Understanding Corn Test Weight
 Corn test weight is a measurement of bulk density, or the weight of a unit volume, of grain.
 Changes in yield and test weight are not always proportional or correlated; yield can be high when test weights are low.
 In general, as grain moisture decreases, its test weight increases. Variation in the amount of increase is due to factors
such as product makeup, grain condition, and drying temperature.
History
Test weight is a measurement of bulk density, or the weight of a
unit volume, of grain (lb/bu). Iowa State University reports that
corn test weight values can range from 45 lbs/bu to over 60
lbs/bu.1 USDA established the standard test weight of a bushel
of corn as 56 lbs/bu based on 15.5% moisture content. Table 1
lists different test weights of corn needed to equal one bushel
of No. 2 shelled corn at 15.5% moisture when corn is harvested
at various moisture levels.2 For some specialty food corn, test
weight is used as an indicator of grain characteristics which are
favorable for processing.
Today in the United States, grain yield is still referenced in
bushels per acre but it is actually traded and sold on a weight
basis. Test weight is still measured when grain is sold. Grain
price can be affected by test weight in certain areas such as
those that have premiums for high test weight or when selling in
the whole food grade market. However, in most cases, only
test weights that are below the minimum for No. 2 yellow corn
(<54 lbs/bu) would result in a discount.3
Is yield influenced by test weight?
Any stress that prematurely stops or reduces grain fill and/or
interferes with photosynthesis has the potential to lower yield
Table 1. Test weight (lbs) of corn needed to equal one bushel of
No. 2 shelled corn (at 15.5% moisture) when corn is harvested at
various moisture levels.
% Moisture
in Corn
Test weight (lbs) of corn needed
to equal one bushel
14
55.02
15
56.67
15.5
56.00
16
56.33
17
57.01
18
57.71
Source: Lauer, J. 2002. Methods for calculating corn yield. University of
Wisconsin. Field Crops 28.47-33. http://www.corn.agronomy.wisc.edu
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potential and test weight. Grain yield is determined by the
number of kernels per acre along with the weight per kernel,
which is shown in the equation below:
Corn yield (bu/acre) =
# of kernels per acre X weight per kernel at 15.5% moisture content
56 lb/bu (standard weight of a bushel)
In comparison, test weight measures the weight of corn in
pounds that will fit into a bushel. Yield is a direct measure of
kernel weight and kernel number. However, test weight is not a
direct factor of grain yield. Test weight is only partially related to
kernel weight because there is also the volume component
associated with the measurement. Factors that affect test
weight but not corn yield are those that influence how kernels fit
or pack together. These may include slipperiness of the seed
coat as well as kernel shape or size. Due to its volume
component, test weight will influence how many bushels a
grower can fit into a bin, wagon or truck, but not yield per acre.
Table 2 compares 3 loads of corn, each weighing 20,000
pounds and consisting of 357.14 bushels (20,000 ÷ 56 lb/bu).
At a consistent moisture content of 14.5% but with varying test
weights, the loads each result in different volumes of grain.
Higher test weight corn takes up less volume because it is
denser and packs more tightly. Price is based on weight, but
can be negatively impacted when test weight is low enough to
incur a price penalty per bushel (Table 2, Load 3). In some
cases, a premium may be paid for a higher test weight. An
increase in price is never due to an increase in the weight of
corn sold.
Correlation between yield and test weight?
Many think yield will always be lower if test weight is lower and
vice versa. If this were true, corn products with high test weight
grain would regularly out-yield corn products with average test
weight grain. Yield can be high and test weight can be low. For
example, if the time for grain fill is shortened, the seed size may
be smaller but the density of the individual seeds remains
unchanged.
Understanding Corn Test Weight
Corn yield is about the accumulation of dry matter in the kernel
and the number of kernels produced per acre. Controlled by
the limitations of the environment and corn product yield
potential, the maximum amount of dry matter possible will be
accumulated within that acre.
In some instances, corn test weights may be lower than
expected due to changes in kernel weight from stresses during
kernel fill. Lower kernel weight can also result in lower yield and
therefore, the test weight can be low as well. When kernel
number is set before stress begins, whether kernels get larger
or stop filling early has a big effect on yield (weight per kernel)
but not always much effect on test weight. Yield and test
weight do not change by equal percentages and unfortunately,
there is no formula to predict how they will be correlated.
Correlation between moisture and test weight?
Although various physical characteristics can influence test
weight, one of the most important is grain moisture. In general,
as grain moisture decreases, its test weight increases. There is
variation in the amount of increase due to various factors such
as product makeup, grain condition, and drying temperature.
