Zeolites : do they have a role in poultry production.

Abstract

ZEOLITES - DO THEY HAVE A ROLE IN POULTRY PRODUCTION ? MICHAEL EVANS* SUMMARY Literature on the application of natural and synthetic zeolites for poultry production is reviewed. A total of 66 experiments were cited involving the application of zeolites to poultry production. Areas where zeolites have potential use are: as a feed additive to improve poultry performance for layers and broilers; to assist in manure and litter management; and to assist in controlling air quality in poultry house environments. These are discussed in detail, including a brief discussion on the general structure and fundamental properties of zeolites. Sdme of the data reviewed is often incomplete and sometimes inconsistent, and this paper indicates areas where research is needed to clarify current findings, and to indicate new areas which are in need of research. Guidelines for the character isation of zeolites in agricultural research are presented and discussed. . INTRODUCTION Zeolites were discovered in 1756 by Freiherr Axe1 Fredrick Cronstedt, a Swedish mineralogist. Since that time, over 45 distinct natural species of zeoli tes have been recognised, and more than 100 species having no natural counterparts have been synthesised in the laboratory. Since the discovery of large mineable deposits in the U. S.A, Soviet Union, Japan and other countr ies, interest in natural zeolites has grown steadily, including Australia. In recent years an increasing amount of effort has been directed towards natural zeolites and their potential applications in industrial and agricultural technology. The potential areas of use for natural zeolites include animal feeds, odour control, horticulture and water treatment and pollution control. A total current market potential for natural. zeolites of 13,000-41,000 tonnes per annum in New South Wales and 33,000-96,000 tonnes per annum in Australia has been estimated (Holmes and Pecover, 1987). The purpose or this paper is to review the ,ootential use of both natural and synthetic zeolites as they apply specifically to poultry production. The paper will highlight areas where information is lacking and indicate those areas where future research should focus. CRYSTAL STRUCTURE AND PROPERTIES OF ZEOLITES Any discussion on the application or potential application of both synthetic and natural zeolites requires an initial discussion on their fundamental physical and chemical properties. It is not the intention of this review to discuss in detail the chemistry and properties of zeolites, since this has been covered and reviewed adequately elsewhere (Mumpton 1984; Holmes and Pecover , 1987), but to indicate and discuss those properties of zeolites which can be exploited and are important in applications for poultry production. Zeolites are crystallin t hyd ated jluminofilicates of alkali (Na+, K+, Rb+, . 5 5 Cs+) and alkaline (Be+ I Mg+ I Ca+ I Sr+ ) earth cations, having infincte, three-dimensional structures. They are further characterised by an ability to lose and gain water reversibly and to exchange constituent cations without a major change in structure. Each zeolite species has its own unique crystal structure, and hence, its own set of physical and chemical properties. *Department of Biochemistry, Microbiology and Nutrition, University of New England, Armidale, NSW 2351, Austalia. 249 Crvstal structure Mumpton (1984) describes in detail the structure of zeolites. Similar to quartz and the feldspar mineral, zeolites are tektosilcates, that is, they consist of three-dimensional frameweorks of Si04 - tetrahedra, wherein all four corner oxygen ions of each tetrahedron are shared with adjacent tetrahedra. This arrangement reduces the overall oxygen:silcon ratio to 2~1, and if each tetrahedron in the framework contains Si as its central cation, the structures are electrically neutral, as is quartz (SiO,). However, in zeolite structures, some of the quadrivalent silicon (Si) are replaced by trivalent aluminium (Al), giving rise to a deficiency of positive charge in the framewo k. This harge is balanced by mono- and divalent cations, such as 5 Na + 8 K+ 8 Ca + 8 and Mg 5 `, elsewhere in the structure. Most zeolite stuctures can be visualised as Si04 and AlO tetrahedra linked together in simple geometrical forms. The geometry of the structure is more easily seen by considering only lines joining the midpoints of each tetrahedron, as shown in figure 1 and are called sodalite units. Individual