Test Weight Issues in Corn – (Bob Nielsen)
Among the top 10 most discussed (and cussed) topics at hometown cafes during harvest season is the test weight of the grain being reported from corn fields in the neighborhood. Test weight is measured in the U.S. in terms of pounds of grain per volumetric bushel. In practice, test weight measurements are based on the weight of grain that fills a quart container (32 qts to a bushel) that meets the specifications of the USDA-FGIS (GIPSA) for official inspection (Fig. 1). Certain electronic moisture meters, like the Dickey-John GAC, estimate test weight based on a smaller-volume cup. These test weight estimates are reasonably accurate but are not accepted for official grain trading purposes.

Fig. 1. A standard filling hopper and stand for the
accurate filling of quart or pint cups for grain test
weight determination.

Fig. 2. Relative grain yield versus test weight for two hybrids and multiple site-years in Indiana, 2006-2009 (Nielsen & Camberato, Purdue University).
The official minimum allowable test weight in the U.S. for No. 1 yellow corn is 56 lbs/bu and for No. 2 yellow corn is 54 lbs/bu (USDA-GIPSA, 1996). Corn grain in the U.S. is marketed on the basis of a 56-lb “bushel” regardless of test weight. Even though grain moisture is not part of the U.S. standards for corn, grain buyers pay on the basis of “dry” bushels (15 to 15.5% grain moisture content) or discount the purchase price to account for the drying expenses they will incur with corn grain wetter than 15 or 15.5% moisture.
Growers worry about low test weight because local grain buyers often discount their offered price to farmers for low test weight grain. In addition, growers are naturally disappointed when they deliver a 1000-bu semi-load of grain with an average 52-lb test weight because they only get paid for 929 56-lb “market” bushels (52,000 lbs ÷ 56 lbs/bu).
On the other hand, high test weight grain makes growers feel good when they deliver a 1000 bushel semi-load of grain with an average 60 lb test weight because they will get paid for 1071 56-lb “market” bushels (60,000 lbs ÷ 56 lbs/bu). These emotions encourage a belief that high test weight grain is associated with high grain yields (lbs. of dry matter per acre) and vice versa. However, there is little evidence in the research literature that corn test weight is strongly correlated with grain yield.
Hybrid variability exists for grain test weight, but also does not necessarily correspond to differences in genetic yield potential. Test weight for a given hybrid can vary from field to field or year to year, but does not necessarily correspond to the yield level of an environment. The graph in Fig. 2 illustrates the absence of a strong correlation between relative grain yield and test weight for two hybrids grown in our nitrogen rate trials over multiple site-years in Indiana.
Conventional dogma suggests that low test weight corn grain results in lower processor efficiency and quality of processed end-use products like corn starch, though the research literature does not consistently support this belief. Similarly, low test corn grain is often thought to be inferior for animal feed quality, though again the research literature is not in agreement on this. Whether or not low test weight grain is inferior to higher test weight grain may depend on the cause of the low test weight in the first place.
Common Causes of Low Test Weight Corn
So far this 2009 harvest season in Indiana, there are more reports of low test weight corn grain than good or above average test weights. There are primarily six factors that account for most of the low test weight grain in 2009 and four share a common overarching effect.
First and foremost, growers should understand that test weight and grain moisture are inversely related. The higher the grain moisture, the lower the test weight. As grain dries in the field or in the dryer, test weight naturally increases as long as kernel integrity remains intact. Test weight increases as grain dries partly because kernel volume tends to shrink with drying and so more kernels pack into a volume bushel and partly because drier grain is slicker which tends to encourage kernels to pack more tightly in a volume bushel.
Therefore in a year like 2009 with many of the initial harvest reports of grain moisture ranging from 25 to 30% instead of the usual starting moisture levels of about 20 to 23%, it should not be surprising that test weights are lower than expected. Hellevang (1995) offered a simple formula for estimating the increase in test weight with grain drying. In its simplest form, the equation is (A/B) x C; where A = 100 - dry moisture content, B = 100 - wet moisture content, and C = test weight at wet moisture content. The author does not say, but I suspect this simple formula is most applicable within a “normal” range of harvest moistures; up to moistures in the mid- to high 20’s.
Example: Dry moisture = 15%, Wet moisture = 25%, Test weight at 25% = 52 lbs/bu.
Test weight at 15% moisture = ((100 - 15) / (100 - 25)) x 52 = (85/75) x 52 = 58.9 lbs/bu
Secondly, thirdly, and fourthly; drought stress (primarily northern Indiana), late-season foliar leaf diseases (primarily gray leaf spot and northern corn leaf blight), and below normal temperatures throughout September all resulted in a significant deterioration of the crop’s photosynthetic machinery beginning in early to mid-September that “pulled the rug out from beneath” the successful completion of the grain filling period in some fields; resulting in less than optimum starch deposition in the kernels. Fifthly, early October frost/freeze damage to late-developing, immature fields resulted in leaf or whole plant death that effectively put an end to the grain-filling process with the same negative effect on test weight.
Finally, there were widespread reports of ear rots (diplodia, gibberella, etc.) throughout many areas of Indiana in 2009. Kernel damage by these fungal pathogens results in light-weight, chaffy grain that also results in low test weight diseased grain, broken kernels, and excessive levels of foreign material. This cause of low test weight grain obviously results in inferior (if not toxic) animal feed quality grain, unacceptable end-use processing consequences (ethanol yield, DDGS quality, starch yield and quality, etc.), and difficulties in storing the damaged grain without further deterioration.
Related References
Bradley, Carl. 2009. Diplodia Ear Rot Causing Problems in Corn Across the State. The Bulletin, Univ of Illinois Extension. [online]. <http://ipm.illinois.edu/bulletin/article.php?id=1233>. [URL accessed Oct 2009].
Hellevang, Kenneth. 1995. Grain Moisture Content Effects and Management. North Dakota State Extension Publication AE-905. [online]. <http://www.ag.ndsu.nodak.edu/abeng/pdffiles/ae905.pdf>. [URL accessed Oct 2009].
Hicks, D.R. and H.A. Cloud. 1991. Calculating Grain Weight Shrinkage in Corn Due to Mechanical Drying. Purdue Extension Publication NCH-61 [online]. <http://www.ces.purdue.edu/extmedia/nch/nch-61.html> [URL accessed Oct 2009].
Hill, Lowell D. 1990. Grain Grades and Standards: Historical Issues Shaping the Future. Univ. of Illinois Press, Champaign, IL.
Hurburgh, Charles and Roger Elmore. 2008. Corn Quality Issues in 2008 - Moisture and Test Weight. Integrated Crop Management News, Iowa State Univ. Extension. [online]. <http://www.extension.iastate.edu/CropNews/2008/1023hurburghrobertsonelmore1.htm>. [URL accessed Oct 2009].
Hurburgh, Charles and Roger Elmore. 2008. Corn Quality Issues in 2008 – Storage Management. Integrated Crop Management News, Iowa State Univ. Extension. [online]. <http://www.extension.iastate.edu/CropNews/2008/1023hurburghrobertson.htm>. [URL accessed Oct 2009].
Nafziger, Emerson. 2003. Test Weight and Yield: A Connection? The Bulletin, Univ of Illinois Extension. [online]. <http://ipm.illinois.edu/bulletin/pastpest/articles/200323h.html>. {URL accessed Oct 2009].
USDA-GIPSA. United States Standards for Corn. 1996. USDA Grain Inspection, Packers and Stockyards Administration (GIPSA). [online] <http://archive.gipsa.usda.gov/reference-library/standards/810corn.pdf>. [URL accessed Oct 2009].
Wise, Kiersten and Charles Woloshuk. 2009. Dealing With Diplodia Ear Rot. Pest & Crop Newsletter, Purdue Extension. [online]. <http://extension.entm.purdue.edu/pestcrop/2009/issue24/index.html>. [URL accessed Oct 2009].

