In fact, the main machines responsible are the harvesting equipment, such as oversized combines and, especially, the grain trailers that move around all over the fields. That's problematic. There are also, as you mentioned, the manure tanks.
Recent studies conducted by Agriculture and Agri-Food Canada have shown that mobile manure spreading booms are technological pieces of equipment that significantly reduce the axle load.
The problem is that equipment that is too heavy will compact the soil to a depth of about one metre. However, very few people measure soil health to this depth. As a result, the problem is difficult to pinpoint. It manifests as a loss of soil drainage capacity and a loss of surface aeration, which, in our opinion, is the cause of the productivity losses we've started to observe.
This phenomenon has been observed not only in Quebec. It's also been observed throughout the “corn belt” region of the U.S.