sodalite units may be connected in several ways; for example, by double fourrings of oxygens as shown in figure 2 the framework structure of synthetic zeoli te A. Generally, cations substitute freely for one another in the exchangeable cation sites in zeolites, the only restriction being overall charge balance. Loosely bound molecular water is also present in the structures of all zeolites as they occur in the natural state or as they are synthesised, and surrounds the exchangeable cations in the large pore spaces of the structure, Fundamental properties Mu@on (1984) describes in detail the fundamental properties of zeolites, All agricultural applications of both natural and syntheic zeolites are basically dependent on one or more of the following physical or chemical propeties: absorption and related lnolecular sieving and hydration-dehydration (a special type of absorption), cation exchange, and extensive properties of the mineral aggregates or r&k particles, such as size, shape, porosity and hardness. As previously indicated, the large cavities and entry channels are generally filled with water molecules that form hydration spheres around the exchangeable cat ions. If the water is removed ,molecules having effective 250 cross-sectional diameters small enough to fit through the entry channels are readily absorbed on the inner surfaces of the dehydrated central cavities. Molecules too large to fit through the entry channels are excluded and pass around the outside of the zeolite particle, giving rise to the well known *molecular sieving' property, of most crystalline zeolites. The surface area available for adsorption ranges up to several hundred square meters per gram, and some zeolites are capable of adsorping as much as 30% by weight of a gas on a dry weight of the zeolite. A more detailed discussion on the adsorption properties of zeolites are found in Mumpton (1984) and Flanigen (1984). Cation-exchange capacity is primarily a function of the degree of Al substitution for Si in the framework structure; the greater the substitution, the greater the deficiency of positive charge and the greater the number of alkali or alkaline cations required for electrical neutrality. In practice however, cation-exchange capacity depends on a number of other factors. In na ural zeolites, the zeolite as mined will contain a mixture of Na+, I<+, and Ca5 + in exchangeable positions in its structure, and, despite whatever pretreatments that may have been performed on it by the supplier, will not be a pure, homoionic species. Secondly, the environment (soil, animal intestinal tract) will contain significant amounts of other cations which may compete with the cation of interest (e.g. NH4), and which may also enter into exchange reactions with the the zeolite. These ions therefore compete with cation of interest for exchange sites within the zeolite. The net result is that cation of interest will exchange onto only apart of the available sites within the zeolite.; other sites will be occupied by the competing cations. The framework of a crytalline zeolite dictates its selectivity towards competing ions. The small amount of Al in the framework of ciinoptilolite, for example I results in relatively low cation-exchange capacity (about 50-220 meq/lOOg); however its cation selectivity is: Cs > Rb > K > NH4 > Ba > Sr > Na > Ca > Fe > Al > Mg > Li (Ames, 1960). Thus clinoptilolite has definite preference for larger cations, and its selectivity for NH4 can be exploited commercially. This inherent selectivity for specific cations influences the effective cation-exchange capacity of a zeolite. The extensive properties of zeolite aggregates are those of the natural particle of rock and are directly related to the manner in which the minerals are formed in nature. The ideal natural zeolite ore for both cation-exchange and adsorption applications should be rich in the zeolite of interest, mechanically strong to resist abrasion and disintegration, highly porous to allow liquids and gases to diffuse in and out of the grain with ease, and soft enough to be crushed to the desired particles size. Not all of these properties are commonly found in the same zeolite ore. Synthetic zeoli tes which are produced with uniform, small (2-10 micron) crystal sizes, bound into high porous pellets do contain contaminants from the manufacturing process which may or may not influence their usage for certain applications. CHARACTERISATION OF ZEOLITES IN AGRICULTURAL RESEARCH One of the major problems encountered