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1 ENZYMES BEYOND PHYTASE IN POULTRY NUTRITION Aaron J. Cowieson, Helen Masey-ONeill1 & Michael R. Bedford1 Poultry Research Foundation, Faculty of Veterinary Science, University of Sydney, NSW 2570 1 AB Vista Feed Ingredients, Marlborough, Wiltshire, UK Summary Phytases have dominated the zootechnical feed additive market for around 20 years and currently account for around half of all feed enzyme sales globally. The reason for this undisputed dominance is of course their tangible and repeatable value to end users. Phytases save the global feed industry an estimated $2bn USD per annum in reduced nutritional input costs, principally through reduced reliance on inorganic phosphates and savings in energy provision. First generation phytases, launched in the early 1990s hydrolysed around 35-40% of dietary phytate, releasing around 0.1% available P at 500 FTU/kg. Today, third or even fourth generation phytases are capable of hydrolysing 60-70% of dietary phytate with 500 FTU/kg matrix values of between 0.12-0.17% available P. Furthermore, increasing commodity prices have encouraged end users to increase phytase inclusion concentrations and it is not uncommon for 750-1250 FTU/kg to be used in many commercial poultry and swine operations. Thus, a combination of evolving technology and increased dosing has to a very real extent resulted in dephytinised diets where all but a relatively low concentration of recalcitrant phytate is hydrolysed and retained by the bird. The question then is, what enzymes are on the horizon for the future and what additional feed cost and/or bird performance advantages may still be untapped? It is the purpose of the current article to discuss opportunities in this area and to set some theoretical limits to give a frame of reference for feed enzyme bioefficacy in the future. Introduction The supplementation of poultry diets with exogenous enzymes to enhance their performance is not a new concept and research articles in this field date back to the early part of the 20th century (Hastings, 1946), with the first published article in 1925 (Rosen, 2010). Since then more than 2500 independent tests on the efficacy of feed enzymes for broilers and more than 450 for layers have emerged, representing one of the most heavily researched fields in avian science (Rosen, 2010). Despite this enormous research effort relatively few publications have led to genuine paradigm shifts in animal production efficiency or in the consistency or magnitude of feed enzyme efficacy. One of the issues that underpins this relatively poor return on investment when some 3000 independent tests are contrasted with a fairly modest expansion of mechanistic understanding, is the law of diminishing returns. As expounded by Cowieson & Bedford (2009) and Cowieson (2010), feed enzymes augment the animals existing digestive processes and as digestibility of any nutrient cannot exceed 100%, the opportunity for an enzyme to elicit a beneficial response is proportionate to the quantity of undigested nutrients that exit the intestine.
2 Over the past 20-30 years (during which time more than 85% of all papers on feed enzymes have been published), broiler genetics has improved dramatically and is predicted to continue with no obvious plateau in sight. De Beer et al. (2011) suggest that whilst a standard 2kg broiler chicken today may be expected to return a FCR of 1.54, in 10 years time (by 2022), at current rates of improvement, this will be in the region of 1.24. Importantly an estimated 85-90% of this improvement will be due to genetic selection, with the remainder being largely nutrition (Havenstein, 2003). The relevance of this staggering improvement in FCR is that as broilers become increasingly efficient converters of feed and water into breast meat, the opportunity for feed enzymes to assist the bird diminishes. This is not to say that the improvements in FCR with genetic selection are driven by improved ileal digestibility of nutrients but rather to indicate that feed enzymes deliver measurable responses more consistently in less efficient populations per se. That said, McDevitt et al. (2006) assessed dry matter digestibility in unselected (essentially 1970s broilers) and selected (around 2002) broilers and observed that selected broilers had significantly higher apparent dry matter digestibility coefficients than unselected (0.52 vs. 0.42; P
