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2.2.3 Determination of Sample Total Dry Matter using the Two-step Procedure

Reference:
Abrams, Stephen M. 1984. Laboratory procedures for determining dry matter, crude protein and acid detergent fiber. Proceedings National Alfalfa Hay Quality Testing Workshop. Chicago, IL.
Mertens, D. R. 1993. Determining dry matter in diverse types of feeds. Proc. NFTA Forage Analysis Workshop, Denver, CO. pp B1-B10.

Scope:
This procedure is applicable for determining total dry matter on all types of forages.

Basic Principle:
Total dry matter is determined in a two-step process. First, the sample is partially dried by one of the methods described in section 2.2.1. Second, laboratory dry matter is determined by one of the methods described in section 2.2.2. Total dry matter is then calculated by multiplying partial dry matter times laboratory dry matter.

Equipment:
See appropriate dry matter methods.

Reagents:
None.

Safety Precautions:
See appropriate dry matter methods.

Procedure:

1. Determine partial dry matter (Section 2.2.1).
2. Determine laboratory dry matter (Section 2.2.2).
3. Calculate total dry matter by multiplying partial dry matter times laboratory dry matter.

Calculation: Percent Total Dry Matter (Total DM)

Partial DM = (W3 - W1 / W6 - W4)
Lab DM = (W2 - W1 / W5 - W4) X 100
% Total DM = Partial DM X Lab DM

• Where W1 = tare weight of container (with cover) in grams
• W2 = initial weight of sample and container (with cover) in grams
• W3 = dry weight of sample and container (with cover) in grams
• W4 = tare weight of container (with cover) in grams
• W5 = initial weight of sample and container (with cover) in grams
• W6 = dry weight of sample and container (with cover) in grams

Calculation: Percent Total Moisture:
% Total Moisture = 100 - % Total DM

Quality Control:
Include at least one set of duplicates in each run if single determinations are being made.

An acceptable average standard deviation among replicated analyses for moisture or dry matter is about ±0.10, which results in a warning limit (2s) of ±0.20 and a control limit (3s) of ±0.30. Plot the results of the duplicate analyses on an R-control chart (Appendix D) and examine the chart for trends. Results outside the 95 percent confidence limits warn of possible problems with the analytical system. Results outside the 99 percent confidence limits indicate loss of control, and results of the run should be discarded. If more than five or six points in succession fall on one side or the other of the 50 percent line, it is a strong indication that something has changed and is cause for investigation.

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