Farm use tags were an idea born out of good intentions. A farmer could use a vehicle dedicated to his farm to transport things along a road from one lot of his land to another without having to get official tags, inspections, etc.. Of course, the exception grew allowing farmers to go to market, church, etc. That might have been pushing the outer edges a little, but generally if it was limited to farmers using the vehicle occasionally it would have been fine. Of course, that's not how things worked out.
Farm use tags were among the most abused thing on the street. As I can personally attest, all sorts of people threw them on all sorts of vehicles that had never been within shouting distance of a farm. I saw trucks and SUV's parked at people's houses in towns with the tags on them. It was so bad that the standing joke was that there are two types of vehicles with farm use tags, one which was held together by bailing wire and a prayer and another that was so shiny new it hurt to look at it. Neither had seen a farm. People just didn't want to (or couldn't) get their vehicle inspected or didn't want to register the vehicle and pay taxes. I know there were actual farmers using farm tags, but I'm not sure any vehicle I saw with farm tags was one of them.
2017
Of course, the General Assembly started trying to fix this in the patchwork fashion that it usually does. In 2017, it made misuse of farm tags subject to a $250 fine [§ 46.2-613(6)(now gone)] and allowed police to ask the driver of the vehicle the address of the farm or the "real property parcel identification number" of the property [§ 46.2-665(C)(now gone)]. Shocking absolutely no one, this did not work. Still, the General Assembly left things static until 2022.
2022
In 2022, the General Assembly did away with the plastic red and white farm use tags you could buy at every gas station in the Commonwealth and required everyone to get the DMV issued, white and blue farm use tags:
However, they also punted to make the change not go into effect until 01 July 2023. This would stop a lot of the silliness. For a $15 fee, the owner gets a permanent farm use tag after showing he actually had a farm and insurance. Things looked up. The plastic plates started disappearing from stores and it seemed the days of watching a brand new "farm use" F-150 blow by me on the highway were numbered.
2023
Rather than letting the law go into effect, the General Assembly bumped back the date of its enforcement to 01 July 2024. It also slipped in an exception for to the DMV plate for "vehicles having a gross vehicle weight rating [gvwr] greater than 7,500 pounds, trailers, or semitrailers used exclusively" for various specific uses. § 46.2-684.2(B).
2024
Finally, the law goes into effect. Immediately, all sorts of questions pop up as to what vehicles are exempt under the exception put in place in 2023. So, here's the best I can figure:
Trucks: When I first heard the 7,500 pound number, I did some research and found that the only street truck at that weight is the Ford F450 Super Duty Crew Cab (an 8,600 pound behemoth). Then, I had someone explain to me that "gross vehicle weight rating" means weight capacity fully loaded. Researching that, I found that my poor Toyota Tacoma doesn't qualify (nor any other Toyota), but it seems like a number of Dodge, Ford, and Chevys do.
Vehicles That Qualify
RAM: The Ram 1500 TRX is the only Ram 1500 above 7,500 gvwr (there seem to be multiple lesser 1500 models).
FORD: Only the very heaviest single model of the V8 and the EcoBoost are above 7,500 gvwr. Each has at least 4 other models that are not.
Vehicle Uses
Assuming someone has bought one of those monsters, the statute limits their use to the following circumstances:
GENERAL: (1) Cross a highway, (2) Go from one plot of farm land to another, (3) Go to mechanic, (4) Take another vehicle to mechanic, (5) Go to trash dump, & (6) Transport farmer between farm and home. § 46.2-665.
TRANSPORT GOODS: Take produce or livestock to "storage house, packing plant, or market." § 46.2-666.
WOOD: Take wood to a "sawmill or sawmill site." § 46.2-670.
COTTON, PEANUTS, FERTILIZER: Take "unginned cotton, peanuts, or fertilizer" to or from "one farm to another, from farm to gin, from farm to dryer, from farm to market, or from fertilizer distributor to farm and on return to the distributor." § 46.2-672.
RETURN TO FARM: (1) Return to farm from market, (2) Bring back food or general shopping products while out for legit purposes (see above), & (3) Transport supplies to the farm. § 46.2-673.
Probable Cause
Probable cause would seem to attach to any vehicle with a moniker less than 2500 (250 for Ford) because while there are a very few, very specific models below that which qualify the probability is that they do not. As this sort of thing would be difficult to determine on the side of the road, it would probably be an affirmative defense as well: "I know there are a hundred Ram 1500 models that don't qualify, but mine does because I specifically bought the only model that does."
It should be pretty easy for an officer to determine if a farm use vehicle is engaging in most of the activities above. If a plastic farm use tag is on a vehicle that is at the movie theater or has been driven to the second job in the city, it will be fairly obvious. A vehicle at the grocery store or fast food joint might not be so obvious; they could be grabbing some things on their way back to the farm from a legitimate trip (understanding this is a grab and go situation - not an eat in).
Conclusion
I think the cost of the mega-pickups that continue to be exempt from the required DMV farm use tag will drastically cut down the abuse of this area of the law. If you're buying a Ram 3500 you are probably using it for real work - not just as a vehicle you are trying to get out of legally registering as you drive around town. Do I think they should have made no vehicles exempt? Sure. Still, 80% of a loaf is better than no loaf at all and if I see a mega-pickup with mud on it I'm more likely to believe it's an actual work vehicle.