There are varying methods for characterizing the nutritional merits of feeds and feedstuff; they include chemical and biochemical analytical techniques.
The assessment of a protein diet is usually progressive, starting from the very simple to the more complex tests. The appraisal starts with nitrogen and amino acid analysis, then through a series of specific chemical evaluations, and finally the biological tests.
Previously, animal experiments have been widely used to assess the protein quality of feed. The results obtained from such experiments are regarded as sufficient in the provision of all the information regarding the protein quality of the feed.
There is however a limit to the quantity and quality of information that can be derived from animal assay procedures which should be highlighted.
Biological techniques are highly recommended in the developing economy, especially in areas where standard laboratories with up-to-date equipment are lacking. Biological assay techniques require fewer and common simple equipment that can be obtained in the open market.
The Significance of Biological Assays
The biological assay of feed is of utmost importance to the identification of feed resources that can be utilized for livestock feeding in developing economies.
The capability of a feed to satisfy the protein requirements of livestock is a function of both the quantity of protein and the nutritional quality of the feed.
The significance of biological assays for assessing protein quality is to measure nutritional quality as a characteristic of a test protein. The method of choice of protein assessment should have values that range from zero to one (O-1) or from zero to one hundred percent (0 to 100 %).
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The Types of Biological Assays
The following are feed protein quality evaluation methods;
1. Protein efficiency ratio (PER)
It is the simplest method of determining the nutritive value of an experimental feed. The growth rate of the young livestock is measured.
The weight gain is related to the amount of protein ingested by the animal, the index that is obtained is then termed as the protein efficiency ratio.
Biological assays are dependent on nitrogen-balance methods such that nitrogen intake and nitrogen excreted are determined for animals fed experimental diets containing the test protein or a protein-free diet, and then the nitrogen retention is estimated indirectly.
The faecal and urinary nitrogen excretion of metabolic and endogenous origin can then be determined. This can be used for the estimations of apparent digestibility (AD), true digestibility (TD), net protein utilization (NPU), and biological value (BV).
2. Net Protein Utilization (NPU)
It is the ratio of amino acids which has been converted to protein to the ratio of amino acids supplied. It could also be expressed as the product of biological value and digestibility.
NPU = BV x D. Experimentally, NPU can be determined by measuring the dietary protein intake and nitrogen excreted by the experimental animal. The equation will thus be;
NPU = {0.16 × (24 hour protein intake in grams)} – {(24 hour urinary urea nitrogen) + 2} – {0.1 × (ideal body weight in kilograms)} / {0.16 × (24 hour protein intake in grams)}
The value of NPU can vary from one (1) to zero (0). A value of one indicates that there is a 100 percent utilization of the dietary nitrogen as protein and a value of zero is an indication that none of the nitrogen supplied was converted into protein. Examples of feedstuff such as milk or egg have an NPU of one (1) on the NPU chart.
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3. Apparent Digestibility (AD)
This is estimated as the difference between nutrients contained in the feedstuff ingested and nutrients contained in the faeces voided by the animal.
AD = Nutrient ingested – Nutrient voided in faeces. Apparent digestibility does not account for nutrients lost as methane gas or metabolic waste in faeces.
4. True Digestibility (TD)
This is the portion of the feedstuff ingested by the animal which has been absorbed into the bloodstream of the animal. True digestibility corrects for the endogenous and microbial effect of nutrients lost in faeces.
Thus, TD could be defined as the difference between nutrient intake and nutrients in faecal output taking into account nutrients that are not derived from digested feed residues (such sources are the cells of the intestinal tracts that have been shed, microbial matter such as bacteria residue and digestive juices or enzymes).
5. Biological Value (BV)
This is the proportion of the absorbed nitrogen that is retained for maintenance and/or growth. When BV is defined in terms of carcass nitrogen, the apparent BV is expressed as a ratio to unity (recommended) or as a percentage.
The determination of the biological value of a protein diet is based on nitrogen-balance methods in which nitrogen ingested and excreted are determined in animals fed diets containing the test protein diet or protein-free diet and nitrogen retention is estimated indirectly.
The procedure allows for the determination of faecal and urinary nitrogen excretion of metabolic and endogenous origin.
In conclusion, a biological assay of diets/feedstuff given to livestock is essential as this will provide relevant information on the protein quality of the feed served to the animal. This in turn will aid productivity due to the knowledge of the protein status and the availability to the livestock.
Biological assay of feedstuff given livestock is essential as it provides relevant information on the protein quality and utilization by the animal. The biological assay of feedstuff for protein value is one of the final tests for chemical/biological evaluation of feedstuff.
A biological assay can be done using a few, common and simple equipment (for example, a weighing scale) to evaluate. There are five (5) types of biological assay considered in this study and they include; PER, NPU, AD, TD, and BV.
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