Amy B. BATAL
Nick M. DALE
Department of Poultry Science
University of Georgia
Athens, GA, U.S.A.
Lysine, the second limiting amino acid (AA) in standard poultry diets, has increased drastically in price during the last months. The price of lysine is directly linked to that of soybean meal (SBM). A weak SBM crop in 2003, increased foreign SBM demand, and a decrease in lysine production are the major culprits for the high current price of lysine. A lysine plant in Mexico that produced approximately 40,000 metric-tons/year closed in August 2003 and other lysine plants are currently not at full production, which has resulted in a reduced supply exactly when demand has increased. It has been speculated that lysine prices will decrease and return to "normal" by spring or late summer as supply and demand reach an equilibrium.
The price spread between corn and SBM is often a good indicator of the relative value of lysine. In general, when the price of corn (energy) is low and price for SBM (protein) is high (i.e., a large price spread) the price for lysine will be high. Atypically, in 2000 and 2001 SBM prices were high but lysine prices did not increase dramatically because of an abundant supply. A relationship between SBM and lysine prices makes sense because as the meal prices increase, poultry producers in general will decrease usage to reduce cost. As SBM is an excellent source of lysine, use of the synthetic form of this AA increases so as to compensate.
Sources
May feed ingredients contain substantial amounts of lysine? In addition, lysine is available in synthetic forms as feed grade L-Lys•HCL (78% lysine) and Biolys™ (50% lysine). The lysine in both products is considered to be 100% available. Listed in Table 1 are the common protein sources used in poultry diets and their lysine concentrations. Soybean meal, fish meal and blood meal are excellent sources of lysine. Fish meal use in the US is limited because of high prices and limited availability. Animal proteins can also be good sources of lysine but there is continued controversy about their future use in animal feeds. Alternative oil seeds, cotton seed meal or canola meal, have much lower levels of lysine than SBM. Corn gluten meal, although high in crude protein, is low in lysine as is distiller's dried grains with solubles. Products of corn origin are not generally good sources of lysine.
Bioavailability – Determining amino acid (lysine) digestibility in feed ingredients
Amino acids (AA) are primarily absorbed in the small intestine of poultry. Due to microbial activity in the cecal the amino acids profile of excreta can be altered from that found at the terminal ileum (end of the small intestine). In order to obtain accurate AA digestibility/availability values the digestive tract of poultry must either be surgically modified (remove the ceca – cecectomy) or digesta samples must be obtained at the terminal ileum (ileo-cecal junction).
Amino acid digestibility can be determined via two methods, to obtain either "apparent" or "true" amino acid digestibility. One needs to be careful when using apparent AA digestibility because values many underestimate the actual AA digestibility. Apparent digestibility values are influenced by the level of feed intake and measures the digestibility of both dietary and endogenous protein. Apparent digestibility values increase as feed intake increases because the endogenous excretion, as a percent of total excretion, decreases proportionally. Endogenous amino acids excretion is made of up enzymatic secretions, sloughing of the intestinal tract, and mucus.
When the intake of the test protein source is low the amount of endogenous excretion as a percent of total amino acids excretion can be high. Thus, the calculated apparent digestibility coefficient will underestimate the actual digestibility. True amino acid digestibility values include a correction factor for endogenous excretion. Consequently, true AA digestibility is not affected by the level of feed intake.
However, uncertainty exists regarding the accuracy of determining endogenous amino acids losses. Endogenous losses can be determined by a variety of methods. The most common is to collect excreta from fasted cecectomized roosters during a 48-hour period. Other methods include crop intubation of a protein-free diet, or a use of control such as soybean meal in which the digestibility is known, to estimate endogenous amino acids losses. There are pros and cons to all the methods because the level of endogenous AA loses will vary due to dry matter intake and the composition of the feed ingredient or diet. Thus, the level of endogenous AA excretion from a fasted bird may be significantly lower than that of a bird fed soybean meal.
Availability/digestibility of lysine in various ingredients can be determined using a precision-fed cecectomized rooster assay (Sibbald, 1979). Roosters of approximately 18 to 20 weeks of age have the cecal surgically removed. They are then allowed 2 to 3 months to recover from surgery prior to feeding studies to determine true amino acid digestibility. In the "precision-fed" assay roosters are fasted for 24 hours and then crop intubated with a certain amount (usually between 30 and 35 g) of the ingredient being evaluated. Excreta are individually collected for 48 hours after crop intubation (time necessary to ensure a complete feed transit). Excreta are also collected from roosters that are fasted so as to make endogenous corrections. Most published values of lysine availability are based on excreta collection from cecectomized roosters because of the simplicity and the fact that the assay can be carried out on a large number of replicates without having to sacrifice the birds.
