United States Grade Standards for Corn

Grading is a two-step process. A lot is first assigned to a class — the standard distinguishes corn by kernel colour, recognising yellow, white, and mixed corn. It is then assigned a numerical grade determined by the grading factors measured on a representative sample. As in the other U.S. grain standards, the poorest factor governs: a lot grades no better than its worst result, so one factor can cap an otherwise sound parcel.

"U.S. Sample grade" is the residual designation. It applies to corn that does not meet the requirements of the numbered grades, or that carries specified objectionable characteristics. It is a defined outcome of the standard rather than an absence of one.

Corn is graded on fewer factors than wheat, and the set is distinctive to the commodity:

• Test weight per bushel — a bulk-density measure used as a general soundness indicator
• Broken corn and foreign material (BCFM) — a combined factor, determined by sieving and hand-picking
• Damaged kernels (total) — kernels materially damaged by any cause
• Heat-damaged kernels — a subset of damaged kernels, limited separately and more strictly

BCFM is worth understanding because it is a single measure covering two quite different things: pieces of corn broken in handling and drying, and material that is not corn at all. Because both pass through the prescribed sieve, they are captured together, and a lot can be discounted on BCFM through mechanical damage alone with no contamination whatsoever.

Heat damage is limited separately from other damage because of what it indicates. It arises from respiration and heating in storage or from excessive drying temperature, and it signals both a processing problem and a history that may have affected the rest of the lot. Moisture is determined and reported but, as in the wheat standard, it is not the basis of the numerical grade itself — a point routinely misunderstood, since moisture is nonetheless central to storage and to how corn is priced in practice.

A grade is a standardised commercial description of a lot. It is not a food-safety determination, not a statement of processing suitability, and not a measure of nutritional value. Buyers routinely contract on attributes the grade does not carry at all — moisture, mycotoxin testing, starch or oil content, or segregation by end use — because the standard was designed to describe soundness and cleanliness rather than to predict performance in a wet mill or a feed ration.

A grade applies to the lot the sample represents, and no further. Official grading depends on prescribed sampling and handling procedures, because a sample that does not represent the lot produces a grade that does not either. Sampling method, sample size, and lot definition are governed by official procedure rather than agreed between the parties.

Corn makes this concrete: broken material segregates during handling, concentrating fines in some parts of a bin or hold and not others. Where and how the sample is drawn therefore bears directly on the BCFM result, which is precisely why the procedure is prescribed rather than left to judgement.