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¶ Introduction
Moisture and dry matter are the most basic of feed tests, but the result is essential to good livestock nutrient management.
Dry matter is defined as the moisture-free content of a feed ingredient or ration. Dry matter contains the various substances and essential nutrients making up that particular feed sample.
Drying removes and separates the water from the original sample (see Figure 1). Feedstuffs and rations vary widely in their moisture content, but the sum of moisture and dry matter contents as a percentage will always equal 100%.
Moisture dilutes the nutrient concentrations, but has little effect on actual dry matter intake Animals consume a certain amount of dry matter per day according to their type, body weight, age, and environmental conditions. They consume this amount of dry matter regardless of the moisture content of the ration. Rations and diets should always be formulated and evaluated on a dry matter basis.
Animals also have a daily water requirement. Some of the daily water requirement can be supplied through the feedstuff moisture. The dry matter results can be used to evaluate whether or not feed or ration moisture content is within expected ranges.
¶ Figure 1. Basic Work Flow of Feedstuff Analysis Procedures
¶ Maintaining Nutrient Quality
Dry feeds usually have less than 15% moisture (greater than 85% DM). Forages have moisture contents between 10% and 90%, depending on the harvest method – pasture, silage, or hay.
Moisture content at harvest and during storage is important to maintain feed and forage quality. Hay should be stored between 15% to 20% moisture. Leaf loss increases with moisture lower than 15%, which reduces forage quality. Moisture higher than 20% promotes mold growth and bacterial decomposition, resulting in loss of forage quality, but also can increase chances for spontaneous fire.
Silage crops and high moisture grains have optimum moisture ranges to promote good fermentation and storability. Moisture above the optimum can result in nutrient loss through seepage or lower nutrient availability due to heat damage. Proper fermentation requires that air must be eliminated from the silage mass. Silage that is below the optimum moisture range does not pack properly, so air remains which allows mold growth and spoilage.
¶ Comparing Nutrient Values
It is important to compare nutrient content of different feeds on an equal basis. Nutrient contents are reported on an "as fed" or “as received” basis (moisture included) and on a “dry matter” basis (moisture excluded). Nutrient content will appear higher on a dry matter basis than on an as fed basis. Feeds having a higher moisture content, like pastures, will appear to have much lower nutrient content than dry, low moisture feeds, like hay, when compared with an as fed basis.
For example, Table 1 shows that the grass pasture has much lower nutrient content on an as fed basis than the grass hay. When the results are corrected for the moisture content, both pasture and hay have equal nutrient content.
¶ Table 1. Comparison of Nutrient Contents Expressed on as Fed (AF) or Dry Matter (DM) Basis |
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| Nutrient Content | ||||
| Example forage | Basis | Dry matter % | Protein % | ADF % |
| Grass pasture | AF | 20 | 2.2 | 8 |
| DM | 100 | 11 | 40 | |
| Grass hay | AF | 90 | 9.9 | 36 |
| DM | 100 | 11 | 40 | |
The as fed results of both feeds must be converted to a dry matter basis to accurately compare their respective nutrient values. Equations to convert from one basis to another are given in Table 2.
¶ Table 2. Conversions for Moisture Basis |
| As fed nutrient content = DM nutrient content x (DM% ÷ 100) |
| Dry matter nutrient content = AF nutrient content ÷ (DM% ÷ 100) |
¶ Comparing Nutrient Costs
Dry matter content should be included when comparing nutrient costs. For example, some livestock operations purchase corn silage based on moisture content in order to pay their silage producers more rapidly.
Table 2 compares two silages with two different prices ($50 vs. $60 per ton) and two different moisture contents (70% vs. 60%). Silage #2 has moisture content that is 10% lower than silage #1, but is $10 per ton more expensive than silage #1.
