When I was in college, the University did a study on the two and three year old cows in their dairy herd. This group of females was split into two equal groups at random, and a magnet was put into all the cows in one group, and the other group was not given magnets. Both were maintained on the same ration. Within a few weeks they saw a 12 % increase in milk production from the cows with magnets over the control group. There were no noticeable ailments in either group of cows.
When I saw this result, I thought that the same should hold true for beef cows, except the increase in milk production may be much smaller. I contacted a buyer at a slaughter plant and asked him if they collected magnets from slaughtered cows. He said that the did, and that I could buy them for $1 each. I purchased 36 dozen ( 432 magnets) for $432 and I try to put them into my bred heifers each fall. Since then , I have purchased another several dozen magnets from the same source. They are cleaned, boxed and look like new ones even though they have been inside of cows ... some of the for a few years. I am told that, on average, there is two handfuls of metal in every cow that is slaughtered. If there are sharp points on any pieces, I am sure there are minor hardware situations in many cows that go unnoticed. I have heard that it is quite amazing what they find in some cows... 6 inch nails, bolts, lots of assorted pieces of wire, occasionally a watch or other jewellery, etc etc
If you are suspicious of hardware, put the cow in a chute. Take a bar or a small fence post and put it under the cows belly, and pry upwards on her stomach. If she grunts significantly, she is probably suffering from hardware. This isn't fool proof but it is a quick test that works on most hardware cases. If in doubt, drop a magnet down her, check her temperature, and give her a fairly high dose of long acting Liquamycin ( or similar product). This will help control any infection that is starting. If the hardware has done too much damage, find a priest and give her last rights.