Dr. William A. Dudley-Cash is a poultry and fish nutritionist and has his own consulting firm in Modesto, Cal. To expedite answers to questions concerning this article, please direct inquiries to
The accumulation of nutrients in soils and the concurrent threat to surface water quality as the result of runoff or leaching of these nutrients is a major challenge facing animal agriculture. The U.S. Environmental Protection Agency has determined that more than 20% of the streams and rivers in the U.S. have been affected by non-point source pollution and has identified animal agriculture has one of the contributors.
The effect of nutrient accumulation in soils as the result of the application of animal manure has been evaluated using the nutrient balance approach. Annual applications of nutrients are offset by the amount of nutrients removed from the soil through crop production. Early estimates assumed that nitrogen accumulation would be a primary problem in the application of poultry litter to farmland. The application of poultry litter would be limited by the amount of nitrogen removed in crop production. However, soil balance studies and experience have shown that phosphorous accumulation is probably the more serious problem.
R. Angel of the University of Maryland presented a paper at the California Animal Nutrition Conference in which she reviewed the role of phosphorous and environmental nutrient accumulation. Phosphorus is present in most poultry feeds at relatively high levels. Only calcium and nitrogen are present at levels higher than total phosphorous. Much of the phosphorous in poultry feeds is poorly utilized. Most crops do not harvest large quantities of phosphorous from the soil. This combination of factors contributes to the importance of phosphorous accumulation in the soil.
Angel reviewed the recent national efforts to minimize losses of particulate and soluble phosphorous from soil to water. These efforts include the Unified National Strategy for Animal Feeding Operations, developed cooperatively by the U.S. Department of Agriculture's National Resources Conservation Service and EPA, and the National Nutrient Strategy adopted by the Natural Resources Conservation Service in 1999.
In Delmarva, new legislation limits the use of litter application to soil (based partly on soil phosphorous content). Legislation in Maryland requires that all poultry feeds contain phytase or other feed additives that decreases phosphorous excretion. With limited land areas for litter application, the poultry industry must find strategies that reduce the phosphorous levels in litter with the least impact on the economics of production. Sooner or later, the challenge of minimizing the excretion of phosphorous will have to be faced by the poultry industry nationwide, according to Angel.
A major part of the Angel presentation was focused on the chemical form(s) of phosphorus in poultry feeds, the factors that affect the biological utilization of this phosphorus and how these relate to the excretion of phosphorus in the litter.
Phytic acid, phytate, phytin
Angel discussed the importance of phytin phosphorus (PP). Phytic acid is a phosphorylated cyclic sugar alcohol, (phosphorylated inositol). Phytate is the anion form of phytic acid. Phytin is the chelated form of phytate. The phosphorous content of phytic acid is 28.2%.
Phytin is the primary storage form of phosphorous in plants. Plant roots contain low amounts of phytin, and vegetative parts of plants (such as the leaves) contain only trace amounts. Most of the phytin is located in the seeds.
The storage location of phytin in the seed varies among species of plants. Ninety percent of the phytin in corn is found in the germ of the kernel. In the case of wheat and rice, most of the phytin is in the aleurone layers of the kernel and the outer bran. In the case of most oilseeds and grain legumes, the phytin appears to be distributed throughout the kernel. Phytin constitutes between 1 and 3%, by weight, of many of the cereals and oil seeds used in animal feeds.
PP accounts for approximately 50-80% of the total phosphorous present in plant feed ingredients. The phosphorous in phytin is not available to monogastric animals (or any other animal) until a phytase enzyme has released it. Phytase enzyme may be naturally present in plant feed ingredients. Bacteria in the intestinal tract may produce phytase enzymes (rumen bacteria produce a lot of phytase). Phytase enzyme may be present in the lining of the intestine. Phytase enzymes may be added to the feed as exogenous sources. The phytase enzyme must come from someplace.
There are adequate levels of total phosphorous in most plant feed ingredients to meet the dietary requirements of most animals, provided that PP is released from the phytin molecule so that it becomes available to the animal. However, in general, monogastric animals poorly utilize PP. PP in feed that is not released, and therefore not utilized, becomes the phosphorous that is excreted and potentially accumulates in the environment.