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Field Drydown of Mature Corn Grain - (Bob Nielsen)
- Weather conditions strongly influence in-field grain
drydown.
- Plant characteristics can also influence in-field grain
drydown.
- Early grain maturation usually means faster in-field
grain drydown.
- Later grain maturation usually means slower in-field
grain drydown.
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Grain moisture content at harvest obviously influences growers’ cost of artificially drying the grain after harvest. An early drydown of the crop also facilitates early or at least timely harvest of the crop prior to the colder and, often, wetter conditions of late fall.
Kernel moisture content decreases as the kernel develops through the blister stage (~ 85% moisture), milk stage (~ 80% moisture), dough stage (~ 70% moisture), dent stage (~ 55% moisture), and finally physiological maturity (~ 30% moisture). Prior to physiological maturity, decreases in kernel moisture occur from a combination of actual water loss (evaporation) from the kernel plus the continued accumulation of kernel dry matter via the grain filling process. After physiological maturity, percent kernel moisture continues to decrease primarily due to water loss from the kernel.
Weather & Timing of Grain Maturation
Grain moisture loss in the field occurs at a fairly linear rate within a range of grain moisture content from about 40 percent down to 15 to 20 percent, and then tapers off to little or no additional moisture loss after that. The exact rate of field drying varies among hybrids and years. Figure 1 illustrates changes in grain moisture content over time for an adapted medium maturity hybrid grown in Indiana in 1992 (unusually cool fall) and 1994 (more typical fall temperatures).

Fig. 1. Example of field drying progress of a mid-maturity corn hybrid in 2 years with different temperature patterns.
Field drying of mature corn grain is influenced primarily by weather factors, especially temperature and humidity/rainfall. Figure 2 illustrates the relationship between the average daily temperature during the drydown period and the rate of field drying. Simply put, warmer temperatures and lower humidity encourage rapid field drying of corn grain.

