The Bell Effect Explained.
For the longest time, we’ve accepted that wheel track damage from sprayers is inevitable without fully understanding how much yield loss it causes. We’ve assumed that most of the damage is found inside the wheel tracks, however that may not be the case.
We can easily calculate wheel track damage by dividing the width of the tires by the width of the boom to get an approximate percentage of wheel track damage. For instance, two 18-inch tires divided by a 90-foot (1080”) boom will leave you with 3% wheel track damage. That doesn’t seem significant until you realize that yield loss can occur outside of the sprayer tracks. The term used to describe this phenomenon is called the bell effect.
The bell effect simply refers to the damage caused to rows that run outside of the wheel tracks. In most cases the damage is caused by compaction and the reduced oxygen, nutrient and water holding capacity that ensues. Australian research has shown an 18% yield reduction from wheel tracks that only covered 8% of the field. The additional 10% yield loss was coming from rows outside of the sprayer tracks. After further investigation, they noticed cracks in the soil caused by compaction that extended out from the wheel tracks to the adjacent rows then disappeared.
Spraying is the most compaction prone operation we have given the weight of our sprayers and the moisture conditions at the time of spraying. Those with high clearance sprayers that can run 24” flotation tires at herbicide timing but have to narrow down to the 15” or 18” tires at fungicide timing. Think about this. It’s wet and you decide to apply a fungicide. You lose 4% from wheel track damage during your high clearance spray application and another 10% from compaction outside of the wheel tracks. You’re trying to gain a 10% to 15% yield advantage from applying a fungicide only to lose 14% from trampling and compaction. You wonder why some folks can’t seem to find a response to fungicides? Perhaps we should take a second look at aerial applications during moist soil conditions. Food for thought!