Blade Change Decisions Are Not Cutting It

Clemson University’s precision ag team searches for an objective way to determine when to change digger blades.


Scooping up and slicing the taproot is the start of peanut harvest, and blades are a primary part of this.

“Blades should be sharp so they can slice through the soil and roots cleanly and smoothly,” says North Carolina State University Extension peanut specialist David Jordan in the 2024 NCSU Peanut Information. “The blade should be nearly flat with a slight lift in the rear, typically ½ inch.

“Most peanut blades have a smooth side and a beveled side. The beveled side offers more aggressive digging action for harder soils,” he says. “The smooth side works best in softer soils. Blades with hardened steel edges are also available. These blades promise extended life and the ability to maintain a sharper edge. Check blade condition frequently, and replace as soon as they become worn.”

Jordan says both blades and the nearby rods that help guide vines onto the shaker belt will wear differently. “They will need to be monitored to ensure that they are working properly.”

Weighing The Costs

Although this may sound fairly straightforward, Kendall Kirk, a Clemson University agricultural engineer says a good, objective way to know when to change out blades doesn’t exist, and the decision comes with costs either way.

“If you change your blades out too early or too soon, then your blade costs on a per-acre basis is going to be higher than it should be. But, if you are getting additional losses because you wait too long or too late to change them out, then your yield losses become higher,” he says. “Is there an opportunity to optimize this?

“That’s what we are working on. Can we objectively figure out when to change blades?” Kirk says of Clemson University’s precision ag team.

Whether it’s a general suggestion, an app or other method, he says the economics of blade replacement make the decision a complex one.

Soil Texture Is A Primary Wear Factor

Although this study found that the blade costs and yield loss intersected at 40 acres for the optimal time to change blades, Kirk says not all wear is the same. It is a much more complicated decision that depends greatly on soil type.

In one study, Kirk says they took 16 sets of blades that had varying levels of wear.

“Some of these were blades from local farmers’ machines that we knew had a good many acres on them from familiar soil types,” he says. Knowing the conditions under which they were used, Kirk says they considered these to be controlled wear in a controlled environment, but there was no relationship between wear and yield loss on the light-textured soils.

“Early data in a light-textured soil showed no real relationship, but heavier-textured soil did show much more of a correlation,” he says.

“On heavier-textured soils, there is a definite reduction in yield for every additional acre you run your digger through beyond optimal blade change. By our calculations, you will have an additional 1.7 pounds per acre of losses. While that’s not that much, it will add up.”

However, Kirk says this was a heavier soil type for peanuts to be grown in anyway.

“If you flip that data around for peanuts, we looked at what the yield loss would be as a function of acres that are on those blades. If you factor in the price of $400 per ton on your peanuts, then you can calculate that loss in dollars per acre,” he says.

Finding The Intersection Of Blade and Loss Costs

“That’s one piece of the puzzle. But there’s also the cost of changing blades too early. Using $270 for six blades and distributing that cost across 10 acres, that’s $27 per acre for all those acres covered with that blade. As you move out in time, those costs on a per-acre basis get lower and lower. The problem is you can’t say, ‘Well, it’s cheaper if I run them longer,’” he says.

“Eventually, losses start creeping up on you, and the further you go, the more losses you are incurring on those blades.”

Kirk says it is the sum of these two lines that is critical.

“These are the blade-associated costs, the sum of the blade costs plus the loss costs,” he says. “In this study, it turned out that 40 acres was the optimal time to change them.”

However, Kirk says he is not saying everyone needs to change blades at 40 acres.

“Our wear and your wear are different. That’s why this is a much more complicated problem,” he says. “In light soils, we saw no relationship; in heavier soils, we saw a relationship. Ten acres in this heavy soil will have a tremendous amount of wear, whereas 10 acres in sugar sand might not look very different than a new blade. So, this gets more complicated.

“That’s what we are trying to dig into, so to speak. We are trying to find an objective way to know, but it is still a work in progress.”

Keep Using The Windrow For Now

Currently, watching the window for proper inversion is the best way to tell when blades need changing. Dull blades will tend to drag plants forward, causing them to ride the inbound on the conveyor, not engage properly with the star wheels and rods and invert improperly.

Kirk says for now, growers can look back at the windrow for signs of a blade problem.

“What tends to happen when a blade gets dull is that it drags the plant forward. Because your blades are banked at a roughly 45-degree angle, it tends to drag plants forward, and when they do, they slide inwards a little bit. So, you’ll see them riding the inbound on the conveyor.”

When the plants do this, Kirk says they don’t hit the star wheels right.

“They don’t engage properly with the star wheels, and they don’t engage properly with the rods, so inversion gets off kilter, and that’s the thing that tips you off, right? As the operator of that machine, you often go by the quality of the windrow, and that’s your tip off that you need to swap the points out.

“However, it may be the case, though, that you should have done that 20 acres ago. But you didn’t know. That’s what we are trying to get at here. Can we objectively figure out when to change blades?”

It may not cost much to go a little beyond optimum wear, but it can start to add up, he says.

Other studies Kirk and his team have done with blades involved weighing the blades, which he says he knows growers are not going to be able to do during the season, but it is part of their learning process in blade wear. Another option may be digital image analysis.

“Maybe this could be something that works out as a cell phone app,” he says. “We’re looking at all the things we can do to give you an objective decision aid to say when to change digger blades.”

Harvest Equipment Considerations

University of Georgia Extension precision ag specialist Simer Virk offers a few considerations for growers to keep in mind for proper digger setup and operation.

“Along with proper timing of digging peanuts, proper setup and operation of peanut harvest equipment is also an important factor to minimize harvest losses and to ensure peak equipment performance and efficiency during harvest,” Virk says. “Here are a few considerations for growers to keep in mind when digging peanuts to prevent any mechanically induced yield losses due to improper digger setup and/or operation.”

• Using an RTK Guidance system/auto-steer on the tractor while digging peanut helps in maintaining the digger path directly over the row center or over the planting path and results in approximately 10% reduction in yield losses compared to digging peanuts with a tractor without an auto-steer system.

• Before beginning harvest and making any adjustments specific to the harvest conditions, inspect the digger carefully for any broken, bent or missing parts as well as the sharpness of the blades. Dull blades that fail to cut the tap root will result in dragging roots or dislodging pods from the plant.

• Adjust the digging angle (and therefore depth) by adjusting the length of the top link on the digger. Digger blades should be set at a slight forward pitch and at the depth where they cut the tap root just below the pod zone. Both an excessively shallower and deeper depth of the digger blade can result in significant digging losses.

• Blade angle/depth is also dependent on soil type and texture. Any considerable change in soil type within or among the fields will also require a change in blade angle/depth adjustments as clay soils usually need a more aggressive angle whereas sandy soils require a less aggressive blade angle.

• Digging speed should be optimized based on the prevalent in-field conditions at harvest. Generally, the optimal ground speed for digging peanuts is between 2 and 3 miles per hour. Speeds above 3 mph can result in an increase in digging losses and should be avoided.

• Set the conveyor speed to match the forward travel speed of the tractor while digging peanuts. Conveyor speeds slower or faster than the tractor speed can both result in increased pod losses.

• The conveyor depth should also be adjusted where it picks up vines with its teeth just clearing the soil. Additionally, if needed, adjust the knocker wheels up or down to regulate the amount of shaking where it is just enough to remove the soil from the vines.

Remember, properly dug and inverted peanut plants will form a uniform, fluffy, well-aerated windrow with very few pods touching the soil. Make sure to keep a close eye on the digger operation in the field and adjust settings accordingly as and when needed. PG

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