Spreader Uniformity & CV from Tray Catches

CV% is the standard deviation of the tray catches divided by their mean, times 100. The tool takes every catch weight you enter, finds the mean, computes the spread around it, and reports CV% along with the mean catch and a uniformity rating. A lower CV means a flatter, more even spread pattern across the swath.

As a rule of thumb, below 15% is good, 15–25% is fair, and above 25% is poor and likely to show as stripes in the crop. Granular nitrogen on visible crops is judged more strictly than lime or P and K, where slightly higher CVs are tolerated. The tool flags the band your result falls in.

Lay a row of identical collection trays at even spacing across the full working width, drive the spreader over them at your normal speed and rate, then weigh the contents of each tray. Enter the catch weights in order. The trays sample the spread pattern, so spacing and consistency matter — use the same trays and a level surface.

More trays give a truer picture; many test kits use a tray every half metre or metre across the swath, giving roughly 15–30 trays for a wide spreader. At a minimum sample enough points to capture the shape of the pattern from one edge, through the centre, to the other edge — too few trays can hide a streak.

Uneven spreading puts too much fertiliser in some strips and too little in others. The over-fertilised strips can lodge, burn or waste nutrient, while the starved strips yield less — and the eye sees the difference as green and pale stripes. Because the crop responds to the low strips, poor uniformity quietly caps the whole field's yield.

Common culprits are worn or wrongly set spinner vanes, the wrong disc or impeller for the product, an off-centre or wrong drop point onto the disc, segregated or damp fertiliser, incorrect bout (swath) width, and driving too fast. The tray test tells you the pattern is bad; adjusting these one at a time and re-testing tells you what fixed it.

Yes — broadcast spreaders rely on overlap between passes to even out a pattern that is heaviest in the centre and tapers at the edges. Run too wide a bout and the tapered edges don't overlap enough, leaving low strips between passes. The right bout width for your machine and product is what turns an individual pass into an even field.

Any consistent unit — grams, or even tray counts — works, because CV% is a ratio of spread to mean and is unit-free. Just weigh every tray in the same unit and enter them in order across the swath. The mean catch is reported in whatever unit you entered.

The CV and mean are exact for the catches you enter, but they only describe that one test run. Pattern can shift with product, humidity, fill level and wear, so re-test after changing product or settings and ideally a few times a season. Use the result to set up and verify the spreader, not as a one-off certificate.