in reviewing the literature on the application of zeolites to agriculture `was the reporting of the full details on the characteristics of the zeolites used in agricultural experiments. This deficiency has contributed to difficulties in interpreting and reproducing experimental results, and therefore has led to some degree of scepticism on the part of many consuming industries. This is in addition to the natural conservatism that alreading exists in the testing of new products. This same problem was also highlighted by Shepherd (1984) and Fredrickson (1987). Both these sources stress the importance of careful detailed chemical 251 characterisation of zeolitic samples under experiments. investigation in agricultural Shepherd (1984) lists and discusses ten criteria that is adequate or desirable for the characterisation of zeolitic material and which should be reported in agricultural experiments. These are as follows: 10 20 30 Name of the zeolite mineral species. Supplier's name and address and product or sample code. Location of the deposit as to the country, state, town from which the mater ial was mined. 4 0 Mesh or particle size. 5 Mineralogic composition of the zeolitic material. 6 0 Chemical composition zeolitic material and the constituent zeolite mater ial. 7 Homogeneity of the zeolite material. 8 0 Crystallite size of the zeolite. 9 0 Cation-exchange and/or adsorption properties, where appropriate. 10. Description of any modifications made to the as-mined material. l l POTENTIAL USE OF ZEOLITES IN POULTRY PRODUCTION Fredrickson (1987) summarises the potential uses for both synthetic and natural zeolites. Other detailed discussions of the various applications can be found (Mumpton, 1977; Sand and Mumpton, 1978; Pond and Mumpton, 1984). The three areas of application where zeolites have potential use in poultry production are: 1, As a feed additive to improve poultry performance for layers and broilers. 2. To assist in manure and litter management. 3. To assist in controlling air quality in poultry house environments USE OF ZEOLITES AS FEED ADDITIVES IN DIETS FOR POULTRY The use of both natural and synthetic zeolites as feed additives for poultry, have been treated separately. The reason for this is because both their physical and chemical characteristics are total different for one another. One critical difference is the ratio of silicon:aluminium which is approx 1:l for the synthetic zeolite, sodium zeolite A (SZA, ETHACAL, Ethyl Corporation, Baton Rouge I LA 70820), and for the natural zeolite clinoptilolite, about 2.50 5: 1. This lower ratio of Si:Al in synthetic sodium zeolite A is responsible for its higher cation-exchange capability due to the presence of more negative charges. This ratio also affects the stability of zeolites under different pH conditions. In general, the lower the Si:Al ratio the more unstable the zeolite will be under conditions of low pH. Synthetic sodium Zeolite A is therefore less stable at lower pH than the natural zeolite, clinoptilolite. The stability or instability will influence the mode of action of the particular zeolite have when it is fed and when it comes in contact with the variable pH that exists in the gut of the bird. Although not proven as yet, SZA may become unstable and breakdown in the gut, with subsequent effects such as aluminium (Al) injected into the gastro-intestinal tract (GIT) and loss of cation-exchange capacity (less material available), whilst the more stable natural zeolites remain intact. This may or may nor be an advantage in certain circumstances. 252 The cation-exchange capability will also influence the exchange of cations in the gut of the bird and therefore the mode of action. These characteristics have been taken into account by the various researchers and is contrasted in the levels of inclusion between the synthetic and natural zeolites in experimental poultry diets. Inclusion levels for synthetic zeolites are generally much lower (< 2%) than the natural zeolites (up to 10%). The consistency of the chemical composition of the zeolite and its associated material may influence the effects that zeolites have on poultry performance. The injection of the various cations and other trace minerals into the gut of the bird may influence poultry performance. Natural zeolites are more variable than synthetic zeolites and this can influence the interpretation of the results obtained. On the other hand, synthetic zeolites do also contain contaminants from their manufacture, such as sodium