(a) If a trustee uses any part of the principal in the operation of a business or, except as provided in subsection (b), an agricultural or farming operation, including the raising of animals or the operation of a nursery, the net profits and losses shall be computed in accordance with generally accepted accounting principles, subject to paragraph (8) of subsection (c) of Section 14. Net profits are income; and if a loss results in any fiscal or calendar year, the loss shall be carried into subsequent fiscal or calendar years for purposes of computing income of the business.
     (b) When, as a part of the principal of an agricultural or farming operation, animals are held partly or wholly because of the offspring or increase which they are expected to produce, all offspring or increase shall be deemed principal to the extent necessary to maintain the original number of such animals and the remainder shall be deemed income, and in all other cases such offspring or increase shall be deemed income.