The task of commercial nutritionists is to meet the nutrient requirements of animals through judiciously selecting and mixing several ingredients, in a most economical, ethical, and environmentally friendly way.
The nutrient requirements of an animal vary depending not only on its biological parameters such as genetic potentials, sex, and growth and reproductive stages, but also on performance parameters that are examined. For example, it is known that a higher dietary methionine content is needed to maximize breast meat yield than body weight for broiler chickens. Although variable and complicated, the nutrient requirements of an animal under given conditions have been extensively investigated and relatively well documented (see, for example, NRC, 1994).
A much greater challenge for a commercial nutritionist, however, is to meet the nutrient requirements of an animal through feed formulation. The problem is with the assessment of true feeding value of feed ingredients. It is a fact that all nutrients in a feed ingredient are not available for utilization by the animal. On the other hand, only those nutrients that can be digested, absorbed, and metabolized (utilized) by the animal for maintenance, growth, and/or reproduction, represent the true feeding value of that ingredient. This portion of nutrients are called available nutrients, and in the case of amino acids (AA), the available AA. Amino acid availability varies not only among different types of feed ingredients, but also among different sources and loads of the same ingredient. Because of this variation in AA availability among various ingredients, the total AA content of a feed or an ingredient is, in a sense, of little relevance to meeting the AA requirements of an animal. When formulating feeds using total AA, nutritionists have to rely on subjective techniques such as using a large safety margin and/or using minimum/maximum limitations to exclude or restrict the use of some ingredients that are known to have low or variable AA digestibility coefficients.
Ideally, nutritionists should be able to evaluate feed ingredients and to formulate feeds using available AA. Amino acid availability is a function of digestion, absorption, and utilization. It can be determined by the growth assay. For example, Batterham (1992) used the slope ratio technique to measure AA availability in pigs. This technique offers a more precise estimation of the true feeding value of a feed than other methods. This technique, however, is very expensive and time consuming. So far, only a few ingredients have been evaluated by this method. It is at present impossible to formulate feeds based on available AA.
The true ileal digestible AA offer a compromise between the more or less irrelevant total AA and the trouble of measuring available AA. The aim of this presentation is firstly to evaluate the feed formulation technique using either total or digestible AA, and secondly, to discuss how to use digestible AA in commercial feed formulation. Before evaluating the formulation technique, it is worthwhile to review the concept and measurement of true ileal digestible AA.
True Ileal Digestibility: Concept, Measurement, And Database
The term "true ileal digestibility" of amino acids comprehends three basic concepts in animal nutritio When a feed/ingredient is consumed, the first tax imposed is represented by that is not digested and is excreted in the feces. This effect is measured by apparent digestibility:
Apparent Digestibility % = (Ingested AA - Excreted AA) x 100 / Ingested AA
There are at least two factors that affect the effectiveness of using the apparent digestibility for the estimation of AA absorption. Firstly, AA are absorbed only in the small intestine, and microbial activities in the hind gut or ceaca change the profile and the amount of amino acids passing through the hind gut or ceaca. This leads to erroneous estimation of AA absorption. To correct this error, AA excretion should be measured at the end of ileum. To measure ileal digestibility, the animals must be surgically modified. In chickens, the caeca are removed (caecetomized), and in pigs, the large intestine is by- passed using the ileo-rectal shunt. This correction yields the ileal digestibility.
Ileal Digestibility % = (Ingested AA - Excreted AA @ end of ileum) x 100 / Ingested AA
The second factor contributing to the inaccuracy of AA apparent digestibility is the fact that all AA reaching the end of ileum are not of the feed origin. The secretion of digestive enzymes and mucus, and the turn-over of gut wall cells contribute to the AA pool at the end of the ileum. These AA are called endogenous AA. Without correction of these endogenous AA, the apparent AA digestibility would be affected by the amount of feed or ingredient used in the digestibility assay. The so-called "True Ileal Digestibility" is thus obtained by correcting the apparent ileal digestibility with the endogeno AA. True Ileal Digestibility of AA is not affected by feed intake (see Figure 1).
