2.2.2 Laboratory Dry Matter of Partially-Dried Samples >>

2.2.1.2 Partial Dry Matter Using Microwave Oven

Reference:
None.

Scope:
This procedure is applicable to all types of forages and is intended for an initial, partial drying for wet samples (less than 85% dry matter, greater than 15% moisture). The procedure has minimal effect on chemical composition, allowing sample to be analyzed subsequently for fiber, lignin, or acid detergent insoluble nitrogen analysis.

Basic Principle:
Moisture is evaporated from sample using microwave energy, and partial dry matter is determined gravimetrically as the residue remaining after oven drying.

Equipment:
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

Reagents:
None.

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.

Procedure:

1. Dry empty containers in microwave for 3 min at full power.
2. Weigh empty container on top loading balance and record weight (W1) 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 sample and container to nearest 0.01 g (W2).
4. Place sample in oven and microwave for about 3 min at full power.
5. Remove sample and mix.
6. Repeat steps 4 and 5 twice for haylage (50% DM) and three times for silage (35% DM).
7. If sufficiently dry (sample crisp, not damp to touch), stop microwave drying. If not, return sample to microwave at 50% power for 1 minute intervals until not damp to the touch. The objective is to dry the sample to 90 to 95% dry matter.
8. Equilibrate sample for 2 hrs at room temperature and weigh sample and container (W3), recording weight to nearest 0.01g.

Comments:

• 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 affects dry matter determination and results of subsequent analysis of fiber, ADIN, lignin analysis and NIR analysis. Be especially careful when drying the entire sample received (no reserve sample).
• 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 oven will reduce the possibility of charring samples.

Calculation: Partial Dry Matter (Partial DM), expressed as ratio of (w/w) of dry matter to total weight.
If container is tared to zero in step 3

Partial DM = W3 - W1 / W7

• Where W1 = tare weight of container in grams
• W7 = initial weight of sample in grams
• W3 = dry weight of sample and container in grams

If container is not tared to zero in step 3

                     W3 - W1

Partial DM = --------  
                     
                     W2 - W1

• Where W1 = tare weight of container in grams
• W2 = initial weight of sample and container in grams
• W3 = dry weight of sample and container in grams

Quality Control:
Laboratory dry matter of partially dried samples should be between 85 and 95%.

2.2.2 Laboratory Dry Matter of Partially-Dried Samples >>

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