CORN GRADES AND FEED VALUE
Morris J. Bitzer and Dan O. Riddell
Corn is traditionally priced on the
basis of U.S. No. 2 grade. However, with the release of CCC-owned corn
stocks for Emergency Assistance Programs, there is an increase in corn
being marketed as No. 4, No. 5 or Sample grade. It is important for producers
to understand corn grading standards and the feeding values of these grades.
Standards for the grading of corn are
provided for and defined in the "Official United States Standards for Grain,"
U.S. Department of Agriculture (USDA) Agricultural Marketing Service, Grain
Division. Requirements for five numerical grades and the Sample grade specify
a minimum testing weight per bushel and maximum limits of moisture, broken
corn and foreign material, and damaged kernels which include both total
damaged kernels and heat-damaged kernels for each of the six grades (Table
1). The grading standards are supervised by the Federal Grain Inspection
Service of the USDA.
Corn is defined in the standards as
any grain which consists of 50 percent or more whole kernels of shelled
dent or flint corn containing not more than 10 percent of other grains
for which standards have been established under the United States Grain
Standards Act. Standards have been established for corn, wheat, barley,
oats, rye, sorghum, flaxseed, soybeans, triticale and mixed grain.
Classes for corn are determined on
the basis of color and are divided into the following: Yellow Corn, White
Corn and Mixed Corn. A mixture of White Corn in Yellow Corn is not as objectionable
as a mixture of the same amount of Yellow Corn in White Corn. A mixture
of more than 5 percent of kernels other than yellow in Yellow Corn or a
mixture of more than 2 percent of kernels other than white in White Corn
causes the corn to be classified as Mixed Corn.
Table 1.-Grade Requirements for Corn1
1Official United States Standards for Grain. USDA. Ag. Marketing
Service, Grain Division
Maximum Limits of
|U.S. No. 1
|U.S. No. 2
|U.S. No. 3
|U.S. No. 4
|U.S. No. 5
U.S. Sample grade corn is that which:
does not meet the requirements for any of the grades from U.S. No. 1 to
U.S. No. 5; 2) contains stones; 3) is musty or sour;
is heating or has heated; 5) has any commercially objectionable
foreign odor; or 6) is otherwise of distinctly low quality.
Discounts on grain not meeting grade
requirements for No. 2 corn will vary according to the quantities of grain
not meeting the requirements for each of the various factors. No. 2 corn
must weigh at least 54 pounds per bushel and not exceed the maximum limits
allowed for the other factors. The price offered will depend on opportunities
for blending the various qualities of corn available to meet certain grades.
Because the commercial value of grain
is not always reflected by its numerical grade alone, the official grain
standards provide for special grade designations. Corn containing weevils
is an example of a special grade. If corn is infested with live weevils
or insects injurious to stored corn, the word "weevily" is made part of
the grade designation. Special grades do not affect the numerical grade,
and, when applicable, are supplemental to the numerical grade assigned.
The main factors used in determining
the feeding value of corn grain are content of total digestible nutrients
(TDN) and crude protein. The test weight per bushel and moisture content
do not affect the feeding value of the grain on a dry matter basis. Broken
corn may not be reduced in feed value. However, broken kernels are more
susceptible to mold invasion and insect infestation and will not store
as well as sound corn. Foreign material may contribute to damage sustained
during storage if it interferes with needed aeration or fumigation. The
effect of foreign material on feeding value would be directly related to
sustained damage and the type and quantity of foreign material present.
Unless certain weed seeds are present, intake by animals would probably
not be affected.
Damaged kernels include those kernels
and pieces of kernels which have been materially damaged by frost, weathering,
disease, mold, sprouting, grinding, heating until discoloration has occurred,
or other means not specified. The percentage of total damaged kernels which
is due to heat is extremely important in determining feed value since heating
can reduce energy and protein availability to consuming animals.
The total amount of damaged kernels
and the type of damage could reduce animal intake and reduce feeding value
proportional to the amount of carbohydrates used up in the damage process.
For precise ration balancing, damaged corn should be tested for feed value
so that any changes in TDN and crude protein content can be considered.
Musty or sour grain would reduce animal intake. Hogs would be more selective
with lower feed palatability than beef or dairy animals.
Unless a very serious mold, noticeable
heat damage or objectionable foreign odor is present, the feeding value
of No. 4, No. 5 or Sample grade corn would not be seriously reduced. On
a dry matter basis, there is very little reduction in feeding value of
lower grade corn. According to the National Research Council, there is
essentially no differences, on a dry matter basis, between U.S. No. 1 corn
and U.S. No. 5 corn for percent crude protein (10.2 versus 10.2), percent
digestible protein (7.6 versus 7.5) or percent TDN (91 versus 91 ). However,
a bushel of U.S. No. 5 corn may contain only 75 percent as much dry matter
as a bushel of U.S. No. 1 corn. Therefore, when compared on a per bushel
basis, the feeding value of lower grades of corn will most likely be reduced.
The magnitude of the reduction will depend on the test weight per bushel
and moisture content of the corn.