trace mineral composition in poultry feed

INTRODUCTION

Trace mineral needs of poultry have received little attention over the last 40 years compared to other aspects of nutrition, such as energy and amino acids. A cursory review of information published in scientific journals over the last 15 years indicates only some 1.5% of research effort expended on trace minerals. Undoubtedly the major reason for this lack of interest lies in their relative economic importance. When there are no overt deficiencies or excess of trace minerals in a diet, as occurs most frequently, then they represent less than 0.5% of total diet cost. This contribution is overwhelmed by the costs of energy and amino acids, and so naturally there is much greater interest in more closely defining the needs for these macronutrients. The resurgence of interest in trace mineral nutrition has been brought about by concerns for the environment and particularly the level of all nutrients in manure. Much has been written about the balance of nitrogen and phosphorus in poultry nutrition, and there are ongoing attempts at limiting their concentration in manure. It seems inevitable that regulatory concerns will be raised about the level of other chemicals in manure, one of which will be trace minerals. Today in Ontario, we find some wells in rural areas with water testing at 0.2 ppm zinc. Such relatively high levels are likely a consequence of various agricultural practices, and zinc content of poultry manure will be but one contributor to this situation. It seems inevitable that trace mineral levels in poultry diets will come under closer scrutiny, and providing amounts excess to requirements will have to be curtailed.

TRACE MINERAL REQUIREMENTS AND INGREDIENT COMPOSITION

Many nutritionists base their estimates of requirement on the NRC (1994) Nutrient Requirements of Poultry. This is something of a unique situation since many ‘requirement’ values within NRC (1994) are often used as a starting point in formulation, and actual diet levels often provide added ‘insurance’. However the NRC (1994) estimates for trace minerals are within the range commonly used in feeding broilers, layers and turkeys. A concern often raised about these values is the somewhat historical data used in their development. All nutrient requirement values within NRC (1994) are based solely on information published in referenced scientific journals. Since there has been little interest in redefining trace mineral needs of poultry, then the NRC (1994) values are necessarily based on somewhat historic data. With changes in genetics of broilers and turkeys, and dramatic changes in performance, the validity of such historic data is often questioned. Table 1 shows the year of the research data used in defining NRC (1994) trace mineral estimates. As shown in Table 1, much of our knowledge of trace mineral requirements is based on information gathered prior to 1980, with a wealth of these data coming from the period 1958-1978. Another point of interest is the type of diets used in these studies (Table 1).

Most often semi-purified or purified research diets are used, since the researcher is often unsure as to the content and/or bioavailability of trace minerals in conventional feedstuffs. Also, the mineral under study is invariably one of the highest purity possible obtained at great cost from a chemical supply company. Table 2 shows recent mineral analyses conducted in spring 2002 on feed ingredients used in some trace mineral studies. The point of interest is the fact that values are quite different to other published values (e.g. NRC, 1994; Leeson and Summers, 1997) and reflects the variable uptake of trace minerals by plants. Of even greater importance in estimating trace mineral needs is knowledge of bioavailability of minerals within feedstuffs such as corn and soybean meal. There is very little information available on this topic. O’Dell et al. (1972) and Nwokolo and Bragg (1980) show 40-70% availability of Mg, Mn, Cu and Zn in canola meal and 50-78% availability in soybean meal. There is virtually no information available to account for such variability, and so predicting the bioavailability of trace minerals in common ingredients becomes a tenuous exercise. Some ingredients can contain very high levels of trace minerals. For example, it has been reported that defluorinated phosphates contain up to 10,000 ppm Fe, and that this is up to 50% bioavailable (Henry et al., 1992). At this level, 1% dietary phosphate will supply 50 ppm biovailable Fe, which is close to the requirement for most classes of poultry. Obviously suppliers of phosphates are not likely to guarantee minimum levels of iron, and so the source is likely to be disregarded or discounted when establishing an ingredient matrix. There is a wealth of information describing the mineral bioavailability from various salts (Ledoux et al., 1991; Smith et al., 1994; Roberson and Edwards, 1994; Pesti and Bakalli, 1996). In general, sulphates are thought to have higher bioavailability than do oxides. Another area of discrepancy in such studies is choice of response criteria. While growth rate and feed efficiency may be the most important from a commercial viewpoint, bone accretion of trace minerals may well be a more sensitive indicator for minerals such as zinc and manganese. Many requirement studies are conducted under idealized environmental conditions, where birds are subjected to minimal stress. It is therefore important to realize that factors such as high environmental temperatures seem to influence the need for trace minerals at the dietary level. Belay et al. (1992) indicated that levels of both fecal and urinary magnesium are increased under hot environments, and that needs for this mineral can be increased by up to 50%.

