The U.S. Department of Agriculture’s Grain Inspection, Packers and Stockyards Administration (USDA/ GIPSA) has established grades, definitions and standards for measurement of many quality factors. The attributes which determine numerical grade are Test Weight, Heat Damage, Total Damage, and Broken Corn and Foreign Material (BCFM). The Corn Grades and Grade Requirements are summarized in the Grade Requirements and Conversions section on page 30. Moisture content is reported on official grade certificates, but does not determine which numerical grade will be assigned to the sample.
Test Weight (weight per volume) is a measure of bulk density and is often used as a general indicator of overall quality and as a gauge of endosperm hardness to alkaline cookers and dry millers. High test weight corn will take up less storage space than the same weight of corn with a lower test weight. Test weight is initially impacted by genetic differences in the structure of the kernel. However, it is also affected by moisture content, method of drying, physical damage to the kernel (broken kernels and scuffed surfaces), foreign material in the sample, kernel size, stress during the growing season, and microbiological damage. When sampled and measured at the point of delivery from the farm at a given moisture content, high test weight generally indicates high quality, high percent of horneous (or hard) endosperm and sound, clean corn. Test weight is highly correlated to true density, and reflects kernel hardness and kernel maturity.
- Average test weight of the U.S. Aggregate of 58.1 lb/bu (74.8 kg/hl) indicates overall good quality and is 4 pounds/bu above the grade limit for No. 2 corn (54 lbs).
- Test weight values in the three ECAs did not vary greatly from the U.S. Aggregate average.
- As corn is comingled moving through the marketing channel, the average test weight in each ECA indicates the U.S. No. 2 minimum for test weight would be met in all ECAs.
- More than 96% of the samples were above the factor limit for No. 2 grade, and over 98% exceeded the factor limit for No. 3 grade (52 lbs).
Broken Corn and Foreign Material (BCFM)
Broken corn and foreign material (BCFM) is an indicator of the amount of clean, sound corn available for feed and processing. The lower the percentage of BCFM, the less foreign material and/or fewer broken kernels in a sample. Foreign material (FM) is defined as any non-corn pieces too large to pass through a 12/64th inch sieve, as well as all fine material small enough to pass through a 6/64th inch sieve. Broken Corn (BC) is defined as everything small enough to pass through a 12/64th inch sieve, but too large to pass through a 6/64th inch sieve. Higher levels of BCFM in farm-originated samples generally stem from combine settings and/or weed seeds in the field.
- Average BCFM for the U.S. Aggregate was 1.0%. None of the ECAs differed substantially from the U.S. Aggregate.
- BCFM levels in almost all corn delivered to the country elevators are well below the maximum of 3% allowed for No. 2 corn – the basis for most discounts in commercial transactions.
- These levels will normally increase during drying and handling, depending on the methods used and the soundness of the kernels.
- The U.S. Aggregate samples showed that the 1.0% BCFM contained 0.8% broken corn and 0.2% foreign material.
Broken Corn (BC)
Broken corn (BC) is more subject to mold and insect damage than whole kernels and can cause problems in handling and processing. When not spread or stirred in a storage bin, broken corn tends to stay in the center of the bin while whole kernels are likely to gravitate to the outer edges. This phenomenon is known as a “spoutline” in the grain business. In some cases, most, if not all, of the spoutline can be removed by pulling grain out of the center draw.
- BC averaged 0.8% in the U.S. Aggregate and 0.7% to 0.9% in the individual ECAs.
- The percent of BC was lowest in the Gulf ECA, in part as a result of harvesting at slightly higher moisture content.
- The levels of BC in farm deliveries in all the areas were very low and would not be an issue in handling and processing.
- The distribution chart as shown to the right, displaying BC as a percent of BCFM, shows that in nearly all samples, BCFM consisted primarily of broken corn.
Foreign Material (FM)
Foreign Material (FM) is of importance in that it has little feed or processing value, it is generally higher in moisture content than the corn and therefore creates a potential for deterioration of corn during storage. FM also contributes to the spoutline and is more serious than BC because of the higher moisture level as mentioned above.
- FM levels below 0.5% seldom create handling problems.
- All ECAs had average FM values of 0.2%.
- High levels of FM found in a few of the samples can be readily cleaned to minimize any significant handling problems.
Total damaged kernels is the percentage of kernels and pieces of kernels that are visually damaged in some way, including heat damaged, frost-damaged, insect-bored, sprout-damaged, diseased, weather-damaged, ground-damaged, germ-damaged, and mold-damaged. Most of these types of damage result in some sort of discoloration or change in kernel texture. Damage does not include broken pieces of grain that are otherwise normal in appearance. Mold damage is usually associated with higher moisture content and high temperature in growing and/or storage. Mold damage and the associated potential for mycotoxins is the damage factor of greatest concern. Mold damage can occur prior to harvest as well as during temporary storage at high moisture and high temperature levels before delivery.
- The average levels in all of the ECAs are well below the limit for No. 1 corn (3.0%) and indicate that Total Damage is not a problem in farm deliveries.
- The distribution chart shows that 94.1% of the samples had 3% or less damaged kernels.
- 97.5% of the samples would grade No.2 (5.0%) or better on the factor of Total Damage.
Heat Damage (HD)
Heat damage (HD) is a subset of total damage and has separate allowances in the U.S. Grade Standards. Heat Damage can be caused by microbiological activity in warm, moist grain or by high heat applied during drying. HD is seldom present in corn delivered at harvest direct from farms and combines.
- There was no heat damage reported in any of the samples.
- The low heat damage was likely in part due to fresh samples coming directly from farm to elevator with minimal prior drying.
Moisture content affects the amount of dry matter being sold and purchased. Moisture is also an indicator for drying that might be needed, has potential implications for storability, and affects test weight. Higher moisture content at harvest increases kernel damage during harvesting and drying, and the amount of drying required will affect stress cracks, breakage, and germination. Extremely wet grain may be a precursor to high mold damage later in storage or transport.
- The U.S. Aggregate elevator-recorded moisture averaged 15.6% with a minimum of 9.5% and a high of 22.0%1.
- 44.8% of the samples contained 15% or less moisture – the base used by most elevators for discounts and a level considered storable for short periods.
- Moisture averages for corn for the Gulf, Pacific Northwest, and Southern Rail ECAs were 16.0%, 14.7% and 14.9%, respectively; however, minimum and maximum values were similar across the ECAs.
- The 1.3% of the samples with very low moisture (< 11%) was associated with regions which suffered from drought.
- As shown to the right, 21.1% of the samples at the point of delivery to the elevator were already 14 % or less, generally considered a safe level for storage and transport without drying.
Grade Factors and Moisture Summary
- Test weight was high with U.S. Aggregate samples averaging 58.1 lb/bu (74.8 kg/hl).
- BCFM of incoming corn was very low with a U.S. Aggregate average of 1.0%, consisting primarily of broken corn.
- Average total damage was extremely low for incoming corn, ranging from 0.6% to 1.3% among ECAs. In addition, no heat damage was reported on any of the samples.
- Of the in-bound elevator samples, 90.9% would grade No. 2 or better on all grade determining factors (the criteria found in most export contracts). Over time, subsequent handling, drying, and storage may cause quality to lower.
- The U.S. Aggregate elevator recorded moisture averaged 15.6% with about 45% of the samples containing 15% or less moisture. These results imply that producers were able to take advantage of in-field drying, resulting in less artificial drying and increasing the overall quality of the 2011 corn crop.