True Ileal Digestibility % = (Ingested AA - (Excreted AA - Endogenous AA @ end of ileum) x 100 / Ingested AA
For the determination of true ileal digestibility, the method developed by Sibbald (1976) for True Metabolizable Energy (TME) and later extended for AA digestibility by Likuski and Dorrell (1978) was adapted by various groups (Rhone Poulenc Animal Nutrition, 1993; NRC, 1994). Basically, caecectomised cockerels are fasted for 24 to 48 hours before being fed the test materials. During this preliminary period, they are only fed 50 g/bird of glucose through the drinking water to minimize the catabolism of amino acids for energy purpose. Following the fasting, the birds are force fed 50 g of the test feedstuff. Excreta are collected for 48 hours after the force feeding. The excreta are pooled and freeze dried. After separation of uric acid from the excreta, the nitrogen and amino acid content of excreta is determined. The endogenous AA excretion is estimated by feeding a nitrogen- free diet (Rhone Poulenc Animal Nutrition, 1993).
A large number of data have been generated using this technique during the past 15 years, and the results agree reasonably well among different laboratories. The primary sources of digestibility data include Sibbald (1986), Parsons (1990a), and Rhone Poulenc Animal Nutrition (1993).
Advantages Of Using True Digestible Amino Acids
The true ileal digestible amino acids can be used by nutritionists in at least two ways to improve their feed formulation as well as the bottom line of economical returns. Compared to total AA, digestible AA allows nutritionists to more objectively evaluate and select feed ingredients. Furthermore, the use of digestible AA enables nutritionists to formulate feeds using a wider range of feed ingredients, to better match the nutrient requirements of the animal, and to lower feed cost.
Using Digestible AA To Evaluate Feed Ingredients
Some important information can be obtained by simply examining the digestibility data. Table 1 listed the mean and coefficient of variation (CV) of the digestibility coefficients of three most important AA for some major ingredients used in poultry feeds (Rhone Poulenc Animal Nutrition, 1993). In general, methionine (Met) has the highest digestibility, followed by lysine (Lys), while Cystine (Cys) is the least digestible. In terms of mean digestibility, major cereals (corn, wheat, barley) have quite consistent and high values (>80%) for all three AA examined. The digestibility of oilseed meals differ greatly among different types with the lowest values observed for cottonseed meal and the highest for high protein soybean meal. The variety of the oilseed affects AA digestibility. The double-zero rapeseed (canola) meal has significantly higher AA digestibilities than the single-zero rapeseed meal. Processing technique also affects AA digestibility. The high protein soybean meal (46-48%) has higher AA digestibilities than the 44% meal, most likely due to a reduction in crude fiber content. The AA digestibilities of animal meals differ greatly among different types of meals, being highest for fish meal, followed by meat and bone meal, and lowest for hydrolyzed feather meal.
The other set of numbers deserving attention is the CV values listed in Table 1. The CV values indicate how consistent of the AA digestibility of different sources or loads of an ingredient. The lower the CV value is, the more consistent a feed ingredient is in its AA digestibility, and the higher the true feeding value. Again, major cereals have relatively lower CV values (<6%) in AA digestibility. Oilseed meals show a low to medium CV in AA digestibility, with cottenseed meal shows the highest variation, followed by single-zero rapeseed meal and sunflower meal, and the soybean meal the lowest CV among oilseed meals. Fish meal has acceptable CV, while meat and bone meal and particularly feather meal are quite variable in AA digestibility. As a result, when these animal by-products are used in feed production, special attention should be paid to evaluate their AA digestibility.
The use of digestible AA also enables nutritionists to evaluate feed ingredients more objectively and precisely than using total AA. Take sorghum as an example. Table 2 listed total AA and true digestibility of two sorghum samples, one with less than 0.5% tanins while the other has more than 0.5% (Rhone Poulenc Animal Nutrition, 1993). The two had the same total AA contents.
Therefore, if evaluated using total AA, the two sample would have the same feeding values. But the low tanin sorghum sample has AA digestibilities of 80 to 92%, while the high tanin sorghum has AA digestibilities of 64 to 79%. Apparently the low tanin sorghum offers a much higher feeding value than the high tanin sorghum does. This difference is only becoming apparent when using digestible AA.
