What Accounts for Variability in Grain Protein Levels in Corn?

(Image Source: C.O.R.N. Newsletter)

We’ve recently heard comments and questions concerning the varying levels of grain protein levels being found in shelled corn. Some feed companies have reported seeing many samples in the upper 6% and lower 7% protein range this year but there are reports of levels that are nearly 9%. Some feed mill operations are using 7% as the default value based on this year and last year’s levels. However, in the past, higher grain protein levels (% +2) have been cited for corn. Are the reports of low levels in 2016 and 2017 an anomaly? What could be accounting for these varying protein levels in corn?

Environmental conditions (esp. those affecting soil moisture), cultural practices (nitrogen fertilization, plant population, drainage) and hybrids genetics all influence grain protein. Production factors and favorable growing conditions that increase grain yield usually increase the starch concentration of grain while reducing the grain protein concentration (except when yield increases as a result of N fertilizer application). We generally had favorable growing conditions in 2017 (and much better than expected yields); however, some areas experienced excessive rainfall in May and July, and other areas were dry in Aug and Sept. and variable soil moisture may have contributed to the varying grain N%.

Soil moisture is a major factor influencing grain protein concentration. Generally, grain protein concentrations are highest in dry years and lowest in years with excess soil moisture. Data from past Ohio Corn Performance Tests indicate that % grain protein (reported as % dry matter or DM, equivalent to 0% grain moisture) may vary by as much as 2 to 3 % points in the same hybrid depending on the growing season (w/drought 10-11% DM, w/good rainfall 8-9% DM). Excessive soil moisture (like that associated with many fields in 2017) can result in loss of soil N through either leaching or denitrification and result in N deficiency – leading to lower grain protein. Moisture stress (drought) can limit a corn plant’s ability to produce dry matter (including starch) and “dilute” the N or protein in the grain tissue. As a result, grain protein concentration in drought stressed corn can often be higher than normal.

Table 1 shows the grain protein levels and grain yields from OSU field trials from 2012-2014 with normal (May) and late (June) planting dates. It seems in a really good year (2013 May planting), protein levels were right at 9.2% DM. However, in a good year that turned dry (June 2013 and all of 2014), protein levels were lower (8.75% DM). In the drought year (2012), protein levels were up (10-10.9%) but yield was down. Again, all of these are on DM basis. Grain protein levels were reduced slightly by the later June planting dates.

If reported at 15.5% moisture basis (Table 2), the same data set as shown in Table 1 drops in protein level dramatically (down 1.3-1.5% closer to the 7% value). This may explain some of the varying protein levels we are hearing about. It may be there is a discrepancy at what moisture protein is being tested/reported at (rather than a major change in values). It’s important to be able to compare grain lots with different protein and moisture levels.

For example, say you want to determine which grain lot below has a higher grain protein:

• Lot 1: 7.3% grain protein at 18.0% grain moisture
• Lot 2: 8.1% grain protein at 9.0% grain moisture

If we convert the grain protein levels of the two lots to a 0% grain moisture or on a dry matter basis (DM) we can make a comparison –

• Lot 1: (7.3/(100-18.0)) x 100 = 8.9% DM protein
• Lot 2: (8.1/(100-9.0)) x 100 = 8.9% DM protein.

Converting these two lots to DM basis shows they have the same level of protein, even though their raw values are almost 1% different.

Nitrogen management practices that minimize N losses and N deficiencies help to optimize protein concentrations. Grain N concentration will respond to N fertilizer application up to a point, but beyond that, increases in N application often have little effect on grain protein concentration. Past Penn State work suggests that the optimum N application rate for corn protein is similar to that required for optimum grain yield. Differences in protein content are present among hybrids. Grain protein levels typically vary by about 1.5 percentage points in corn hybrid trials. (Some specialty corns developed for enhanced nutritional content may exhibit protein levels greater that our commodity grain hybrids.) Usually the variability in corn grain protein is greater from year to year (environmental effects) than it is within a performance trial (genetic effects). Of course, within a performance test (with hybrids in close proximity and pollen drift), we have the “xenia” effect to deal with. The hybrids we plant today require high populations to optimize yields and this has has resulted in grain with a higher starch content and lower protein content.