Late winter is bale-opening season on most mixed farms, and that means tramp metal season. The silos are drawn down, the bale count is rising, and somewhere in that stack of round bales is a bolt that fell off a wrapper arm two years ago, or a length of tie wire that got folded into the bale at the press. Most operators know this in the back of their minds. Not all of them have a plan for it.
The loader is not the last line of defence. It never was. By the time a loader bucket has picked a bale off the pad and tipped it into the tub, anything small enough to be embedded in the outer wrap is already in your mix.
Where Tramp Metal Actually Comes From
Silage and baled hay carry metal in predictable ways, but the sources are easy to underestimate. Round bales pick up wire from old fencing during raking. Square bales carry remnants of twine knives and broken tying needles. Silage clamps hide dropped fasteners from repair work done mid-harvest, and the inoculant spray bar is not the only thing that goes over that face during the season.
Loader buckets add their own contribution. A worn cutting edge sheds small steel fragments. A tooth that has been cracked for a month will eventually give up a piece. On concrete-floored feed pads in the Western Cape, where buckets work over hard surfaces daily, edge wear is faster and more irregular than on dirt. The fragments are small, angular, and magnetic. They stick to the base of a bale and travel with it.
Hay processing is where the geometry gets worse. The auger cuts long-stem material by folding it over the knife edge, which is exactly how you want it to work. But that folding action also traps small metal objects against the flighting and drives them into the mix. A loose piece of wire that came in on top of the bale will not float off. It moves with the material.
Wire is the most common problem. It wraps around auger flighting, causes vibration, and eventually damages knife edges or flights. Bolts and bucket teeth are less common but cause more immediate harm: swallowed hardware can kill a cow from hardware disease, a condition every dairy operator in KwaZulu-Natal knows by its consequences rather than its name.
The Magnets Auger Catch Approach: Position in the Flow Path
The best place to intercept metal is not at the discharge door, though that is better than nothing. The best position is as close to the auger flighting as possible, where material is moving in a controlled, repeating path and any ferrous object will pass the magnet face multiple times before discharge.
Auger-mounted magnets sit in the active mixing zone. Material lifts, tumbles, and re-contacts the auger on every revolution. A magnet mounted at or near the base of the auger flight intercepts the same load repeatedly. Wire that entered at the top of the tub will work its way down through the mix and eventually reach the magnetic field. That repeated pass is what makes auger placement more reliable than a single-point door check.
Door frame magnets serve a different purpose. They are the second screen, positioned at the point where mixed material exits the tub. Anything the auger magnet did not capture has one more chance to be pulled out before it flows into the delivery wagon or straight to the bunk. The two positions are complementary, not interchangeable.
On LEO Agriculture‘s mixer range, the Generation 2 magnets mount to the door frame and are available on all models. Machines in the 7002 through 7006 VT range have one magnet position; the 7008 VT, 7010 VT, 7012, and larger models carry two magnet positions. Where two positions exist, both should be filled. A single magnet on a high-capacity machine leaves half the door width unguarded. The blank-off plates that ship in place of magnets when the option is not ordered are a practical reminder of how the system is designed: the position was engineered in, not bolted on as an afterthought.
Magnets can be added to machines already in service. If your current mixer did not ship with them, that is a retrofit worth doing before the next bale season runs hard.
What Magnets Do Not Catch
Aluminium, brass, and hardened stainless steel are non-ferrous and pass straight through. Plastic twine, net wrap fragments, and rubber all go through too. This is not a criticism of the magnet system; it is a reason not to treat magnets as a complete solution on their own.
A silage additive applicator tube that fell into the clamp face will not stick to anything. A length of bale net wrap can wrap an auger flight just as effectively as wire can. Good housekeeping at the feed pad, a habit of checking loader bucket edges regularly, and consistent bale inspection before loading are not replaced by any in-mixer screen. They are still the first filter.
Inspection After a Suspect Batch
One tramp metal find does not mean the problem is solved. It means the magnet worked once. The question is what else came in on the same load.
A simple post-batch routine takes about four minutes and prevents a lot of downstream problems.
- After discharge, before the next load, pull each magnet and inspect the collected material. Lay it on a white surface if you need to see small fragments clearly. Identify the source if you can: tie wire has a characteristic twist, bucket-edge fragments are irregular and chunky, wrapper wire is thinner and more uniform.
- Note which magnet position caught the material. Auger position only, or door frame too? If both are loaded, the incoming contamination was heavy and you need to find the source bale or silage section.
- Check the knife edges and auger flighting for scoring or deformation. A bolt that passed through a knife edge leaves a mark. Catching that early means a blade swap rather than a flight repair.
- If wire was found, run the next batch dry for a short cycle and re-inspect. Wire fragments can snap and leave secondary pieces behind. One piece found rarely means one piece total.
Wipe the magnets clean before re-installing. A magnet face that is already loaded with fine iron dust has significantly reduced capacity to hold new material on the next batch.
Frequency and Seasonal Timing
Most operators check their magnets weekly during low-risk periods and forget about them during high-risk ones. The seasons are backwards. Bale-intensive periods, late winter into spring when hay reserves are being worked through, are exactly when magnet inspection should move to daily or per-batch.
In the Free State, where large-scale backgrounding operations run big loads through mixers at high throughput, the window between loading a contaminated bale and pushing ration to the pen can be under an hour. Daily inspection at minimum is not excessive in that context.
Magnet condition matters too. Permanent magnets lose holding strength gradually, particularly if they are repeatedly dropped or exposed to high heat. If your magnets are several seasons old and you have not tested their pull strength recently, it is worth doing. A magnet that looks fine visually may be holding a fraction of its original rating.
System Thinking: The Mixer as the Last Screen
There is a useful way to think about this. Every handling step between the field and the bunk is a potential metal entry point, and every handling step after that is a chance to intercept it. The loader is not a screen. The tub is one. The door is another. If you are choosing a vertical TMR feed mixer and magnet positions are not in the specification, they should be a condition of purchase rather than an afterthought.
Feed delivery after the mixer is a separate risk layer. Once the ration has discharged cleanly, it needs to stay clean through the delivery wagon and into the bunk. Any hardware that made it through the mixer without being caught is now in free-flowing mixed feed. The auger kicker action that improves mix quality also distributes any remaining metal evenly through the batch. That is an argument for catching it before discharge, not after.
The auger innovations that improve cutting efficiency and material flow through a vertical tub also change where metal travels inside it. Understanding that flow path is part of getting the most out of a well-designed auger system and is directly relevant to magnet placement.
Magnets are not expensive. They are not complicated. What they require is consistent attention, particularly now, when bale stocks are running low and operators are pulling from the back of the stack where bale quality and unknown history are harder to track. The wire that has been sitting in a net-wrapped round bale for eighteen months is just as capable of causing hardware disease as a fresh contaminant. The magnet in the door frame does not know the difference.