In addition, nutritionists can use the digestible AA data to assign shadow prices to alternative ingredients so that the shadow prices more closely reflect the true feeding values. As shown in Table 3, for instance, the shadow price of rapeseed meal would be 72 and 66% of that of soy 48 if evaluated using crude protein and total lysine contents, respectively. Based on digestible lysine, however, the feeding value and thus the shadow price of rapeseed meal should only be 51% of that of soy 48 due to a much lower lysine digestibility in rapeseed meal. Therefore, using crude protein and/or total AA content would over- estimate the feeding value and shadow price of rapeseed meal.
Formulating Feed Using Digestible Vs Total Amino Acids
Most commercial feeds formulated on total AA basis have a large safety margin, say 5 to 10%, in order to minimise the variation in nutrient content and animal performance. As a result, the impact of formulating using either digestible or total AA on animal performance is easier to be demonstrated with AA deficient diets, or with poorly digested ingredients.
As lysine is the most variable AA in terms of digestibility, a broiler experiment was done using lysine deficient diets (Uzu, 1985). Two sets of broiler feeds were formulated to contain 0.75% lysine based on either total or digestible lysine. The test was carried out using broilers from 21 to 42 days of age. Soybean meal, sunflower meal or rapeseed meal was used as the main source of protein and synthetic lysine was used to equalise the lysine content when formulating on digestible AA.
With feed formulated on total lysine, growth and feed efficiency of broilers were greatly reduced when soybean meal was replaced by sunflower or rapeseed meals (Table 4). On digestible AA basis, replacement of soybean meal by sunflower meal did not reduce performance. Replacement of soybean meal by rapeseed meal reduced weight gain but did not affect feed efficiency. However, the reduction in weight gain was lower (11% versus 28%) with the digestible AA formulation than with the total AA formulation. With practical diets, such an effect of formulating technique on performance can be observed more easily with poorly digested ingredients. Two separate broiler studies have recently been published.
Fernandez et al. (1995) recently compared the two formulation techniques using cottonseed meal. The adverse effect of gossypol in cottenseed meal was minimised by the addition of FeSO4 (1:1 Fe to free gossypol). As shown in Table 5, with total AA formulation, the growth and feed conversion efficiency were reduced when cottenseed meal was included at 15% and particularly at 20% level. Using digestible AA, cottonseed meal could be used up to 20% in broiler diets without affecting the performance. For inclusion levels higher than 20%, however, adverse effects on growth and feed efficiency were observed despite formulating feeds on digestible AA.
Rostagno et al (1995) fed three sets of diets to male broilers for 6 weeks. The Control (Diet A) was corn-soya based with high AA digestibility. Diet B was formulated to contain the same levels of total AA as those of Diet A but included by- products such as rice bran, meat & bone meal, poultry by-products, and feather meal. Diet B thus had lower digestible AA. Diet C was similar to Diet B but supplemented with synthetic lysine and methionine to the same levels of digestible lysine and methionine of Diet A (Table 6).
As shown in Table 6, formulating on total AA using low digestible ingredients (Diet B) significantly reduced weight gain, feed conversion efficiency, and breast meat yield. Supplementing Diet B with synthetic lysine and methionine (Diet C) improved the live performance to the same level as achieved with Diet A, although breast yield was still slightly lower. In terms of cost ratios, Diet A resulted in the highest feed costs per kg live weight or per kg carcass weight, and Diet B the highest feed cost per kg breast meat. Diet C generated the lowest feed costs for all three parameters examined. These results demonstrated that the performance of poultry could be maintained and the profitability of poultry production improved by including low digestible ingredients such as cereal and animal by- products, if the diets are formulated based on digestible rather than total AA.
The same effects of formulating on digestible AA on performance were also demonstrated in laying hens by Bougon and Joly (1990).
In summary, the use of true digestible instead of total AA enables nutritionists to evaluate and select feed ingredients more objectively, to use a wider range of ingredients such as by-products in feed formulation, to formulate feeds that meet the nutrient requirements of animals better, and to improve the profitability of poultry operation. In addition, when formulating diets based on digestible AA, the crude protein content of diets could often be reduced by using synthetic amino acids without affecting the performance. This reduction in dietary crude protein may lead to a decrease in nitrogen excretion, which in turn, will improve the welfare of the animal (better litter and air quality) and reduce nitrogen- related pollution problem.
