Even if his program was almost perfectly efficient he could only cut the cost of diesel by about a third for one year. While this could help a little, it is not going to save the farm because that doesn't include the diesel used for shipping.
The reality is this is an ill thought out plan that will likely be corrupted and abused. Farmers might only see a small decrease in fuel bills while Trump and his cronies take the rest home through their schemes. Eight billion more dollars gone like it was nothing.