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Grain Moisture

The drying of grain following physiological maturity while the ear is on the stalk is critical to a profitable yield. More than 25% of a grower's profit depends on the value of grain moisture at the time of harvest. Here's why:

The standard grain moisture for corn is 15.5%, and for soybeans is 13%. If a grower harvests his grain when it is wetter than this amount, he has two options

dry it using natural gas or propane before selling it to the elevator. Fuel (particularly in 2000 and 2001) is expensive and commodity prices are low. Drying costs cut into profits significantly.

sell it to the elevator and receive a price penalty. Profit margins are small to begin with, so a lower price has a large effect on final profits.

If the grower harvests his grain when it is drier than the standard moisture content he is penalized as well:

grain that is very dry shatters during the harvest process and is lost. The farmer's yield is smaller, reducing profits.

in addition, elevators typically do not pay more for grain drier than the standard moisture content. They pay by weight. Since dry grain will weigh less than wetter grain, the farmer receives less money.

So it is imperative in times of low commodity prices that the farmer harvest his grain when it is as close as possible to the standard moisture content. This is a challenge for a grower who has many fields, or whose fields contain variability in soils or topography (and therefore differences in plant maturity stage and grain moisture). Information on the spatial distribution of yield and grain moisture on each of his fields would be very valuable to the farmer at harvest time.

Corn grain moisture

Models of in-situ grain drying are rare and it is a challenging variable to predict. The primary variables are the water content of the kernel, atmospheric humidity, wind, solar radiation and its penetration into the canopy, the moisture release characteristics of kernels (Brooker et al. 1974), the resistance of the sheath to water vapor transport, and kernel characteristics.

PALMS uses the approach of J. Ritchie (personal communication) for the various stages of the grain fill/grain drying for corn. After the initial expansion phase of the infant kernel, the kernel enters a linear grain fill phase. The kernel begins at about 85% moisture content (wet basis) and linearly replaces moisture (water) with dry matter (starch). The rate of fill/drydown in this stage is a function of temperature. After the linear phase is a brief non-linear phase in which the kernel remains at a constant moisture content. At the end of this stage the plant shuts down all transfer of moisture and mass to and from the kernel. Then the kernel dries down according to the relative humidity, wind speed, and leaf temperature in the canopy and the resistance of the pericarp (kernel skin) and husks to vapor transport. We use evaporation from a coated cylinder as the scheme, in which the vapor pressure difference between the kernel and the canopy atmosphere is damped by the bulk resistance of the pericarp and husks. Rehydration of the kernels takes place when the canopy air humidity exceeds that of the grain, or when the cob is wetted by rain for an extended amount of time.

PALMS computes grain moisture on a daily basis beginning from the silking date. Some examples of simulatd grain moisture compared to results are shown.