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Animal-Based Analyses

As stated, laboratory-based analyses provide quantitative information on the nutrient composition of a feedstuff, however, do not provide consistent information on the efficiency of nutrient utilization by the animal. To provide more information on the nutritional value of a feedstuff, laboratory-based analyses are used in conjunction with animal-based analyses such as feeding, digestion, and balance trials.

Feeding trials are designed to compare production results between two or more feeding schemes. Feeding trials are classified as applied research, as the trials are designed to simulate a specific production environment. In nutrition research, feeding trials are used extensively. Often, feeding trials are preliminary trials and will be followed with additional trials, such as digestion and balance trials, to more specifically estimate nutrient utilization. To compare feeding schemes, feeding trials are designed with a control and with a treatment. The treatment may be administered at one or more predetermined levels. The treatment of a feeding trial may be a specific feedstuff, the processing of a feedstuff, the level of the feedstuff, the use of additives, etc. Generally, the effect of a feedstuff is measured and stated as differences in production between control and treatment(s). Production may be measured as total production, rate of production, or change in production over a period of time.

Digestion trials provide an estimate of the total amount of a specific nutrient absorbed from the gastrointestinal tract. Digestion trials can be performed to measure the digestibility of a range of feed components including the general nutrient class or a specific component within the class. The two measures of digestibility are apparent and true digestibility. Apparent digestibility does not account for endogenous sources and true digestibility does account for endogenous sources of the nutrient of interest. Endogenous materials include intestinal cells, enzymes, and microbes. Apparent digestibilities have limited use for nutrients significantly influenced by endogenous materials. Generally, digestion trials measure apparent digestibility. A digestion trial will have an initial adjustment period followed by a sampling period.

The initial step is to determine the total amount of the feedstuff consumed and the concentration of the specific nutrient in the feedstuff. One method to measure the total amount consumed is to individually feed the animals and to measure the amount fed and the amount refused for each animal.

Figure 3.9 is a photograph illustrating individual feeders commonly used for dairy and beef animals.

Figure 3.9 - Individual feeders for cattle

The next step is to determine the total amount of feces excreted and the concentration of the specific nutrient in the feces. One method to measure the total amount excreted is to collect and measure the total amount of feces excreted.

Figure 3.10 is a photograph illustrating the use of a fecal collection bag.

Figure 3.10 - Fecal collection bag

Measurements are expressed on a dry matter basis (DM basis) to correct for differences in moisture content. Finally, the values are plugged into the following equation to determine apparent digestibility of a nutrient: digestibility (%) = ((nutrient in nutrient out) / (nutrient in)) x 100.

An additional method to measure apparent digestibility of a nutrient uses a marker or indicator. Markers are classified as either external or internal. External markers are compounds supplemented to the feedstuff such as chromic oxide. Internal markers are compounds inherent to the feedstuff such as lignin. The marker must be an inert compound. The following equation is used to determine the apparent digestibility of a nutrient: digestibility (%) = 100 (100 x (marker concentration in feedstuff / marker concentration in feces) x (nutrient concentration in feces / nutrient concentration in feedstuff)). If total intake and output cannot be measured, markers are used to estimate digestibility of a nutrient.

Balance trials are another method to determine nutrient utilization of a feedstuff. Compared to digestion trials, balance trials provide a more complete and accurate picture of nutrient utilization of a feedstuff in the animal. Balance trials attempt to estimate the net retention of a specific nutrient in the animal. Balance trials may be performed on a range of feed components. Similar to digestion trials, balance trials measure total amount of a nutrient consumed and total amount of a nutrient excreted and use an equation to measure the resultant value. In a balance trial, individual intake is measured similar to the first method in a digestion trial. However, in a balance trial the measured sources of excretion are feces, urine, respired air, and sloughed materials such as skin, hair, and feathers. For lactating animals, milk yield and components are also included as a source of excretion. To collect the required measurements, balance trials require the use of a metabolism crate.

Figure 3.11 is a photograph of a swine metabolism crate.

Figure 3.11 - Swine metabolism crate

Balance trials also require an adaptation and sampling period. The sampling period is dependent upon the time required to receive an accurate estimate of net retention of a specific nutrient.

In addition to providing information on the value of specific nutrients in a feedstuff, digestion and balance trials provide information on the energy value of a feedstuff. As stated previously, there are a number of terms to describe the energy value of a feedstuff.

Figure 3.12 illustrates the relationships between the various terms used to describe the energy value of a feedstuff.

Figure 3.12 -  Energy flow diagram

In comparison to gross energy, digestible energy (DE) is a more accurate estimate of the energy value of a feedstuff. To estimate the DE of a feedstuff, a digestion trial is performed and the total or gross energy of the feed consumed and the feces excreted are measured with the use of a bomb calorimeter. The feed energy is abbreviated GE. The fecal energy is abbreviated FE. Fecal energy is the single greatest energy loss for any given feedstuff. The apparent DE is estimated by the following equation: DE = GE FE. The actual DE values are available for a number of the domestic species and a broad range of feedstuffs. In comparison to DE, metabolizable energy (ME) is a more accurate estimate of the energy value of a feedstuff. To determine ME, the total or gross energy of a feed consumed, the feces excreted, the urine excreted, and the gases excreted are measured. The urinary and gaseous energy are abbreviated UE and GE*, respectively. The primary gas produced by microbial metabolism in the GI tract is methane. The ME is estimated using the following equation: ME = GE (FE + UE + GE*). For poultry, ME is used to define the energy value of a feedstuff and to formulate rations. For ruminants, ME is used to help calculate net energy values. The most accurate term to describe the energy value of a feedstuff is net energy (NE). Net energy estimates the energy available for maintenance and productive functions in the animal. As the efficiency of energy utilization varies by function, the NE values vary by function. The three NE values are net energy of maintenance (NEM), growth (NEG), and lactation (NEL). The initial step to estimate NE is to estimate ME via a balance trial. The second step is to estimate the heat increment (HI) via calorimetry. The heat increment is the energy loss associated with the consumption, digestion, absorption, and metabolism of the nutrients for maintenance and productive functions. The energy is in the form of heat. The NE is estimated using the following equation: NE = ME HI. Rather than via direct determination, the majority of NE values were estimated with the use of equations. For ruminants, NE values are used to formulate rations.

Total digestible nutrients (TDN) is another method that estimates the energy value of a feedstuff. It has been argued, the TDN system is not the most accurate system available. However, as the TDN system is relatively easy to understand and has been available for a long period of time, the TDN system remains the most common system used on farms. To estimate TDN, the total individual digestibilities of protein, carbohydrates, and lipids in the feedstuff are determined using digestion trials in combination with proximate analysis. Then, to estimate TDN, the individual digestibilities are plugged into to the following equation: TDN = (digestible crude protein) + (digestible crude fiber) + (digestible nitrogen-free extract) + (digestible ether extract x 2.25). The multiplier of 2.25 is used to adjust for the greater energy density in a unit of ether extract compared to a unit of carbohydrate or protein. TDN is expressed in units of concentration or mass. TDN values account for fecal and urinary energy losses, therefore TDN is intermediate between DE and ME values. The primary advantage of the TDN system is its simplicity. The primary disadvantage of the TDN system is it tends to overestimate the energy value of fibrous feedstuffs in comparison to nonfibrous feedstuffs.

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