EFFECT OF USING PHYTASE

Phytic acid affects the birds’ metabolism of trace minerals as well that of calcium and phosphorus. Use of a phytase enzyme is therefore likely to increase the bioavailability of trace minerals in ingredients such as corn and soybean meal. Figure 1 shows the structure of phytate, in which Zn, Fe and Mg are bound much as is calcium. While the effects of phytase on the liberation of P and Ca from phytate is well recorded, there is less information available on trace mineral release and bioavailability. Theoretically, as much Zn, Mg and Fe should be released as is Ca. However, it is not clear if the increased bioavailability of Ca due to phytase results from Ca release from phytate or from greater bioavailability of diet Ca per se. Roberson and Edwards (1994) conclude that if phytase is used, then it may not be necessary to use supplemental zinc while levels of other cationic minerals such as Mn and Fe can also be reduced. Yi et al. (1996) show improved retention of zinc from ‘natural’ ingredients when phytase is added to the diet (Table 3). Using regression analyses, they suggested that 100 U of phytase releases about 1 mg of zinc. Therefore 600 U phytase will provide around 10% of zinc needs for a young broiler.

TRACE MINERAL PROTEINATES

It seems difficult to predict with any great degree of accuracy the bioavailability of trace minerals in diets containing inorganic supplements. To date this has not been a major concern since the obvious solution has been to over-formulate to ensure requirements even under the most stressful situations. However if there is need to minimize trace mineral excretion in manure, then we need greater confidence in bioavailability of ‘reduced’ diet inclusions. In this context, the mineral proteinates may be of more use, since their bioavailability is more consistent. Proteinates are chelates of protein/amino acids containing minerals, whose consistent bioavailability equates more closely to that of amino acids i.e. 90-95%. Mineral proteinates usually contain amino acids, dipeptides, tripeptides or proteins per se, and are thought to enhance digestibility and availability of the mineral sequestered by the ligand. Improved digestibility may be due to better solubilization, greater stability in the lumen and/or perhaps the ligand serves as an efficient carrier for the mineral across the brush border. Once absorbed, there is also the potential for greater retention, since there is less likelihood of secretion or excretion prior to incorporation in end-product molecules. Proteinates therefore seem ideal choices in formulating diets containing minimal levels of trace minerals. Table 4 shows the stability constant (Log Kf) of some trace minerals with various protein ligands and EDTA. EDTA is an exceptional ligand with high affinity for most minerals, and is used as a standard for comparative purposes. The metal ion, having a higher stability constant, theoretically can replace a metal ion of lower stability constant in a chelate or at least the chelate with the higher stability constant will be formed before that of a lower constant. With a higher level of copper in the diet, this element may replace the ions having lower stability constant, such as zinc, iron or manganese, in chelates of these metal ions. The very high stability constants for EDTA with all of the mineral elements shown above clearly demonstrate the reason that EDTA can so successfully act as a metal scavenger and, when present in a system in adequate concentration, pick up all of the polyvalent transition cations even in competition with most other chelating agents. In conventional diets, chelating agents with zinc stability constants below 11 are thought to be of little benefit since they cannot compete with phytic acid. However, this concept has not been studied since the introduction of phytase enzymes. As an initial step in investigating the role of metal proteinates in reducing trace mineral levels in manure, we have conducted a study with caged broiler chickens. Broiler diets were formulated with mineral sulphates using 100 ppm Zn, 90 ppm Mn, 30 ppm Fe and 5 ppm Cu. These sulphates were assured to be 70% bioavailable. The same level of bioavailable minerals were contributed by BioplexTM minerals, and then this level further reduced to 80%, 60%, 40% or 20% of this level (Table 5). All other trace minerals and vitamin levels were constant across the five treatments. Each treatment was tested with 6 replicate groups of 8 caged birds to 17 days and thereafter 5 replicate groups of 5 caged birds to 42 days. Table 6 shows the body weight and feed efficiency of broilers fed the various levels of trace minerals. There were surprisingly few effects of trace mineral supplementation, where even birds fed just 20% of a normal level of BioplexTM trace minerals grew quite well considering the cage environment. There were no overt problems with bird health, and no differences seen in mortality. During the 15-17 day period and the 39-42 day period, a balance study was conducted, and feed and excreta analyzed for trace minerals. Based on this data, trace mineral excretion for a farm housing 100,000 male broilers was calculated for each of the trace minerals under study. Use of BioplexTM minerals calculated to be at the same level as contributed by the inorganic salts resulted in reduced mineral output. To some extent this situation reflects the 70% bioavailability assigned to the inorganic salts, and that diets with chelates therefore contained some 30% less mineral per se. Alternatively the BioplexTM minerals may be even more bioavailable relative to the inorganic salts, although the trial was not designed to answer this question. It seems as though trace mineral needs for nonheat-stressed broilers may be substantially less than current industry recommendations, and so there is potential to reduce manure loading of these nutrients. As trace mineral levels are reduced, then chelated minerals become more attractive, since their bioavailablity is very high, and very consistent. Using chelated minerals at lower levels also makes them more economically attractive.