Phytin is often referred to as anti-nutritive because of its ability to chelate cations in the diet, which renders these chelated cations partially or completely unavailable to the animal. At all pH values normally encountered in feeds and digesta, phytin will carry a strong negative charge and is capable of binding di- and trivalent cations including calcium, cobalt, copper, iron, magnesium, manganese, nickel and zinc.
The phytin forms very stable complexes reducing the availability of these minerals to the animal. Phytin also forms complexes with proteins and starches. The binding of phytin with proteins and starches may also reduce the availability of these nutrients.
Most recent studies with chickens show that PP utilization is variable. Dietary factors may influence the hydrolysis of PP in the intestinal tract. These dietary factors include the levels of calcium, non-phytin phosphorous, total phosphorous and vitamin D as well as feed processing and feed ingredient particle size. PP utilization by chickens is reported to be as low as zero to 15% by some researchers and as high as 70-75% by others. Ballam et al. (1984) reported PP hydrolysis in four-week-old female broilers ranged from 3 to 42% and depended primarily on dietary calcium level.
The effect of high levels of calcium on intestinal pH may be partly responsible for some of the deleterious effects that high dietary calcium has on PP hydrolysis. High dietary calcium concentration (2.53 versus 1.07%) was reported to increase the pH in the crop (5.32 versus 4.89) and ileum (7.39 versus 6.62; Shafey et al., 1991). The pH of the contents of the gastrointestinal (GI) tract increased as the digesta moved distally, starting at the proventriculus. Digesta pH in birds fed 1.07% calcium was 4.89 in the crop, 1.98 in the proventriculus, 3.14 in the gizzard, 5.53 in the duodenum, 6.06 in the jejunum, 6.62 in the ileum and 6.48 in the ceaca.
The increased pH of the GI contents causes the PP molecule to be ionized and thus more readily forms complexes with divalent metal cations like calcium, zinc, magnesium and iron. These complexes, at high pH, are less soluble. Solubility is an important factor in the overall utilization of PP because phytase cannot act efficiently on PP unless it is in a soluble form.
The degree to which PP is utilized by the animal will depend to a large extent upon its hydrolysis in the GI tract. The hydrolysis of phytin by phytase occurs when phytin is in solution. When phytin chelates with calcium and other cations, it may form soluble complexes, especially at lower pH. However, as the pH increases, these complexes precipitate as insoluble complexes at the higher pH found in the lower intestinal tract. These insoluble complexes reduce the ability of phytase to hydrolyze the chelated complex, thus decreasing the availability of both PP and the cations associated with this insoluble complex.
When the dietary calcium and phosphorous concentrations are increased, the proportion of chelate complexes that precipitate also increases. The solubility of calcium was found to be 16.9% when the calcium and available phosphorous levels were 1.53 and 0.53%, respectively. The solubility of calcium was only 8.3% when the levels of calcium and available phosphorous were increased to 2.26 and 0.83%, respectively. Increasing the dietary calcium and available phosphorous levels reduced the proportion of soluble minerals and thereby further decreased the availability of these minerals.
The inhibitory effect of high levels of calcium on PP hydrolysis can be prevented or reduced by the addition of a calcium chelator like ethylene di-amine tetra acetate (EDTA). The mode of action of EDTA may be that it is competing with phytin to bind with calcium, thereby preventing the precipitation of the phytin-calcium complex.
Phytases are enzymes that are able to catalyze the hydrolysis of the phosphate ester bonds of phytin. The action of phytases ultimately leads to the production of inorganic phosphorous and free myo-inositol. Phytases are widely distributed in plants, animals and microorganisms. IUB currently acknowledges two general classes of phytase enzymes: 3-phytases and 6-phytases. These phytases initiate the hydrolysis on the 3 and 6 position of the inositol ring, respectively.
Wodzinski and Ullah (1996) reported that the 3-phytases do not always completely dephosphorylate phytic acid while the 6-phytases do. It has been reported that microorganisms normally produce 3-phytases while 6-phytases are usually found in plants. However, exceptions to this general rule have been discovered.
According to Angel, there are four possible sources of phytases that could be found in the intestinal tract of animals: (1) phytases present in feed ingredients, (2) exogenous microbial phytases added to the feed, (3) phytases produced by endogenous microflora in the intestinal tract and (4) membrane-bound phytases in intestinal mucosa.
