(a) Except as provided by Subsection (a-1), it is an affirmative defense to prosecution of, or an action under Subchapter F for, the offense of operating a vehicle with a single axle weight or tandem axle weight heavier than the axle weight authorized by law that at the time of the offense the vehicle:
(1) had a single axle weight or tandem axle weight that was not heavier than the axle weight authorized by law plus 12 percent;
(2) was loaded with timber, pulp wood, wood chips, or cotton, livestock, or other agricultural products that are:
(A) in their natural state; and
(B) being transported from the place of production to the place of first marketing or first processing; and
(3) was not being operated on a portion of the national system of interstate and defense highways.
(a-1) The affirmative defense provided by Subsection (a) does not apply to the excess weights authorized under § 623.421(b).

(b) It is an affirmative defense to prosecution of, or an action under Subchapter F for, the offense of operating a vehicle with a single axle weight, tandem axle weight, or gross weight heavier than the weight authorized by law that at the time of the offense the weight enforcement officer failed to follow the weighing procedures established under § 621.402(e) when determining the weight of the vehicle.