coating ends up on each seed, so variation in size makes uniform coverage harder unless you calibrate for surface area or size.