This year’s delayed maturity of corn because of late planting may translate into delayed drydown before harvest.
Bob Nielsen, Purdue University Extension corn specialist, explains how temperature, stress and hybrid selection all affect moisture levels in corn at harvest.
Know moisture content
Before 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, identified by presence of the kernel black layer, percent kernel moisture continues to decrease primarily because of water loss from the kernel.
Grain moisture loss in the field occurs at a fairly linear rate within a range of grain moisture content from about 40% down to 15% to 20%, and then tapers off to little or no additional moisture loss. The exact rate of field drying varies among hybrids and years.
Analyze your situation
Here are three conditions that may affect corn drydown.
1. Temperature. Field drying of mature corn grain is primarily influenced by weather factors, especially temperature, humidity and rainfall. Simply put, Nielsen says, warmer temperatures and lower humidity encourage rapid field drying of corn grain.
A corn crop that matures in late August will dry down faster than one that matures in mid-September, because of grain drydown rates being greater when the drydown period is warmer. In fact, there is a close relationship between the date when the grain nears physiological maturity (half-milk line or 2 to 3 weeks before kernel black layer) and the subsequent average daily drydown rate.
Daily drydown rates will range from about 0.8 percentage points per day for grain that nears maturity in late August to about 0.4 percentage points per day for grain that nears maturity in mid- to late September.
2. Stress. Farmers often question whether field drydown will occur "normally" after severe weather-related stress damages the crop before physiological maturity or causes premature death of the plants. Examples of weather stress include damage caused by severe drought plus heat, late-season hailstorms, and frost or killing freeze events before physiological maturity. The answer in all cases to whether grain drydown will occur "normally" is essentially "yes," but this requires a bit of explanation.
Lingering severe stress such as drought, or foliar disease such as gray leaf spot, during the latter stages of the grain filling period can cause premature death of entire plants, smaller than normal kernels, and premature formation of kernel black layer. The latter two factors usually result in earlier than expected drydown of the grain. Moisture content in these areas of a field will be much drier at harvest.
The fact that grain drydown of "prematurely mature" grain begins earlier usually means it occurs in relatively warmer time periods, and so grain drydown rates per day are higher than would be expected if the grain had matured "normally" at a later date. However, the rate of grain drydown is "normal" for the time period during which the grain is drying.
The effects of a sudden single stress event such as hail or lethal cold temperatures before physiological maturity often create an optical illusion of sorts relative to subsequent field drying of the grain.
Because leaf or plant death of an immature crop may occur rapidly in response to severe hail damage or lethal frost or freeze events, the moisture content of the yet immature grain will "appear" to be high given the appearance of the now dead plants. The appearance of the dead plant tissue gives the illusion that field drydown was slowed by the damage from the hail or frost.
3. Hybrid choice. The seed industry uses grain moisture content data to assign relative hybrid maturity ratings based on relative moisture differences among hybrids at harvest. Two hybrids that differ by one "day" of relative maturity typically will vary by about one-half a 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 favorable for rapid grain drydown, hybrids tend to dry at similar rates. When weather conditions are not favorable for rapid drydown, then hybrid characteristics that influence the rate of grain drying become more important.