TAXATION ‑- EXCISE ‑- FARM CUSTOM WORK ‑- ASSESSMENTS FOR PRIOR YEARS Persons who farm and who also engage in the business of doing custom farm work are liable for business and occupation tax upon the gross income of such custom work.In the absence of fraud, the commission may assess taxes for all years subsequent to 1946 against taxpayers who should have, but failed to register as required under the revenue act.