3 retain DM. The effect of feed enzymes on water consumption and retention and on FCR on a DM basis is not known and should be explored to better understand how feed enzymes enhance efficiency. In addition to a contemporary genetic and nutritional environment that does not necessarily favour feed enzyme efficacy, there is an additional challenge associated with the sheer number of enzymes and other digestibility enhancers that are available, and which compete for space in the diet. In the 1980s when glucanases and xylanases were initially introduced they were somewhat unique in their role and there was a dearth of competition from alternative microingredients. FCR improvements in this introductory phase with glucanase and xylanase reached 20-30 points, a magnitude that would be unheard of today. Broiler diets today often contain multiple enzymes including xylanase, amylase, glucanase, phytase and protease (and others such as pectinase, mannanase etc) and other digestibility enhancers such as organic acids, various phytochemicals, pre- and pro-biotics etc. Though mode of action differs from product to product there are overlapping regions where the efficacy of one will cannibalise another. This cannibalistic environment is explored in detail in Cowieson (2010) and should inform strategic end user selection of feed microingredients. For example, the value of xylanase, in the absence of phytase, may be $4.50/tonne treated through a reduction in added lipid where in the presence of phytase this is reduced to $3.50/tonne as the energy digestibility effects are not fully additive. The reason for this rather historical introduction is that it is of importance to understand where we have come from and where we are currently to set a frame of reference for possible future direction. There is no doubt that the feed enzyme market has a very bright future and that the feed additive market in general is, and will continue to, grow rapidly. Indeed, the feed additive market as a whole is predicted to grow from $13.5bn in 2011 to $17.5bn in 2018 (World Poultry, 2012). However, the vast number of competing additives, the continued gains in genetic efficiency in animals and plants, and improvements in our understanding of nutrition will result in diminished returns and more challenging circumstances into which to launch new feed enzymes. Hence, the present review will focus on areas of blue sea where there are opportunities for further expansion of the feed enzyme market whilst acknowledging that each new additive we add to our diet reduces the value of the next. The future of phytase and calcium/phosphorus nutrition As discussed above, contemporary phytases are capable of hydrolysing around 60-70% of dietary phytate at economically sensible dose rates, especially when dietary Ca concentrations are moderate (
4 2003) then there is still around 0.7g/kg phytate-P that is available to the animal provided the right phytase can be produced. Alternatively, with phytase prices today in some regions of only $0.6USD for 500 FTU/kg it may be more cost effective (compared with a $5-10m research program to discover a more efficacious phytase) to simply add more of an existing phytase to the diet. Phytase efficacy follows a log-linear dose response curve for most nutrients (Shirley & Edwards, 2003), especially when the intake of digestible nutrients is considered (as P drives intake). Thus, increasing phytase doses from 500 to 1000 FTU will generate an increase in efficacy of around 30% (with an additional 15-20% possible from 1000 to 2000 FTU/kg), reducing the pool of undigested phytate-P to around 0.1-0.2g/kg, a residue that in economic terms may be unimportant. In the early 2000s Zyla and colleagues ran a series of studies to explore whether the efficacy of phytases could be enhanced by augmentation with other enzymes including acid phosphatases, xylanases and pectinases (Zyla et al., 2000; Zyla et al., 2004). Zyla et al. (2004) concluded that addition of either a 3-phytase or a 6-phytase to a diet that already contained an acid phosphatase resulted in different physiological responses in the bird. These data suggest that the esters and indeed isomers of lower inositol phosphates generated by the action of different phytases may be physiologically important. This was recently confirmed by Yu et al. (2012) where InsP5(1,2,3,4,5) generated by a 6-phytase was less powerful in aggregating protein than InsP5(1,2,4,5,6) generated by a 3-phytase. Furthermore, Zyla et al. (2004) found that the addition of 0.1% myo-inositol to a broiler diet resulted in a significant improvement in BWG, FCR (10 points) but actually reduced P retention. Collectively, these pieces of information suggest that in the future, combinations of phytases with different substrate specificities, varying acid phosphatase activity and ability to generate myo-inositol in vivo may be more effective compared with engineering of progressively more aggressive conventional phytases with a single activity. Cowieson & Bedford (2009) summarised the apparent lack of full additivity between phytase and other enzyme classes. As mentioned briefly above, one of the single most influential factors that dictates the magnitude of the response to an exogenous enzyme is the inherent digestibility of nutrients in the control diet. As phytases enhance the digestibility of various nutrients, including amino acids, energy and various minerals (Selle & Ravindran, 2007) the pool of undigested nutrients is reduced and this in turn reduces the opportunity for further enhancement from other feed enzymes. Cowieson & Bedford (2009) suggest that the efficacy of exogenous xylanase will be reduced by 15- 20% when phytase is present in the diet. These effects presumably extend to other enzyme classes such as protease, amylase and glucanase and all these enzymes are interactive. For example, Cowieson et al. (2010) observed cannibalism of efficacy of glucanase by supplementary xylanase (and vice versa), presumably due to overlapping mode of action. Due to the consistent return on investment with microbial phytase it is likely to always be the incumbent in a diet and so the efficacy of additional zootechnical additives must be proven in addition to, not in isolation from, phytase. This