Questions have been raised about the use of adult cecectomized roosters for the determination of amino acids digestibility of feed ingredients for broilers, which are much younger and usually only reared for 36 to 56 days of age. Reports (Fernandez et al., 1995) have concluded that true amino acids digestibility values determined with adult cecectomized roosters are similar to the values obtained for 3 to 6 week old broilers. However, true AA digestibility values obtained with adult cecectomized roosters many not be accurate for young broilers from 0 to 10 days of age.
To determine amino acids digestibility/bioavailability of feed ingredients in young broilers one can either perform a chick growth assay or collect digesta samples at the terminal ileum. As a separate chick growth assay has to be performed to determine the bioavailability of each individual AA, this assay is extremely time consuming and expensive. The collection of digesta from the terminal ileum can be used to determine amino acids digestibility for a number of AA. However, this method requires a large number of birds to be euthanized to obtain an adequate sample of digesta. Nevertheless, the general assumption today is that the analysis of ileal contents rather than excreta is a more reliable method for assessing amino acid digestibility in poultry.
In our laboratory we determined the digestibility and bioavailability of lysine in distiller's dried grains with solubles (DDGS) using the precision-fed cecectomized rooster assay as well as a chick growth assay. The true digestibility of lysine in DDGS determined with cecectomized roosters was 75%. The lysine bioavailability value (80%) determined in a chick growth assay (8 to 21 days) using slope-ratio methodology was very similar to the value determined with cecectomized roosters.
Factors affecting requirements
Many factors such as age, breed/strain, sex, protein level, amino acid balance, physiological functions, and environmental stress can affect the lysine requirements of poultry. These requirements are well known to decrease with age. However, nutritionists generally do not have a good grasp of the rate of this decrease. In addition, it is easy to overlook the critical posthatch period, the first 7 to 10 days. It is generally recognized that the NRC (1994) 0 to 3 week lysine requirement (1.10%) is too low. It has been speculated that broilers benefit from a much higher level of lysine (1.20 - 1.30%) during the immediate posthatching period. The benefit of increased lysine levels early posthatch may not be seen immediately but is evident latter in the production period. Further research is needed to determine an initial lysine requirement and how this changes during the first 3 weeks of age.
Breed and strain also have a clear effect on lysine requirements. It appears that industry nutritionists have a good appreciation of the differences in lysine requirements for various breeds. Leaner birds, those that deposit more protein per unit of feed intake, have a higher lysine requirement. This also explains the differences in the lysine requirements between males and females. As male broilers deposit more protein than females their amino acids needs are greater.
Factors that affect feed intake also affect the lysine requirement, expressed as percent of the diet. In general, if the energy value of the diet increases the lysine requirement expressed as percent of the diet will increase, because "in theory" poultry eat to meet their energy needs. Thus, as the energy in the diet increases birds will decrease their feed intake. During heat stress broilers decrease feed intake, so nutrient intake decreases. The lysine requirement as a percent of the diet may be increased, in part to meet the bird's requirements. But also to reduce the heat increment associated with protein catabolism.
It often seems to the nutritionist that reducing the feed cost is the major mission of some poultry companies. This is usually done by decreasing nutrient specifications such as lysine level. However, feeding these low nutrient density diets may comprise product yield and quality. Researchers have concluded that increasing the nutrient density of the diet, by increasing the amino acids levels (increasing lysine, and by using ideal AA ratios the levels of all other AA increase) will optimize breast meat yield, decrease feed conversion (3 to 5 points), and reduce fat deposition. The positive benefits of higher nutrient density diets have been observed throughout the production period, and generally results in increased profitability
Conclusion
Because of the current high prices of lysine and SBM, many nutritionists may consider the use of alternative ingredients to meet the bird's lysine needs. A variety of methods can be used to determine the amino acids digestibility/bioavailability of these ingredients. However, each method has its pros and cons, which must be appreciated in order to employ determined lysine digestibility/availability values to their optimum advantage. Analysis of ileal contents (collected at the terminal ileum) rather than excreta is thought to be the most reliable and accurate method for assessing amino acids digestibility. However, the balance technique with precision fed roosters offers by far the most rapid means of assessing AA digestibility.
Lysine needs during the first week posthatching clearly need to be better established. Benefits many well be observed when increased levels are fed during the first 7 to 10 days posthatching. The benefits of increased levels of lysine early posthatching on production need to be explored further. Attempts to reduce feed cost by lowering nutrient levels may likely lead to marginal deficiencies that depress production.
As many nutritionists are aware, it is extremely difficult to distinguish between a diet adequate in a given nutrient and one that is slightly deficient. While a few extra grams of breast meat per bird may be almost impossible to definitively detect, its value certainly exceeds that of the small amount of extra lysine needed to optimize tissue synthesis. Thus, prior to setting the lysine minimum for a specific diet, careful consideration should be given to factors such as lysine level and availability in each ingredient, existing conditions which may affect the lysine needs of the flock, and the specific marketing objectives of the firm.
From Proceedings of the "Midwest Poultry Federation Convention", St. Paul, Minnesota, U.S.A.
