19 U.S. Code § 391 - Ascertainment of duties on grain
For the purpose of estimating the duties on importations of grain, the number of bushels shall be ascertained by weight, instead of by measuring; and sixty pounds of wheat, fifty-six pounds of corn, fifty-six pounds of rye, forty-eight pounds of barley, thirty-two pounds of oats, sixty pounds of peas, and forty-two pounds of buckwheat, avoirdupois weight, shall respectively be estimated as a bushel.
Source(R.S. § 2919.)
R.S. § 2919 derived from act July 18, 1866, ch. 201, § 38,14 Stat. 187.