REFERENCES

Albert, A., 1962. Stability constraints of trace minerals. Fed. Proc. 20:137.
Belay, T., C.J. Wiernusz and R.G. Teeter. 1992. Mineral balance and urinary fecal mineral excretion profile of broilers housed in thermoneutral and heat distressed environments. Poultry Sci. 71:1043-1047.
Henry, P.R., C.B. Ammerman, R.D. Miles and R.C. Littell. 1992. Relative bioavailability of iron in feed grade phosphates for chicks. J. Anim. Sci. 70:228. Ledoux, D.R., P.R. Henry, C.B. Ammerman, P.V. Rao, R.D. Miles. 1991. Estimation of the relative bioavailability of inorganic copper sources for chicks using tissue uptake of copper. J. Anim. Sci., 69:215. Leeson, S. and J.D. Summers. 1997. Commercial Poultry Nutrition 7th Rev. Ed. National Research Council. 1994. Nutrient Requirements of Poultry. 9th Rev. Ed. NAS-NRC, Washington, D.C. Nwokolo, E. and D.B. Bragg. 1980. Biological availability of minerals in rapeseed meal. Poultry Sci. 59:155-158 O’Dell, B.L., C.E. Burpo and J.E. Savage. 1972. Evaluation of zinc availability in foodstuffs of plant and animal origin. J. Nutr. 102:653-660. Pesti, G.M. and R.I. Bakalli. 1996. Studies on the feeding of cupric sulfate pentahydrate and cupric citrate to broiler chickens. Poultry Sci. 75:1086. Roberson, K.D. and H.M. Edwards. 1994. Effects of 1,25(OH)2 D3 and phytase on Zn utilization in broiler chicks. Poultry Sci. 73:1312-1326. Smith, M.O., I.L. Sherman, L.C. Miller, K.R. Robbins. 1994. Bioavailability of manganese from different sources in heat-distressed broilers. Poultry Sci. 73:163. Yi, Z., E.T. Karnegay and D.M. Dembow. 1996. Supplemental microbial phytase improves zinc utilization in broilers. Poultry Sci. 75:540-546.