5 is true for xylanases, proteases, pectinases, amylases and mannanases but also for additional phytases such as the addition of a 3-phytase to a diet that already contains a 6-phytase or the use of acid phosphatases and so on. The key here is to understand the quantity of the undigested starch, protein, phytate, fat and other nutritionally meaningful nutrients in the diet and to select feed enzymes accordingly. Whilst the search for increasingly efficacious phytases continues, apparently unabated, a complementary body of work is emerging on calcium and phosphorus nutrition that is critical to set the scene for phytase-induced modifications to dietary specifications. A novel approach that our lab is currently using to do this is termed the Geometric Framework (Cowieson et al., 2012; GF). The GF is a graphical approach to nutrition where outcomes of interest are mapped in a geometric space created by two or more axes that represent nutritional, environmental or other variables. In the case of the study expounded below the two factors of interest were total Ca and av.P though the GF was originally used to explore digestible carbohydrate and digestible protein intakes in insects (Simpson & Raubenheimer, 1995). To meet nutrient requirement, especially in non-domesticated species, animals must consume a range of nutritional sources, as any one food is unlikely to completely satisfy the requirement for all nutrients simultaneously. Provided that the animals immediate environment contains nutritionally imbalanced but complementary foods, in the majority of cases the animal can regulate intake to a particular target point in order to maximise fitness. In intensive animal production the objective of the nutritionist is to formulate a nutritionally balanced ideal diet that will meet the requirement for all nutrients simultaneously. However, these diets are typically formulated to meet the nutrient requirement of the average bird in a flock and are likely to be over- or under-specified for many individuals. When an animal is presented with a single nutrient source (such as a commercial pelleted broiler diet) that is not ideally balanced for all nutrients it is forced to compromise by over- consumption of some nutrients and under-consumption of others. This rule of compromise will be dictated by a number of physiological mechanisms including taste receptors, more general nutrient receptors, the priority order of nutrients and of course the animals ability to post-ingestively process nutrient excesses and overcome insufficiency by regulation of absorption. In the case of digestible carbohydrate and digestible protein, Simpson & Raubenheimer (2012) have shown that for the majority of animal species it is digestible protein that is the most carefully regulated and animals (and humans) will readily overconsume digestible carbohydrate to meet a protein intake target but not vice versa. These principals apply equally to commercial poultry (Shariatmadari & Forbes, 1993). In the case of Ca and av.P it is not clear what the rule of compromise is for broiler chickens. If the diet contains excess Ca but deficient av.P will the bird be prepared to overconsume Ca (and so other dietary nutrients) to meet an av.P intake target? Conversely, if a diet contains excess av.P but insufficient Ca, will it be prepared to overconsume av.P to meet a Ca intake target? These questions
6 are relevant, especially in instances when meat and bone meal and phytase are used, where variable limestone sources are introduced or where Ca concentrations of dietary ingredients or water sources are not monitored. A full description of the Ca/av.P geometric study can be found in Cowieson et al. (2012). However, a truncated view can be found below in Figs 1-5. The effect of dietary Ca and av.P on bodyweight gain (BWG), FCR and feed intake is presented in Figs. 1 & 2. Mortality (not shown) was low and unrelated to treatment. Bodyweight gain increased rapidly from 0.1 to 0.2% av.P, more slowly from 0.2-0.45% av.P and was largely unaffected by dietary Ca over this range. However, at av.P concentrations above 0.40%, Ca rather than av.P became more influential resulting in a (P1.0% Ca) and was most compromised by low av.P and high Ca. However, feed intake between d14-21 was strongly depressed by low Ca and low av.P and was significantly increased by increasing av.P beyond 0.50%. It is well accepted that av.P drives feed intake both in broiler chicks and also in pigs (Onyango et al., 2005; Jendza et al., 2006). These effects were observed in the present experiment where feed intake was highest, over the duration of the experiment, at high av.P concentrations. These effects were particularly apparent across the range 0.24-0.45% av.P. Interestingly, at high av.P concentrations (0.50-0.60%), increasing dietary Ca concentrations from 0.80-1.1% resulted in an increase in feed intake that was not apparent at low av.P concentrations. These effects did not seem to be associated with a particular ratio of Ca to av.P, though it is true that the scope of the experiment did not adequately cover a 2:1 ratio at all av.P concentrations, but it is possible that at av.P concentrations essentially at or above requirement a