Fig. 2. Average daily grain moisture loss (percentage points/day) relative to average daily air temperature during the drydown period for three corn hybrids planted late April to early May, 1991-1994, west central Indiana.
Because grain drydown rates are greater when the drydown period is warmer, it stands to reason that a corn crop that matures in late August will dry down faster than one that matures in mid-September. In fact, there is a close relationship between the date when the grain nears physiological maturity (half-milkline or 2-3 weeks prior to kernel blacklayer) and the subsequent average daily drydown rate. Average daily drydown rates will range from about 0.8 percentage point per day for grain that nears maturity in late August to about 0.4 percentage point per day for grain that nears maturity in mid- to late September (Fig 3).

Fig. 3. Relationship between field drying rate and the date at which the grain nears maturity (half-milkline) for three corn hybrids planted late April to early May, 1991-1994, west central Indiana.
Bear in mind that grain moisture loss for any particular day may be quite high or low depending on the exact temperature, humidity, sunshine, or rain conditions that day. It is not unheard of for grain moisture to decline more than one percentage point per day for a period of days when conditions are warm, sunny, windy and dry. By the same token, there may be zero drydown on cool, cloudy, rainy days.
Hybrid Variability for Field Drying
Hybrid variability for the rate of grain moisture loss during post-maturity drydown and the eventual grain moisture content at harvest are of great interest to grower and seed industry alike. Growers desire hybrids with superior yielding ability (maximum gross income) that also dry very quickly in the fall (minimum drying or grain shrinkage costs).
The seed industry uses grain moisture content data to assign relative hybrid maturity ratings on the basis of relative moisture differences among hybrids at harvest. Two hybrids that differ by one “day” of relative maturity will typically vary by about one half percentage point of grain moisture content (an average daily loss of moisture) if planted and harvested on the same days. Recognize that relative hybrid maturity ratings are most consistent within, not among, seed companies.
When weather conditions are great for rapid grain drydown, hybrids tend to dry at fairly similar rates. When weather conditions are not favorable for rapid drydown, then hybrid characteristics that influence the rate of grain drying become more important.
Researchers have identified the following traits or characteristics as ones most likely to influence grain drying in the field. The relative importance of each trait varies throughout the duration of the field drydown process and, as mentioned earlier, is most influential when weather conditions are not conducive for rapid grain drying.
• Kernel Pericarp Characteristics. The pericarp is the outermost layer of a corn kernel (botanically; the ovary wall). Thinner or simply more permeable pericarp layers have been associated with faster drying rates in the field.
• Husk Leaf Number. The fewer the number of husk leaves, the more rapid the grain moisture loss. In fact, modern hybrids have fewer husk leaves than those commonly grown years ago.
• Husk Leaf Thickness. The thinner the husk leaves, the more rapid the grain moisture loss.
• Husk Leaf Senescence. The sooner the husk leaves senesce (die), the more rapid the grain moisture loss.
• Husk Coverage of the Ear. The less the husk covers the tip of the ear, the more rapid the grain moisture loss.
• Husk Tightness. The looser the husk covers the ear, the more rapid the grain moisture loss.
• Ear Declination. The sooner the ears drop from an upright position after grain maturation to a downward position, the more rapid the grain moisture loss. In particular, husks of upright ears can “capture” rainfall.
Final Trivia For Coffeeshop Conversations
Interestingly, there is little, if any, documented evidence that moisture loss occurs through the pedicel (kernel connection to the cob) of the kernel through the cob tissue. Post-maturity grain moisture loss occurs primarily by evaporative loss from the kernel itself. Research many years ago established that post-maturity moisture loss through the kernel connective tissues (placental tissues) back to the cob is essentially non-existent (Kiesselbach and Walker, 1952; Crane et. al., 1959). As those tissues cease to function (associated with the onset of kernel black layer and physiological maturity), the moisture and nutritional connection between kernel and cob is essentially broken.
Related References
Cavalieri, A.J. and O.S. Smith. 1985. Grain Filling and Field Drying of a Set of Maize Hybrids Released From 1930 to 1982. Crop Sci. 25:856-860.
Crane, P.L., S.R. Miles, and J.E. Newman. 1959. Factors Associated With Varietal Differences in Rate of Field Drying in Corn. Agron. J. 51:318-320.
Kiesselbach, T. A. and Elda R. Walker. 1952. Structure of Certain Specialized Tissue in the Kernel of Corn. Am J Bot 39:561-69
Nielsen, R.L. (Bob). 2008. Grain Fill Stages in Corn. Corny News Network, Purdue Univ. [online] <http://www.kingcorn.org/news/timeless/GrainFill.html> [URL accessed Sep 2009].
Troyer, A.F. and W.B. Ambrose. 1971. Plant Characteristics Affecting Field Drying Rate of Ear Corn. Crop Sci. 11:529-531.
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