Load cells, GPS, and rate controllers have changed the game in a good way for solid manure application, there’s still one piece of the puzzle they don’t solve. They don’t tell you how wide your spread pattern
We treat swath width as if it’s a fixed property of the spreader. But it isn’t. It’s a property of the material, the day, the setup, and the physics of throwing irregular particles into a crosswind. That’s why the single simplest thing you can do to improve manure uniformity, is to pick the right width for the material you’re spreading right now, not what you spread last year.
The Pattern Isn’t a Rectangle, It’s a Curve
When you watch a spinner or beater from behind, it looks like it’s flinging material in a straight band. But if you take a dozen catch pans and run a pattern test, the truth appears fast:
• Every dry manure pattern is a curve; generally heavy in the center, tapering at the edges.
Sometimes that curve is broad and smooth. Other times it looks like a mountain peak with almost no “shoulders” at all. Cattle bedded pack tends to be clumpy. Turkey litter might be fluffy in one load and sticky the next. Layer manure can be powdery or soupy. And because the pattern is a curve, the effective swath width; the width where two adjacent passes overlap enough to even things out, is always narrower than how far the spreader can physically throw material.
Figure 1. Example spread patterns, both at the same application rate, but one (in blue) with a perfect, uniform spread pattern and a second (in orange) with a more common spread pattern.
A machine that throws 45 feet often has an effective width of 25–30 feet, but it also depends on the material. That’s why a single “standard width” is fiction.
Why the Same Spreader Behaves Differently Every Day
Think about what determines where a manure particle lands:
• its size
• its density
• its shape
• its moisture content
• the velocity and angle it leaves the spinner or beater
• the wind it encounters
We pretend manure is consistent because that’s convenient. The reality is that even within one pile, all those variables shift. Move to a new farm, where they manage bedding and ventilation differently, and all bets are off. Swath width is not only a machine setting; it’s a material property that changes over time.
You Can’t Outsmart a Bad Width with Technology
GPS lines will keep the tractor straight. Load cells will keep the average tons per acre accurate. Rate controllers will put it together keep the gate or chain feeding consistent amounts. All good things. But none of them can fix a spread pattern that is inherently too narrow or too uneven for the width you’re driving.
Technology can keep the average rate correct. Only the correct swath width keeps the distribution correct. It’s the difference between “I applied 4 tons per acre” and “I applied somewhere between 2 and 6 tons per acre in alternating stripes.” If you’ve ever flown a drone over a dry-manure field and seen stripes, swath width, i.e., application uniformity is a potential culprit.
The Real Question: How Do You Pick the Right Width?
Pattern testing is still the gold standard, followed by fancy math to make it as good as it can be, but here’s the part we often don’t say. You don’t need perfection; it’s manure. What you do need is a sense of the shape and to understand how you are using that manure as part of your fertility program. It’s providing all your nitrogen; you better be pretty uniform. Providing about 2/3 of your nitrogen you are probably hoping everything gets covered, but it doesn’t need to be perfect. Spreading for P and K, uniform enough every plant gets some of the goodness, but trusting your soil to buffer the rest of the nutrient supply.
A simple line of tarps (or catch pans gives you the curve). Once you see the curve, pick a width where the wings from one pass overlap strongly with the wings from the next. You’re trying to turn two curves into a flat line. That’s the entire game.
Looking back at our example in figure 1, if we are using the blue spreader our effective width is easy, 45 feet. If we move over 45 feet every pass, we’ll uniformly cover the field. But what if we are that orange spreader, how far do we move over? In figure 2, I provide the uniformity of the application with different effective swaths. As path width gets smaller, uniformity bets better, but in truth at a path width around 30 to 35 feet the application gets relatively uniform.
Figure 2. Estimated application uniformity when using different effective swath widths. Closer swath widths would require lower application rates with each pass. Despite this we assumed uniformity would be similar width even with lower application rates.
Why You Shouldn’t Use the Same Width for Poultry, Cattle, Compost, or Anything Else
Each material has its own physics:
• Poultry manure tends to be finer, spreads farther but has lighter wings.
• Cattle manure often spreads short and chunky, great center, weak edges.
• Composted materials can throw evenly until you hit a clod.
• If the spread pattern changes, the effective width changes.
The Bottom Line: Swath Width Is Your Cheapest Precision Tool
When we say manage for every plant, sometimes precision agriculture has told us to think about how to manage differently for every plant, but to a large degree the base of that still starts the same. Try to treat every inch of your field the same, then pick that precision equipment and precision fertilizer to manage to that plant’s specific needs. With manure, we aren’t that precision fertilizer, at least not yet, but I’m not sure there is anything better to provide that baseline fertility as long as we are managing it right.
