It would seem that getting an accurate estimate of whole plant dry matter would be somewhat straight forward. However, experiences over the past two years have prompted some questions. Here is why:

Exhibit 1:

In 1997 the Miner Institute reports that Koster tester readings for corn silage and alfalfa haylage were 3 to 5 percentage units higher in dry matter than the same samples run through forced air or convection ovens. Drying time reported was one hour for the Koster and 36 hours for the ovens.

Exhibit 2:

Whole plant corn silage samples from local "burndown days" analyzed with NIR have consistently been 5 to 7 percentage units lower in dry matter (higher in moisture) than the same samples analyzed using only a microwave oven. This was true in 1997 and 1998.

Exhibit 3:

Same whole plant corn silage samples obtained at local "burndown days" and analyzed with a Koster Tester have been about 3 percentage units higher in dry matter (lower in moisture) than when sent to a lab and analyzed with NIR.

Is the 4 to 7 points difference in dry matter determination between testing methods something to be concerned about? Certainly for corn silage it’s too large of a swing when we consider that harvest recommendations are based on whole plant moisture for optimum yield and quality. A difference in moisture of this magnitude translates into a 1 to 2 week difference in optimum harvest date.

Can we account for why these differences occur? Certainly to some degree we can. Most forage labs dry samples to a point using a microwave or conventional oven and then use NIR to read the remaining moisture. The NIR machine reads water chemical bonds very well and easily accounts for ALL of the moisture in a sample. With other methods it is nearly impossible to account for all of the water. Even long duration oven drying (as most research samples are done and most management recommendations are based upon) leave a few points of moisture in the sample.