Microbial phytases tend to have pH optima in the range 2-6, while plant phytases tend to have a pH optima at 5. Most research indicates that all sources of phytases have phytate hydrolyzing activity. However, it is also clear there is a wide range of activity from one experiment to the next. The difference in activity may be related to the source of phytase or the composition of the experimental diets.
It would appear that microbial phytase, with a wider range of pH optima, may have an advantage. With pH optima near 2, the microbial phytases would be active in the proventriculus and gizzard where phytin tends to be most soluble and therefore available for enzyme action. Plant phytases, with pH optima near 5, would be most active in the small intestine where phytin tends to be insoluble and therefore not accessible for hydrolysis.
In addition, the plant phytases, like those that are found in different varies of wheat, may be largely denatured by the low pH encountered in the proventriculus, or destroyed by protease enzymes, before they reach the small intestine.
Estimation of nutrient excretion based on models has become necessary due to the recent emphasis on environmental management and regulation of animal feeding operations that rely on the prediction of nutrient excretion. As federal, state and local regulation increases in the nutrient management arena, the accuracy of these nutrient excretion models is imperative. Angel discussed two of these models.
The National Research Council (NRC; 2003) model is based on a process-base mass balance approach. Excreted nitrogen is calculated by subtracting the quantity of nitrogen produced in the animal from the quantity of nitrogen consumed in the feed. This model assumes broilers are fed to meet NRC (1994) nutrient recommendations, and this model uses bodyweight and feed efficiencies as given in the NRC (1994) bulletin.
The NRC (2003) model, thus, is using broiler information published between 1942 and 1990 for amino acid requirements and between 1952 and 1983 for phosphorous requirements. There clearly are differences in efficiency in nutrient use, growth and skeletal development between current broilers and those of more than 10 years ago.
A model developed by Applegate et al. (2003) bases nutrient excretion on industry average feed formulation and average performance. The primary nutrients estimated are nitrogen, phosphorous and dry matter. The model may be used to calculate manure nutrient excretion per bird to market weight, based on average diet formulation, average performance and average published dry matter, nitrogen and phosphorous retention for broilers, turkey males, turkey hens, ducks and laying hens.
A balance trial was conducted with broilers (Angel et al., 2003) to validate the model developed by Applegate et al. (2003). The results of the balance trial indicated that the model over-estimates dry matter, nitrogen and phosphorous excretion by approximately 19.4, 14 and 13%, respectively.
While modeling programs are necessary in order to provide information that is not available from direct experimentation, it is essential that the components for feed formulation, bird genetics and bird performance be continuously updated to reflect current production.
Calcium, phosphorus in feed
An area that Angel emphasized in her oral presentation was the importance of feed components and feed formulation on the reduction of excreted phosphorous. It is clear that reduced levels of phosphorous and especially calcium can improve the utilization of PP and consequently reduce the levels of excreted phosphorous as well as reduce the need for supplemental inorganic phosphorous. It may be possible to reduce the levels of calcium and total phosphorous in the diet to the extent that the addition of inorganic phosphorous is not necessary, even in the absence of supplemental phytases (at least in finisher feeds).
With the growing importance of excreted phosphorous, this area clearly requires more careful evaluation.
The original paper in the proceedings is 15 pages long and contains 62 references. For the serious nutritionist, this is an excellent paper, and the reference list alone justifies the cost of the proceedings.
The Bottom Line
The poultry industry eventually will have to face the challenge of minimizing phosphorous excretion (and probably other feed nutrients). The total phosphorous content of most poultry diets would be adequate to meet the phosphorous requirement if all PP were utilized. The levels of total phosphorous and calcium that are contained in the diet significantly impact the availability of PP.
The nutritional requirements for calcium and phosphorus may be lower than the levels provided in current commercial feeds. Reducing the levels of these nutrients will reduce their excretion in the litter. In addition, reducing the levels of calcium and phosphorus will increase the utilization of PP. Careful adjustment of the total phosphorous and calcium levels may allow the industry to significantly reduce the amount of phosphorous excreted into the litter while maintaining economic performance.
Copies of the proceedings of the California Animal Nutrition Conference may be obtained ($20 each) by contacting Ann Quinn, California Grain & Feed Assn., 1521 I St., Sacramento, Cal. 95814; phone, (916) 441-2272 or fax, (916) 446-1063.