Having your own semi is very important during harvest: your combine will fill up in 15 minutes. That means every 15 minutes you need to unload it. 2 combines worth of grain is as much as your can haul in a semi (because of road weight restrictions). When the farmer is harvesting they don't want to stop: every delay is a chance that a storm latter will destroy the rest of the crop. Farmers often hard hauling to a elevator 20 minutes away where there is a 20 minute line to dump. As a result it isn't unusual for a farmer to need 2 semis for every combine.

Now you can rent semis, but it isn't cheap. For a farmer the rental company will look at it as a semi that won't be rented for 10 months so they charge for it.

At least for my family's farm in OK, a lot of the labor moves and takes the equipment with them. They move north to south with the season and the farm owners don't have to own as much equipment (in our case, none of us are farmers and haven't been for a few generations so a different situation). The farm manager still farms his own land, but he uses this same labor and their equipment with own a little of his own for tending during the growing season (the most labor and equipment intensive parts are harvest and planting).

Point being, the expensive equipment that is effectively being rented (with the labor in our case) isn't sitting idle most of the year. It's being used in different areas.

That is also a common way to do it. I know of farmers who own all their own equipment. I know of farmers who hire everything done. Both are very valid and correct ways to do things. There are different tradeoffs involved in both.

Tesla Semis owned by a coop would be killer for this use case, with charging facilities at the elevator. Very similar to the runs Tesla is going to use them for from their lithium evap pond partner in NV and the Gigafactory.

How does that solve the problem that "all the farmers" need it at the same time of year, and thus one shared one isn't enough? What makes a Tesla Semi notably better than a conventional one?

Doesn't solve the contention issue by itself, but a reduction in maintenance costs (million mile warranty) and running costs make the shared coop model more tenable. Sorry I wasn't clear in my original post!

Funny fact, Tesla wanted to use a grain trailer at the presentation but couldn't find a pristine looking one.

Like clockwork.