hydroxide in SZA manufacture, which is a strong alkali and which may influence the pH of the gut. The effects of these synthetic and natural contaminants is unknown. The balancing of diets for protein (amino acids), energy and minerals is critical to ensure that the true charcterisitics of the zeolites are evaluated and not confounded with dilution effects and effects from imbalanced minerals. Use of natural zeolites in the diets of laying hens Ten experiments were cited where natural zeolites had been included in diets for laying hens. In all cases description of the zeolite used was inadequate and in two of the experiments the zeolite used was not even named. In all of the experiments where the zeolite was named, clinoptilolite was used, with the exception of one experiment where mordenite was used as well. In many of the experiments no information was supplied on the mesh or particle size, mineralogical composition, chemical composition including exchangeable cations, homogenicity, crystalite size, cat ion exchange and adsorption properties or any modification that may have been made to the zeolite. The importance of describing the natural geolite material can not be over emphasised, and it is unfortunate that from these ten experiments very little can be deduced as to what characteristics of the zeolites are responsible for the results observed. Questions such as how important is the cation and absorption properties, particle size, the type of cation present initially in the zeolite can not be answered from the data presented in these experiments. Out of the 10 experiments cited, two were conducted on birds that were early in lay (16 to 35 weeks) I two mid lay (35 to 65 weeks) and two late in lay (over 65 weeks). Four did not indicate the age of the birds. Inclusion levels of zeolite varied from 0% to 10% and in half of the experiments, inclusion was by direct substitution for another feed ingredient, usually grain. In only one experiment was the diet isonitrogenous, only one was isocaloric, and three were both isonitrogenous and isocaloric. Of these three, two were balanced for minerals as well. The balancing of diets for protein (amino acids), energy and minerals is critical to ensure that the true characteristics of the zeolites are evaluated and are not confounded with dilution effects and effects from imbalanced minerals. (i) Effect on body weight and growth rate. Out of the 10 experiments, only three reported information on growth rate (Kvashali et al, 1980; Kvashali & Mikautadze, 1980; Nakaue & Koelliker, 1981) and four reported information on body weight (Nakaue 6 Koelliker, 1981; Olver, 1986; Vest & Shutze, 1984, experiments l& 2),(3 different). In all cases there was no difference between zeolite and control diets, except in two experiments where one experiment reported an higher growth rate for zeolites but supplied no data details for verification (Kvashali et al, 1980), and in one experiment where the increased growth rate was observed in diets where the zeolite was included in granular 253 form (1 to 2.5 mm) rather than as a powder (Kvashali & Mikautadze, 1980). (ii) Effect on egg production. Eight of the 10 experiments reported information on egg production. Four reported no difference between zeolite and control diets (Roland, 1988; Szabo et al, 1983; Berrios et al, 1983; Vest 6 Shutze, 1984, experiment 1) I two found a positive effect due to zeolites (Merabishvili et al, 1980; Olver, 1986) and two found a negative effect due to zeolites (Nakaue & Koelliker, 1981; Vest 6 Shutze, 1984 in experiment 2), although Nakaue & Koelliker (1981) reported inconsistent results, with egg production being depressed at 2.5% and 5% but not at 10% zeolite inclusion. (iii) Effect on egg weight, egg mass and internal egg quality. Five out of the 10 experiments reported information on egg weight (Nakaue & Koelliker, 1981; Roland, 1988; Szabo et al, 1983; Berrios et al, 1983; Olver, 1986) and one of these five (Olver , 1986) reported information on egg mass and three of these 5 (Nakaue & Koelliker, 1981; Roland, 1988; Olver, 1986) reported information on internal egg quality as well. In all of these experiments there was no influence of zeolites on egg weight, egg mass or internal egg quality. (iv) Effect on feed consumption. Out of the 10 experiments, five only reported information on feed consumption. Two reported no difference between control and zeolite diets (Roland, 1988; Berrios et al, 1983), one reported an increase in feed consumption as zeolite