Actually 25 hp is the cutoff. Kubota sells a 24.8 without the DPF and a 31 with it. Note that neither uses DEF (diesel exhaust fluid). A mower is about the best application to have a regen filter on. It spends 99% of its time at full throttle, operating temp and a decent load on it. Very little PM (particulate matter, the soot that the filter catches) is produced like that. Slow speed, cool engine temps and throttling up and down are what gives a DPF fits.