Here is the case for R1/ag tires:
1) Bar tires are designed for grip in mud. Grip well on wet flatland. Grip marginally better than R4s on side slopes. Standard R1 ag tires are 4-ply. R1/ag tires on a Grand L may be 6-ply ~~ I do not know, but Kubota does not skimp on Grand L components.
Here is the case for R4/industrial tires:
R4s are at least 6-ply. The front R4s on my
L3560 came standard with 10-ply tires.
Six-ply tires are 50% stronger than 4-ply tires. R4s squat very little under heavy Loader loads, a time when R1/ag tire bulging sidewalls are vulnerable.
Tougher construction is good working around burn piles.
Both R1 and R4 rear wheels provided on Grand Ls have two part rimes, so both R1 and R4 rear wheels can be adjusted for rear wheel spread.
R4s are much smoother cruising at 12 - 18 mph on hard surface roads, relative to ag tires, which vibrate. R4s are much more wear resistant on hard surface roads, relative to ag tires. ( I wore out my original R4 front tires at 1,800 engine hours. Fronts, of course, revolve much faster than larger rear tires. At 1,800 engine hours R4 rear tires were 70%.)