level increased (Nakaue 61 Koelliker, 1981), but this was clearly a dilution effect. Where diets were balanced for protein and energy, one experiment reported an increase in feed consumption as the zeolite increased (Olver, 1986) and one reported a slight decrease in feed consumption with the zeolite diet (Vest & Shutze, 1984). (v) Effect on feed efficiency. Seven out of the 10 experiments reported information on feed efficiency. In five of these seven experiments (Kvashali, 1980; Kvashali & Mikautadze, 1980; Berries et al, 1983; Olver, 1986; Vest & Shutze, 1984 experiment 2) feed efficiency was improved by zeolites and in one of these cases feed efficiency was better when zeolites were included as a granule (1 to 2.5 mm) rather than in powdered form (Kvashali & Mikautadze, 1980). One experiment gave no difference between control and zeolite diets (Vest & Shutze I 1984 experiment 1) and one gave a negative response (Nakaue 6 Koelliker, 1981). The experiment which gave a negative response was confounded by an energy dilution effect resulting in a significant increase in feed consumption. In this experiment the diets were balanced for protein. (vi) Effect on mortality. Of four experiments, two reported a reduction in mortality with zeolites but gave no details as to the extent of the reduction, (Kvashali et al, 1980; Merabishvili et al, 1980) and two showed no difference (Nakaue & Koelliker, 1981; Vest 6 Shutze, 1984 experiment 1). (vii) Effect on shell quality. Seven out of the 10 experiments reported information on shell quality. Five of these experiments showed no difference in shell quality. Three of these five experiments measured shell quality by specific gravity of eggs, (Nakaue & Koelliker I 1981; Roland, 1988; Szabo et al, 1983), one experiment used an unspecified shell resistance measurement, (Berrios et al, 1983), and another experiment used shell thickness measurement to measure egg shell quality, (Olver, 1986). Two experiments by Vest & Shutze, 1984, experiments 1 and 2, gave conflicting results. In experiment 1 the zeolite diet (2% clinoptilolite) improved shell quality as measured by a deformation evaluation method, but showed no difference in specific gravity or % cracks observed; and in experiment 2 the zeolite gave poor shell quality as measured by the same deformation evaluation method, but showed no difference in specific gravity, although specific gravity tended to be better on the zeolite diet. 254 (i) Effect on body weight and growth rate. Out of the 19 experiments only one reported information on growth rate (Roland et al, 1985 experiment 2) I which showed no effect due to SZA and seven reported information on body weight. The results are conflicting, with four experiments showing no effect on body weight due to SZA (1.0% & 1.5%) I (Ingram et al, 1987c; Ingram et al, 1987a; Ingram et al, 1987d; Ingram et al, 1987b experiment 2) ; one showed a decrease in body weight due to SZA (0.75%, l.S%)(Roland et al, 1985, experiment 1); one showed an increase in body weight due to SZA (1.5%)(Ingram et al, 1987b experiment 1) and one showed that SZA reduced the effect of high temperatures (heat stress) on body weight loss. Birds which had experienced a body weight loss (the group with no SZA) due to heat stress recovered this body weight loss when placed on a diet with SZA at 1.5%. (ii) Effect on egg production. Seventeen of the nineteen experiments reported information on egg production. Thirteen experiments reported no difference between control and SZA diets with inclusions of 1.0% SZA (Roland et al, 1985 experiments 1 & 2; Miles et al, 1986; Ingram et al, 1987a; Ingram et al, 1987d; Roland, 1988a experiments 1 to 4; Skinner et al, 1988; Roland, 1988b; Ingram, 198713 experiments 1 & 2). Seven of these experiments showed no effect on egg production at levels of up to 1.5% SZA (Roland et al, 1985, experiments 1 & 2; Roland, 1988a, experiments 1 & 3; Skinner et al, 1988; Ingram et al, 1987b experiments 1 6 2) I but two showed a reduction in egg production at the 1.5% SZA inclusion (Miles et al, I 1986; Roland, 1988a experiment 2) and one showed a reduction at the 1.5% level only when the calcium level was low (2.75%)(Roland, 1988a experiment 4). One of these experiments only showed an effect on egg production due to SZA (1.0%) when the total phosphorus (P) level in the diet was below 0.43% (Roland, 1988b). Above a total P of 0.43 up to 0.7% P, SZA up to 1.0% did not affect egg production. Two experiments showed a linear reduction in egg production