Propane is an excellent alternative fuel for farm equipment and was often offered as an OEM tractor fuel system. The conversions available to automotive applications are also applicable to tractors. These conversions are also simpler to do because farm equipment often still use carburetors rather than fuel injection and hood clearance is less of an issue.
Another advantage with agricultural propane conversions is the absence of emission controls and testing. Open-loop (no exhaust O2 sensor & electronic controls) conversions are the simplest conversions to do. Air valve and venturi carburetor conversions are available in both mono and dual-fuel systems. Closed-loop operation can potentially result in reduced fuel consumption with the resulting quicker payback.
A simple way to fuel tractors with propane is to add a DOT lift truck propane cylinder. DOT cylinders are available from Manchester Tank in a variety of sizes in both steel and aluminum construction. The largest tank holds 43.5 lbs of propane and an aluminum tank weighs 25 lbs for a total weight of 68.5 lbs while the steel tank weighs 87.5 lbs. A 43.5# tank holds 9.9 gallons of propane and the equivalent energy of 7.2 gallons of gasoline. These tanks could be changed out with fresh cylinders in the field as they are consumed. For larger equipment that would use several 43.5# tanks per day, it might make more sense to use larger ASME tanks that are readily available in sizes as large as 87 gallons of propane capacity.
Many farms do not have natural gas available to them and instead have a number of bulk propane cylinders on-site for heating purposes. It is possible to add a propane station motor-fuel dispenser to the bulk tanks to refuel propane-fuelled farm equipment. This would allow the farmer to buy propane at bulk rates and would allow the refuelling of much larger automotive-style ASME tanks. While the capital cost of a motor fuel dispenser is another yet another expense, the lower cost of bulk fuel can potentially pay for itself quite quickly. Sometimes, the cost of the dispenser can be rolled into the cost of the fuel and this should be discussed with your propane supplier. Custom-built propane stations are also available.