For example, grain that has a large percentage of damaged
kernels will experience less of an increase with drying than high
quality grain would. The drying temperature also plays a role.
Drying grain at high temperatures (in excess of 180° F) will also
result in a lesser increase in test weight. Table 3 shows an
average expected increase in test weight as corn grain dries to
15% for mature corn harvested between 18 and 28% kernel
moisture.
Table 2. Example calculations of corn value, with varying test weights.
Load #1
Load #2
Load #3
Weight (lbs)
20,000
20,000
20,000
# of bushels to be
sold (=weight/56)a
357.14
357.14
357.14
Moisture contentb
14.50%
14.50%
14.50%
Test weight (lb/bu)
54
59
51
461
422
488
Price ($/bu)
$3.25
$3.25
$3.25 - .04
= $3.21c
Calculation of value
357.14 bu X
3.25/bu
357.14 bu X
3.25/bu
357.14 bu X
3.21/bu
Value ($)
$1,160.71
$1,160.71
$1,146.42
Volume
(ft3)
a
56 lb/bu is the standard weight of a bushel established by USDA.
b
Moisture discounts and drying charges can occur when the moisture content
exceeds 15%. This example has no moisture discount or drying charges.
C
Includes a test weight discount of $0.04/bu due to a test weight below 54 lbs/bu.
Modified From: Bern, C. and T. Brumm. 2009. Grain test weight deception. Iowa
State University Extension. PMR 1005. http://lib.dr.iastate.edu/
Table 3. Increase in test weight for mature corn harvested between
18 and 28% kernel moisture as corn grain dries to 15%.
Harvest moisture content
Increase in test weight
%
lbs/bu
18
1.5
20
2.0
22
2.5
24
3.0
26
3.5
28
4.0
Source: Rankin, M. 2009. Understanding corn test weight. University of
Wisconsin Extension. http://fyi.uwex.edu/
Corn maturity also plays an important role in test weight
determination. When corn maturity is challenged, test weights
will naturally be lower because of the grain moisture content.
While drying grain will help increase test weight, the amount of
increase will vary based on kernel moisture, grain quality, and
the maturity level reached. Research done at the University of
Minnesota demonstrated that if corn was well-dented or
beyond in maturity, some increase should occur with drying.
However, in the soft dough through early dent stages, drying
actually decreased the test weight.4
If very low test weight is a concern, using corn as silage or
chopping for feed might also be considered. However, it is
important to evaluate the stage and health of the corn to
determine if chopping for corn silage is still a viable option.
Corn product selection and test weight
In addition to environmental stresses impacting test weight,
corn product selection may also influence test weight results.
Corn products with higher vitreous (hard or flinty) endosperm
tend to have higher test weights than ones with floury (soft or
dent) endosperm due to vitreous kernels having higher
densities. This potential difference in endosperm content does
not necessarily correlate to differences in genetic yield potential.
For additional agronomic information, please contact your local seed representative.
Sources: 1 Hurburgh, C. and R. Elmore. 2008. Corn quality issues in 2008 – moisture and test
weight. Iowa State University Extension. Integrated Crop News. www.extension.iastate.edu/
(verified 9/15/2014); 2 Lauer, J. 2002. Methods for calculating corn yield. University of
Wisconsin. Field Crops 28.47-33. http://corn.agronomy.wisc.edu/ (verified 9/15/2014);
3
Bern, C. and T. Brumm. 2009. Grain test weight deception. Iowa State University Extension.
PMR 1005. http://lib.dr.iastate.edu/ (verified 9/15/2014); 4 Hicks, D. 2004. Corn test weight
changes during drying. Minnesota Crop News. http://blog.lib.umn.edu/efans/cropnews/ (verified
9/15/14). Additional sources: Nafziger, E. 2003. Test weight and yield: A connection?
University of Illinois Extension. The Bulletin. http://bulletin.ipm.illinois.edu/ (verified 9/15/2014);
Nielsen, R.L. 2012. Test weight issues in corn. Purdue University. Corny News Network.
http://www.agry.purdue.edu/ (verified 9/17/2014); Rankin, M. 2009. Understanding corn test
weight. University of Wisconsin Extension. http://fyi.uwex.edu/ (verified 9/17/2014); Nafziger,
E. 2010. Is low test weight a cause for concern? University of Illinois Extension. The Bulletin
Issue No. 20, Article 4. http://web.extension.illinois.edu/ (verified 9/17/2014).
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