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2.2 Two Step Total Dry Matter Determination of Wet Samples >>

2.1.3 Total Dry Matter by Microwave Drying to Constant Weight


This procedure is applicable for hay and other samples with low volatile acid content. Samples dried by this procedure are not appropriate for subsequent fiber, lignin, acid detergent insoluble nitrogen analysis or NIR analysis.

Basic Principle:
Moisture is evaporated from sample by microwave radiation. Total dry matter is determined gravimetrically as residue remaining after drying.

Microwave oven with minimum of 600 watts, turntable preferred but not essential Top loading electronic balance, accurate to 0.01 g Containers, paper or microwavable glass or plastic, sufficient in size to hold 100 to 250 g coarse forage with maximum sample depth of 1.5 inches


Safety Precautions:

  • Periodically test the oven for radiation leaks. Microwaves are absorbed by the body and can produce damaging heat effects, especially on the lens of the eye.
  • Cardiac pacemakers may fail in presence of microwave radiation.
  • Do not place any metal item or aluminum foil into the oven. Sparking or crackling in the oven indicates the presence of metal.


  1. Dry empty container in microwave for 3 min at full power.
  2. Weigh empty container on top loading balance and record weight (W4) to nearest 0.01 g.
  3. Tare empty container to zero and weigh forage (100 to 200 g) into container, recording weight to nearest 0.01 g (W7), or add 100 to 200 g forage to the container and record weight of container and sample to nearest 0.01 g (W5).
  4. Place sample in oven and microwave at full power for about 3 min.
  5. Remove sample and mix.
  6. Repeat steps 4 and 5 twice for haylage (50% DM) and three times for silage (35% DM).
  7. Weigh sample and container recording weight to 0.01 g.
  8. Return sample to microwave at 50% power for 1 minute and re-weigh sample and container, recording weight to nearest 0.01 g.
  9. Repeat steps 7 and 8 until no weight loss occurs during drying interval. Final weight is W6. Do not allow sample to char or burn.


  • Do not place any metal item or aluminum foil into the oven. Sparking or crackling in the oven indicates the presence of metal.
  • Be careful not to char or burn samples. As samples approach dryness reduce drying time and/or power setting. Discard darkened or burnt smelling samples and start over. Charring causes weight loss other than moisture and results in low dry matter values.
  • Samples with higher moisture contents require longer drying times. Increase the number of drying intervals, not the time per drying interval or microwave power level, as these may lead to charred or burned samples.
  • A glass of water placed in the microwave will reduce possibility of charring samples.

Calculation: Percent Total Dry Matter (Total DM)
If empty container is tared to zero in step 3

               W6 - W4

% Total DM = -------- X 100

  • Where W4 = tare weight of container in grams
  • W7 = dry weight of sample in grams
  • W6 = dry weight of sample and container in grams


<< 2.1.2 Total Dry Matter by Oven Drying at 100oC for...

2.2 Two Step Total Dry Matter Determination of Wet Samples >>

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