Is a local elevator still paying a bonus or overlooking higher moisture contents to get corn in the door. If so, that might influence you to start harvest or continue harvest, as the case may be even on fields with moisture contents in the high 20% moisture range. Or maybe you've got some fields that were stressed and they're already starting to fall over. Or maybe you've just got so many acres to harvest and bushels to handle that you want to get started as soon as possible.
Those can all be legitimate reasons for starting corn harvest. However, for some this year, especially those with corn planted later, as late as early June, it may boil down to numbers. How wet is the corn? Can you harvest it by adjusting combine settings to avoid excessive breakage and grinding of kernels? That's already been a problem in some areas of the country, perhaps due to the stressed nature of the corn crop and the unusually early death of corn plants in some locations.
Suppose it comes down to moisture. How much are you losing by harvesting early in terms of elevator dock or shrink? Different elevators may impose their own dock per point of moisture. Some use a combination of dock and shrinkage. Based on the Purdue University Corn & Soybean Field Guide, 2011 edition, here are some examples that illustrate how to calculate shrinkage. They may be useful, especially if you're going to dry corn on the farm.
A common formula uses SF, a shrink factor, and PR, for points of moisture removed. That's calculated by subtracting final moisture content from the original moisture content for the incoming corn.
The formula is: Dry bushels=wet bushels – (wet bushels x SF x PR)
Here's an example that shows how this formula works. If you harvest 1,000 bushels, or 56,000 pounds, of corn at 26% moisture, how many bushels will you have left once you dry them to 15% moisture?
The formula becomes: Dry bushels= 1,000 – (1000 x 0.01176 x 11) = 1,000 minus
129.36 = 870.64 bushels. The bottom line is that if you shrink the corn from 26% to 15%, you're going to have about 870 bushels, not 1,000 bushels.
If you know your cost in your own drying system to remove a set number of points per bushel, then factor that in to determine costs. The number of bushels you will have left to sell are the dry bushels. That's typically considered dry bushels of #2 corn. Yield should be calculated on the dry bushel figure divided by acres harvested.
The shrinkage factor varies depending upon the % moisture you're dropping the grain to by drying. Here are typical shrink (SF) factors: 15.5 =0.01183; 15 = 0.01176; 14= 0.01163; 13 = 0.01149; 12= 0.01136.
Note that a mistake in drying that takes corn to 12% not only costs extra in drying energy you didn't need to spend, but also results in more bushels lost and fewer dry bushels remaining to sell.