The Application Of True Digestible AA In Feed Formulation
There are three questions that have to be addressed before using digestible AA to formulate feeds. Firstly, the additivity of digestibility coefficients of various ingredients has to be demonstrated. Furthermore, a database of digestible AA contents of all ingredients that are used in the feed formulation has to be established. Finally, the nutrient requirements of animals have to be expressed in digestible amino acids.
Additivity Of AA Digestibility Coefficients
The additivity of the digestibility coefficients can be demonstrated by comparing the determined AA digestibilities of practical feeds to those calculated based on coefficients of individual ingredients.
A number of feeds were tested in Rh?ne-Poulenc laboratory, based on ingredients currently used in the feed industry at usual inclusion rates (Rhone Poulenc Animal Nutrition, 1993). As shown in Table 7, there was in general an agreement between the determined and calculated digestibility coefficients, although the determined coefficients were often slightly lower than those calculated, with a larger variability.
Establishing Ingredient Database Of Digestible AA
It would be cost prohibitive to build an ingredient database on digestible amino acids from scratch. Instead, the digestibility coefficients published by major laboratories could be used to convert the database of total AA to digestible AA. As mentioned before, the three major sources of this information are Sibbald (1986), Parsons (1990a), and Rhone Poulenc Animal Nutrition (1993), and the results agree reasonably well among these major sources. The work on true digestible energy and AA in Sibbald¡ãOs laboratory had been discontinued. Parsons at University of Illinois, Rhone Poulenc Anima Nutrition in France, and a few other labs, are still actively involved in determining true AA digestibility of ingredients. Therefore, the digestibility of any new and specialty ingredients can be determined when needed.
Establishing Nutrient Requirements In Digestible AA
This is a challenging area because the AA requirements of animals are reported in most cases (e.g., NRC, 1994) in total instead of digestible AA. So far, two approaches have been reported to solve this problem, i.e., direct determination and indirect estimation.
The digestible AA requirements of an animal can be directly determined by growth assay using diets containing graded levels of the digestible AA to be examined. Rhone Poulenc Animal Nutrition (Dalibard and Paillard, 1995) had carried out several such tests to determine the digestible lysine and digestible sulphur AA requirements of broiler chickens during various growing periods. In one of such tests, for example, the requirements of broilers for digestible sulphur AA were found to be 0.81% and 0.77%, for body weight gain and for feed conversion efficiency, respectively, during the first three weeks of age. Han and Baker (1994) reported the digestible lysine requirement of male and female broiler chickens during the period of three to six weeks of age.
Alternatively, the digestible AA requirements can be estimated indirectly by applying the AA digestibility coefficients to the total AA requirements reported in various sources. Parsons (1990b) re-evaluated 28 published studies on the lysine and sulphur AA requirements of broilers, turkeys, and laying hens. Firstly, using the AA digestibility coefficients, he calculated the digestible AA contents of the basal feed ingredients used in the requirement studies. Then he added the calculated digestible AA to the amount of supplemental crystalline AA (assumed to be 100% available) to derive the digestible AA requirements. He found that the results were consistent for the 28 studies reviewed. The digestible AA requirements were 8 to 10% lower than the determined total AA requirements. Similar efforts have also been taken by Rhone Poulenc Animal Nutrition (1993). The digestible AA requirements of various animal species were estimated by applying the digestibility coefficients of feed ingredients to total AA requirements.
The two methods generated quite close results (Table 8), indicating that both approaches can be used to establish digestible AA requirements.
The concept and methodology of true ileal digestible amino acids has gained wide acceptance in academics as well as in industry. A number of laboratories world-wide have generated and continue to generate a large number of data which are available to the feed industry. The AA digestibility data allow nutritionists to more accurately and objectively evaluate the true feeding value of feed ingredients. Formulating diets using digestible AA enables nutritionists to improve the precision of formulation, to use with confidence of poorly digestible ingredients, to lower feed cost while maintaining animal performance. This will increase the competitiveness and the profitability of the operation.
To switch from total to digestible amino acid formulation, the digestibility coefficients data published by major laboratories (Sibbald, 1986; Parsons, 1990a; Rhone Poulenc Animal Nutrition, 1993) can be used to convert the database. The database of Rhone Poulenc Animal Nutrition is constantly expanded by new feed ingredients coming from various countries. The digestible AA requirements of animals could be readily estimated by applying the digestibility coefficients to total amino acid requirements.
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