as the the level of SZA increased (Littleton, 1988; Roland, 1988a experiment 5; ) . However I birds subjected to heat stress (two experiments) I benefited from the inclusion of SZA (1.5 & 1.0%) in their diets (Ingram et al, 1987c experiments 1 & 2). The adverse effects of heat stress were reversed when control birds which had experienced a drop in egg production were switched to SZA diets and bird feds diets containing SZA prior to heat stress and which were continued on these diets did not experience an egg production drop when subjected to heat stress. (iii) Effect on egg weight and egg mass. Fifteen of the experiments reported information on egg weight. Thirteen of these fifteen experiments showed no effect on egg weight due to SZA. (Roland et al, 1985 experiments 1 & 2; Ingram et al, 1987c experiment 1; Ingram et al, 1987a; Ingram et al, 1987d; Littleton, 1988; Roland, 1988a experiments 1, 3, 4 61 5; Roland, 1988b; Ingram et al, 1987b experiment 1 6 2). One showed no ef feet on egg weight up to 0.75% but decreased egg weight at 1.5% (Miles et al, 1986) I and another showed a decrease in egg weight two weeks after birds were placed on a diet containing 1.0% SZA (Harms & Miles, 1987 ) . One experiment which showed no significant effect on egg weight, showed a trend to reduced egg weight as the level of SZA increased up to 1.5% (Littleton, 1988). In this same experiment, the only experiment that reported on egg mass, showed no significant effect on egg mass, but again the trend was for egg mass to decrease as the level of SZA increased. (iv) Effect on feed consumption. Three experiments with broiler breeder hens had their feed consumption fixed. Of the remaining 16 experiments, 13 recorded information on the effects of SZA on feed consumption. Eleven experiments showed no effect on feed consumption due to SZA (Roland et al, 1985 experiments 1 & 2; Miles et al, 1986; Ingram et al, 1987c, experiment 2; Ingram et al, 1987a; Ingram et al, 1987d; Littleton, 1988; Roland, 1988a, experiments 1 to 4) and two showed a significant decrease on feed consumption 256 as the level of SZA increased (Roland, 1988a, experiment 5; Roland 1988b). Of the eleven experiments that showed no effect on feed consumption, three showed a trend to reduced feed consumption as SZA increased in the diet, but this was not significant (Littleton, 1988; Roland, 1988a, experiments 3 & 4) and one showed a significant decrease in feed consumption at the 1.5% inclusion rate, but not at the 0.75% level (Miles et al, 1986). (v) Effect on feed efficiency. Three experiments reported information on feed efficiency. One experiment found no effect of SZA (levels up to 1.5% SZA) on feed efficiency (Littleton, 19881, although there was a trend for feed efficiency to deteriorate as the level of SZA increased. One experiment reported an improvement in feed efficiency at the 1.5% inclusion level but not up to the 0.75% level (Miles et al, 1986). In this experiment the diet was balanced for protein, energy and minerals. Another experiment reported an improvement in feed efficiency at a 2% SZA inclusion rate (Roland & Dorr, 1987), unfortunately information on whether the diet was balanced or not was not repor ted. (vi) Effect on shell quality. All nineteen experiments cited reported information on shell quality. In all of the experiments shell quality was determined using specific gravity of the eggs as the measure. In seventeen of the nineteen experiments SZA increased specific gravity (Roland et al, 1985, experiments 1 & 2; Miles et al, 1986; Ingram et al, 1987c experiments 1 & 2; Harms & Miles, 1987; Ingram et al, 1987a; Ingram et al, 1987d; Roland & Dorr, 1987; Littleton, 1988; Roland, 1988a experiments 3, 4 6r 5; Roland, 1988b; Ingram, 1987b experiment 1 6 2). In one experiment where specific gravity was not generally improved by SZA, balancing the chloride level with hydrochloric acid did allow SZA to improve specific gravity. Where balancing was done with other chloride salts, SZA had no effect on specific gravity compared to control diets (0% SZA) (Roland, 1988a experiment 1). However, in another experiment I regardless of how chloride was balanced, SZA did not improve specific gravity (Roland, 1988a experiment 2). The improvement in specific gravity was generally better in diets containing reduced calcium (2.75%) (Roland et al, 1985) and where the trend for low specific gravity eggs was observed, (Ingram et al, 1987d). In one experiment, the improvement in specific gravity was reflected in a