Silage #2 is actually a better buy than silage #1 because a ton of “wet” #2 silage has 200 pounds more dry matter (i.e., nutrients) than a ton of silage #1. A truckload of silage will have the same amount of wet forage, but a truckload of silage #2 has a higher nutrient value than a
truckload of silage #1 because it contains more dry matter and less water.
The purchase cost of the “wet” silage is 2.5¢ and 3.0¢ per pound for silage #1 and #2, respectively. The extra dry matter per ton in silage #2 potentially means 200 pounds more nutrients in each purchased ton. Therefore, the dry matter cost of silage #1 is 8.3¢ per pound and silage #2 is 7.5¢ per pound.
The same approach can be used to compare cost of any nutrient between two feed sources. Divide the price per purchased unit on an as fed basis by the content of the nutrient per purchased unit on a dry matter basis.
¶ Measuring Moisture, Dry Matter
Oven drying is the standard method for determining dry matter content of a feed. There are other drying methods that use microwave ovens or forced air, but oven drying is the standard.
Oven drying involves weighing a sample of a “wet” or asreceived feed, then heating the sample in a convection drying oven for a specified time to remove the moisture. Drying is complete when the sample reaches a constant weight. The dried sample is weighed back after cooling.
Standard laboratory methods specify the temperatures and drying times for different commodities and/or different equipment. A common drying temperature is 105°C (221°F), slightly above the boiling point of water.
Drying times may range from 3 to 48 hours, depending on the method.
Drying a sample at a temperature that is too high can cause fractions in certain feeds and ingredients to volatilize or “burn off”. This includes some organic materials, like fats or oils, and inorganic compounds, like ammonia or urea. This loss of true dry matter content would be falsely included in the total “moisture” content.
Samples for nutrient analysis are typically dried at lower temperatures to avoid altering the feed composition.
¶ Reporting Moisture, Dry Matter
Dry matter content is expressed as a percentage of the original sample weight (including both moisture and dry matter). The moisture percentage is the difference between the dry matter percentage and 100%.
For example, a “wet” feed sample originally weighed 150 grams. It weighed 50 grams when fully dry. The dry matter percentage would be 33.3%:
- dry matter % = (dry weight ÷ wet weight) x 100
- 33.3% = (50 g ÷ 150 g) x 100
The moisture content of this feed would be 66.7%:
- moisture % = 100 - dry matter %
- 66.7% = 100 - 33.3%
¶ Table 3. Dry Matter Content Affects Nutrient Cost |
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| Purchase Cost* | Lab Analysis | Pounds per Ton | Dry matter cost per pound** | ||
| Water | Dry Matter | ||||
| Silage #1, $50/ton | Moisture | 70% | 1400 | ---- | ---- |
| Dry matter | 30% | ---- | 600 | $0.083 | |
| Silage #2, $60/ton* | Moisture | 60% | 1200 | ---- | ---- |
| Dry matter | 40% | ---- | 800 | $0.075 | |
| *$50 ÷ 2000 lb. = $0.025/lb | **$50 ÷ 600 lb. = $0.083 | ||||
| *$60 ÷ 2000 lb. = $0.030/lb | **$60 ÷ 800 lb. = $0.075 | ||||
¶ Air-Dry Basis
The guaranteed analysis on a feed tag is commonly assumed to be an "as received" value with 90% dry matter, unless specified differently. A standard 90% dry matter (10% moisture) is referred to as the “air dry" basis. To convert the 90% air-dry basis result to a 100% dry matter basis, divide the air-dry results by 90% (dry matter basis % = air-dry basis % ÷ 0.90).
¶ References
Van Saun, R. 2013. Determining Forage Quality: Understanding Feed Analysis. Penn State Univ. Exte. https://extension.psu.edu/determining-forage-qualityunderstanding-feed-analysis
Rasby, R.J., et. al.2008. NebGuide G1892 - Understanding and Using a Feed Analysis Report. Univ. of Neb. Coop Ext. accessed 19July2022 https://extensionpublications.unl.edu/assets/html/g1892/build/g1892.htm