NDSU Extension - Ramsey County


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August 20, 2012 Agriculture Column


The rains the last weeks has lowered test weights in our small grains.  Have you ever wondered why test weights lower as the rains come?  My neighboring county agent called and asked if I knew the reason for that and although I thought I knew the answer I thought I had better do a little research and found what I knew but am more comfortable talking about the issue.

The most desirable time to harvest HRSW is in the 13-15 percent moisture range; however most of us would like to harvest in dry conditions (meaning 13.5 moisture in HRSW).  There has been talk about why test weight drops after a rain in spring wheat.  Not knowing the answer I went searching and found this:

Rain Damage Reduces Grain Quality

Grain that is re-wet by rainfall after it is dried down at harvest suffers quality loss through either lowered test weight or sprouting. The damage can occur in both swathed and standing grain.

Test weight loss on grain that has had rain after it has matured is usually 2 to 3 pounds per bushel but can be worse as grain wets and dries several times. This occurs because grain swells as it absorbs water, then fails to re-dry to it's original size.

Apparently, inside the kernel, it's like getting your roof layers wet and then they kind of peel and stick up and don't come back together. The berries stay puffed up or enlarged and have a slightly irregular shape so going back to the density of the kernal, you have the same weight but it is taking up more space. Once that kernel size and density is set, no yield loss in terms of total pounds takes place. It is strictly that it will take more bushels at less pounds per bushel.

Kansas state University has data showing that in some cases, the test weights at maturity were fine, but rains at harvest time lowered the test weights by 2-3 points or more almost overnight. In this case, the reason for the low test weight is that the rain causes the wheat kernels to swell something like puffed wheat cereal. Test weight is a measure of how much grain weight can be placed in a given volume. Prior to the rain, the wheat kernels could be packed well into a bushel. When it rains, kernels swell and upon drying, the kernels do not shrink back to their original volume, shape, and smoothness. This results in more space between kernels, and they will not pack into a bushel as well as they did before the rain. The result is a lower test weight. But this does not reduce the total number of pounds of grain produced per acre. There is no real grain yield loss. The number of bushels per acre will be increased but the weight per bushel will be reduced.

USDA research at Mandan shows that a standing grain crop that will yield 50 bushels per acre will saturate with the equivalent of .04 inch of rain on one acre. Assuming that only 25 percent of the rain that falls hits and is absorbed by the standing grain, the rain required to saturate the crop increases to .16 inch.

More rain is required to re-wet swathed grain. Windrowing concentrates plant tissue and exposes bare ground. The rain required to re-wet swathed grain varies with the size of the windrow, but the dominating factors are grain yield and the width of the swath cut. For example, a 45 bushel crop in a yard-wide windrow cut with a 24 foot header will require about .35 inches of rain to re-wet.


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