decrease in the number of cracked eggs (29%) I a reduction in egg breakage (28%) and a reduction in the number of egg body checks (44%) (Roland & Dorr, 1987). Where specific gravity was adversely affected due to heat stress, SZA reduced the effects of the heat stress by maintaining the specific gravity of the eggs. The levels of SZA used were 1.0 and 1.5% (Ingram et al, 1987c experiments 1 & 2). It should be pointed out however, that despite these positive observations on specific gravity by using SZA, one would have to question whether these improvements in specific gravity are economical. Specific gravity in most of the experiments was reasonably good (above 1.080), both in control and SZA diets. Only one experiment reports information on actually shelldefects and breakage. Only two experiments reported information on and this was done on birds exposed to heat stress. Under mortality in layers, heat stress conditions diets containing 1.5% SZA reduced mortality associated with the heat stress, (Ingram et al, 1987c experiment l), but not when SZA was included at 1% of the diet, (Ingram et al, 1987c experiment 2). (vii) Effect on mortality. (viii) Effect on plasma calcium and phosphorus levels. Three experiments measured Ca plama levels (Roland et al, 1985 experiments 1 & 2; Miles et al, 1986) and one of these measured plasma P as well (Miles et al, 1986). In all experiments SZA did not influence plasma Ca or P levels. 257 Information was not reported in any of the 19 experiments on water consumption, internal egg quality, moisture or ammonia levels in manure, influences on GIT microflora or effects on nutrient utilisation. Use of natural zeolites in the diets of broilers Twenty five experiment s were cited in which natural zeolites had been included in diets for broilers and young chickens. Seventeen experiments cited clinoptilolite as the natural zeolite and eight did not report the zeolite used. In all cases description of the zeolite used was inadequate. Sixteen of the experiments were conducted on birds beginning at less than one week of age, seven did not report the age of birds and two were conducted on birds 3 weeks of age. In twenty two of the experiments where meat chickens were used, all except one, were taken to marketable ages i.e. between 7 to 12 weeks of age, depending on strain. Inclusion levels of zeolite varied from 0% to lo%, with the majority of experiments having inclusion levels between 1% and 5%. Only two of the experiments balanced dietary nutrients; one balanced protein and the other balanced protein and energy. Fifteen experiments did not balance dietary nutrients, but substituted the zeolite for grain or feed. Two exper iments I balancing was not applicable and six did not report. (i) Effect on growth rate and body weight. The results were conflicting. Twenty one experiments reported information on growth rate or body weight. Four experiments reported an increase in growth rate on zeolite diets compared to controls, (Onagi, 1966; Dzhen Sun Din, 1987; Lon-Wo et al, 1987; Kempton, 1988). Six experiments reported an increase in market body weight, (Chung et al, 1978; Hatieganu et al, 1979; Nestorov, 1983; Vest & Shutze, 1983; Karadzhyan 6 Chirkinyan, 1986; Karelina, 1985) ; two experiments reported a decrease in body weight, (Zavadsky et al, 1985; Lenkova, 1985), one at levels of 7% and 10% inclusion levels only, (Lenkova, 1985); six reported no difference in body weight, (Nakaue et al, 1981; Dion & Carew experiments 1 & 2; Waldroup et al, 1984 experiments 1 & 2; Dion & Carew 1984, experiments 1 6 2); one experiment reported an increase in body weight at 3 weeks but this disappeared at market age (7 weeks) (Hayhurst & Willard, 1980); and another reported no difference at 3 weeks but an increase in body weight at market age (7 weeks) f or one source of zeolite (K type)! but not for another source (Na type) (Willis et al, 1982). For those experiments where zeolite supplementation improved body weight the improvement was from 1% to 16%. For those experiments where a decrease in body weight was reported, the extent of the decrease was not reported. (ii) Effect on feed consumption. The results were conflicting. Seven exper iments repor ted information on feed consumption. In two experiments zeolite diets depressed feed consumption (Hayhurst 6 Willard, 1980; Lon-Wo, 1987). Three experiments reported an increase in feed consumption on zeolite diets compared to controls (Dion & Carew, 1983 experiment 1; Dion & Carew, 1984 experiment 1; Zavadsk