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Corn test weight changes during drying

Dale R. Hicks, Department of Agronomy and Plant Genetics, University of Minnesota

Some of this year's corn crop will not reach normal maturity before the next killing freeze. As a result, maximum yield potential and normal test weights will not occur. Test weights in the low 50's(lb/bu) may be common in some areas the state, especially in the northern half of MN. Test weight can increase with artificial drying if the drying temperature is maintained below 180°F.

Research at the University of Illinois and Iowa State University showed mature corn increased in test weight when dried to 15% kernel moisture (KM). The size of the increase depended upon the hybrid, mechanical condition of the grain, and drying temperature. Greatest increases in test weight occurred with good quality corn (less than 10% cracked kernels) dried at temperatures less than 180°F. The summary of that research showed that test weight increases could be expected as shown in Table 1 when drying mature grain to 15% KM.

Table 1. Increase in test weight during drying for mature corn harvested between 18 and 28% kernel moisture content.
Harvest moisture content Increase in test weight
Source: University of Illinois and Iowa State University

We conducted a study to measure the effect of stage of kernel maturity and drying temperature on the change in test weight during drying. Ear samples were harvested from normally growing plants beginning August 27 and continued at approximately 10-day intervals until October 13. Grain was shelled and wet test weight and KM were measured. Kernel samples were split and put into either a high (120°F) or low temperature (80°F) oven for drying. Samples were removed from the oven every 1 to 4 days to measure test weight and KM. Samples were replaced in the oven to continue drying and subsequent measurements of test weight and KM content.

corn grain test weights
Figure 1. Wet and dry test weights for grain harvested at soft dough through mature kernel stages and dried to 15.5% kernel moisture with temperatures of 80° or 120°F.
Wet test weight and test weights after drying to 15.5% moisture are shown in Figure 1 for corn harvested on six calendar dates. Moisture contents given across the bottom of the graph correspond to the six harvest dates; kernel development for the six harvest dates were soft dough, early dent, well dented, nearly mature, mature, and mature. The three bars in the figure for each kernel moisture level shows the initial wet test weight (bar 1) when the grain was harvested and the test weight after drying to 15.5% KM with 80°F temperature (bar 2) or with 120°F (bar 3).

Wet test weight for corn in the soft dough stage at 52.5% KM was 52 lb/bu, but dropped to less than 46 when dried to 15.5% KM. When corn was in the soft dough and early dent stages for the first two harvest dates, test weight did not increase when drying to 15.5% KM.

After grain had reached the well-dented stage, test weight increased about 4 lb/bu when dried to 15.5% KM with either drying temperature. Moisture content was 17.5% for the last harvest date and there was no change in test weight during drying because the change had already occurred as kernels dried in the field.

These drying temperatures were below 180°F recommended by Illinois and Iowa to achieve the increase in test weight during artificial drying. However, the test weight increase when drying kernels in the well-dented stage was greater using 80° than when drying with 120°F (third group of 3 bars in Figure 1). Temperature had no effect on the test weight increase during drying for nearly mature and mature kernels (last 3 groups of bars in figure 1).

For corn that is not mature this year when frozen, the test weight will be lower than it would have been with normal growing to maturity, even after careful drying. But some increase in test weight should occur during drying for corn that has progressed in maturity to or past the well-dented stage. Dry test weight should be 54 or higher, particularly if the drying temperature is less than 180°F. For other corn that has not well-dented when frozen, the dry test weight is not likely to reach 54.

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