Organic matter digestibility (OMD) is a crucial metric in animal nutrition, representing the proportion of organic matter in a feedstuff that an animal can actually digest and utilize. Understanding how to calculate OMD is essential for formulating balanced diets, optimizing feed efficiency, and ensuring animal health.
Understanding Organic Matter Digestibility (OMD)
Organic Matter Digestibility (OMD) quantifies the percentage of the total organic matter in a feed that an animal's digestive system can break down and absorb. This is vital because only the digested portion contributes to the animal's energy and nutrient supply for maintenance, growth, and production.
The calculation of OMD primarily involves two key components:
- Total Organic Matter (OM) present in the feed.
- Digestible Organic Matter (DOM), which is the fraction of total organic matter absorbed by the animal.
The Calculation Formula for Organic Matter Digestibility
To calculate organic matter digestibility, you first need to determine the total organic matter and then ascertain the digestible fraction.
Step 1: Determine Total Organic Matter (OM) in Feed
Total organic matter is the portion of a feed material that is not inorganic (ash). It includes carbohydrates, proteins, lipids, and vitamins.
The formula to calculate total organic matter is:
Total Organic Matter (OM) = Dry Matter (DM) - Crude Ash
- Dry Matter (DM): This is the feed sample after all moisture has been removed, typically by drying in an oven.
- Crude Ash: This represents the inorganic mineral content remaining after burning the feed sample at high temperatures.
Example: If a feed sample has 90% Dry Matter and 5% Crude Ash, its Total Organic Matter is 90% - 5% = 85%.
Step 2: Determine Digestible Organic Matter (DOM)
The Digestible Organic Matter (DOM) represents the amount of organic matter in a feed that is actually digested and absorbed by the animal. This value is derived by multiplying the total organic matter by its specific digestibility coefficient.
The formula for Digestible Organic Matter (DOM) is:
Digestible Organic Matter (DOM) = Total Organic Matter (OM) × Digestibility Coefficient of Organic Matter (%)
The Digestibility Coefficient of Organic Matter is a percentage value (e.g., 70% or 0.70) that represents how much of the organic matter in a specific feed is digestible by a particular animal species. This coefficient is not calculated from the feed's chemical composition alone but rather determined through experimental methods (discussed below).
Example: If a feed contains 85% Total Organic Matter and its Digestibility Coefficient of Organic Matter is 70%, then:
DOM = 85% × 0.70 = 59.5%
Step 3: Calculate Organic Matter Digestibility (OMD%)
Finally, the Organic Matter Digestibility (OMD) as a percentage is calculated by dividing the Digestible Organic Matter (DOM) by the Total Organic Matter (OM) in the feed and multiplying by 100.
Organic Matter Digestibility (OMD%) = (Digestible Organic Matter (DOM) / Total Organic Matter (OM)) × 100
Example (continuing from above):
OMD% = (59.5% / 85%) × 100 = 70%
This confirms that the OMD percentage is essentially the digestibility coefficient when calculated this way. In practical animal feeding trials, the OMD is often directly calculated as:
OMD% = ((Organic Matter Intake - Organic Matter in Feces) / Organic Matter Intake) × 100
Where:
- Organic Matter Intake: Total organic matter consumed by the animal.
- Organic Matter in Feces: Total organic matter excreted in the feces.
This latter method is how the "Digestibility Coefficient of Organic Matter" itself is experimentally determined.
Practical Methods for Determining Digestibility Coefficients
The accuracy of OMD calculations heavily relies on reliable digestibility coefficients, which are typically determined through scientific experiments.
1. In Vivo (Animal Feeding Trials)
Considered the "gold standard," in vivo trials involve feeding animals a controlled diet and meticulously collecting all feed consumed and feces excreted over a specific period.
- Process: Animals are kept in metabolism crates, and intake is measured. Feces are collected, weighed, and analyzed for dry matter and ash content.
- Calculation: The difference between organic matter intake and organic matter excreted in feces represents the digestible organic matter.
- Advantages: Provides the most accurate representation of true digestibility in a living animal.
- Disadvantages: Time-consuming, expensive, requires specialized facilities and animal care.
2. In Vitro (Laboratory Methods)
These methods simulate the digestive process in a laboratory setting, offering a faster and more cost-effective alternative to in vivo trials.
- Rumen Fluid Incubation: Feed samples are incubated with rumen fluid (collected from cannulated animals) under anaerobic conditions to mimic microbial digestion in the rumen.
- Enzyme Digestion: Specific enzymes (e.g., cellulase, pepsin) are used to simulate enzymatic digestion in the monogastric digestive tract.
- Advantages: Faster, less expensive, allows for screening of many samples.
- Disadvantages: May not perfectly replicate the complex biological processes in a living animal, requiring calibration with in vivo data.
3. Predictive Equations
These equations estimate digestibility based on the chemical composition of the feed, such as fiber content (e.g., Neutral Detergent Fiber - NDF, Acid Detergent Fiber - ADF) or crude protein.
- Method: Chemical analysis of the feed provides input for equations developed from extensive in vivo and in vitro data.
- Advantages: Quick, inexpensive, requires only chemical analysis.
- Disadvantages: Less accurate than direct experimental methods, as predictions rely on statistical relationships.
Example Digestibility Coefficients (Illustrative)
The following table provides hypothetical digestibility coefficients for organic matter in common feedstuffs for cattle. Actual values can vary significantly based on feed quality, processing, and animal factors.
Feedstuff | Average OMD Coefficient (%) |
---|---|
High-Quality Alfalfa Hay | 65 - 75 |
Corn Silage | 60 - 70 |
Barley Grain | 85 - 90 |
Soybean Meal | 80 - 85 |
Wheat Straw | 40 - 50 |
(Note: These values are illustrative and should not be used for actual feed formulation without consulting specific, up-to-date feed composition tables.)
Importance of Accurate OMD Calculation
Accurately calculating organic matter digestibility is fundamental for:
- Diet Formulation: Ensuring animals receive adequate digestible nutrients to meet their production goals (e.g., milk, meat, eggs).
- Feed Efficiency: Maximizing the conversion of feed into animal products, which is crucial for economic viability.
- Environmental Impact: Optimizing nutrient utilization can reduce nutrient excretion (e.g., nitrogen, phosphorus), thereby minimizing environmental pollution.
- Animal Health: Preventing nutrient deficiencies or excesses that could impair health and performance.
By understanding the methods to calculate total and digestible organic matter, and the ways in which digestibility coefficients are determined, nutritionists and producers can make informed decisions to optimize animal feeding strategies.