Moisture Testing Method Affects Accuracy

The current dry conditions across Ontario make moisture-testing essential this year for corn silage. What methods are available? Which is the most accurate?

The 4 moisture-testing methods are NIR (Near Infrared) moisture, lab oven dry matter, Koster tester and microwave.

Some labs will test moisture using an NIR machine with a corn silage moisture calibration. This is a very accurate method. All labs will offer an oven dry matter which is also quite accurate. In a lab oven analysis, an 80 to 125 gram sample is dried in a 55° C oven overnight. Drying overnight will not remove all the moisture. Even under controlled lab settings, there will be 1.3 to 2.2% residual moisture in the sample after drying in an oven (Figure 1). Sample results from oven drying at a lab will be available in as little as 1 - 2 days.

Koster testers and microwaves can also be used. Researchers' Ballweg and Rankin compared a lab oven, Koster and microwave moisture testing methods in a laboratory setting. The researcher's experienced operator found about 2% residual moisture in the samples (Figure 1). Also, it is important that the sample be finely ground to reduce the residual moisture (Figure 1). This can be achieved by running the whole corn plant into a bedding chopper, leaf chipper or forage harvester. Chopping by hand is not recommended. Fine chopping allows the sample to dry more completely, reducing residual moisture.

| Top of Page |

Ballweg and Rankin, 1998

Ballweg and Rankin, 1998

| Top of Page |

An on-farm study looked at residual moisture in samples tested by Koster or microwave using a single experience operator or multiple operators on the same sample of corn silage (Figure 2). The highest residual moisture (6%) was found in microwaved samples using multiple operators. Error levels of this magnitude are unacceptable for growers needing to make a corn silage harvest decision. Ensure that one operator does all the moisture testing, especially when using a microwave (Figure 2). If you are not an experienced operator at using a microwave for moisture testing, use a Koster tester to improve the accuracy of the test.

If you know there will be residual moisture, account for this! For example, if a sample of corn silage tests 37% moisture, there will still be residual moisture in the sample. This moisture could range from 2 to 6%, depending on the testing method and the experience of the operator. This means the actual moisture of the sample is 39 to 43%!

Koster testers are available from many farm supply dealers in Ontario. Using a 100-gram sample in a Koster tester will take over 30 minutes if the silage is very wet. When you think the sample is dry, weigh it. Return it to the Koster for 2 to 3 additional minutes, then weigh it again. If the moisture has not changed, the sample is dry (assume at least 2% residual moisture). Digital postal scales are more accurate than the scale provided with the Koster tester.

For microwave testing, weigh and record the weight of an empty paper plate or bag. Record this weight as A. Spread an 80 to 125 gram sample evenly on the plate or bag. Weigh and record the exact weight (sample plus container). Record this weight as B. Don't forget to put a glass of water in the microwave. Dry on high power for 2 minutes (or up to 3 to 4 minutes if the sample is very wet). Remove, record weight, gently stir the sample and return it to the microwave. Don't burn your fingers! Continue for intervals of 30 seconds to 1 minute on medium-low until the sample weight is stabilized. When the weight remains the same for 3 drying intervals, record the weight as C. If the sample becomes charred, start again with a fresh sample.

To calculate moisture, use the following formula:

[(B - A) - (C - A)] ÷ (B - A) x 100 = % Moisture

Accuracy of Moisture Testing

Fine > coarse (Figure 1)
Lab oven > Koster > microwave (Figures 1 and 2)

Accurately determining the moisture of corn silage on the farm is a difficult process. A consistent protocol is essential. Even with a good protocol, residual moisture will be at least 2%. A better option is to submit samples to a lab for an oven dry matter analysis. A lab analysis reduces sample-to-sample variability and improves accuracy. When an accurate moisture is known, the operator can make reasonable management decisions on when to harvest corn for silage.

| Top of Page |