7 further increase in intake is possible by increasing Ca concentrations in the diet. Further assessment of the interim periods (d7-14, 14-21 and 21-28) is required in order to elucidate the age-dependent effects. The effect of dietary Ca and av.P on latency to lie and tibia ash is presented in Fig. 3. In accord with the BWG and FCR data, tibia ash was increased (from around 40% to 45-50%) by increasing dietary av.P from 0.25-0.45% where dietary Ca had no effect at this range but increased tibia ash only at av.P concentrations above 0.45% resulting in a significant Ca*av.P interaction. The highest (best) latency to lie scores (expressed in minutes to sit, Fig. 3) were associated with 2:1 Ca:av.P ratios and particularly with intermediate av.P concentrations and either low or high Ca. For example, 0.4% av.P and 0.8% Ca returned latency to lie times of around 5 minutes i.e. the average bird took 5 minutes to sit in the tepid water whereas at 0.2% av.P and 1.2% Ca the average bird sat in the water after only 2 minutes. Of relevance is that excessively high av.P concentrations were as deleterious as low av.P concentrations in terms of the birds mobility. When considered with the tibia ash % results (Fig. 3), this suggests that at low av.P broiler mobility may be impaired by poor bone mineralization whereas at high av.P mobility may be impaired by another mechanism, perhaps unrelated to bone, or related to bone mineral composition or matrix architecture rather than mineral content per se. These mechanisms warrant further exploration but may include hyper-phosphoric inflammatory effects and/or reductions in systemic alkaline phosphatase activity. Calcium digestibility coefficients were maximized at high dietary Ca and low av.P concentrations. The ileal digestibility coefficient of Ca at the geometric centre (0.90% Ca and 0.45% av.P) was approximately 0.45 which is in agreement with previous work in corn/soy-based diets for this age of broiler (Cowieson et al., 2006). Furthermore, an improvement in the digestibility of Ca with a reduction in av.P has also been previously reported, where reducing av.P from 0.48% to 0.29% resulted in an increase in the retention coefficient for Ca from 0.54 to 0.71 (Cowieson et al., 2006). Presumably the improvement in the apparent ileal digestibility coefficient for Ca in the present study with increasing dietary Ca concentration is due to the higher digestibility of Ca from limestone than the endogenous Ca in the corn and soybean meal. The ileal digestibility of P was maximized around 0.5% av.P (formulated) and at high dietary Ca concentrations (>1.0%). Particularly for P these effects were not entirely expected as it has previously been observed that P digestibility is enhanced by low dietary Ca concentrations (Tamim et al., 2004). Data from the present experiment (Fig. 4) suggests that both dietary av.P and Ca concentrations dictate measured ileal P digestibility coefficients but the effects of Ca are more pronounced at low av.P levels and that of P more pronounced at intermediate levels Indeed, at 0.70% av.P (formulated), increasing total Ca from 0.60-1.20% increased ileal P digestibility coefficients from around 0.40 to over 0.50. The reason for this effect is not clear though it appears that P recovery from the small
8 intestine may be maximized with both high av.P (>0.50%) and high dietary Ca (>1.0%). The ratio of Ca to av.P appears unimportant in the ileal digestibility coefficients of most minerals, including P. Thus the effects of Ca and P are equally important at low Ca and P levels but both become less important as the av.P level increases above 0.55. For both the ileal digestibility of Ca and P the fact that at high dietary concentrations there was an increase is not entirely intuitive. It may be expected that if a bird has an intake target for Ca and P that is relatively inflexible then a diet that contains excess would be met with post-ingestive processes that reduce absorption or increase excretion or both. In this case, in the diets with high Ca and low av.P the ileal digestibility of Ca was actually maximized and this is apparently pathological as this would exacerbate the Ca excess (and exaggerate the av.P insufficiency). The intake, absorption and excretion mechanisms for both Ca and av.P are complex and still poorly understood but clearly involve not only digestibility coefficients but digestible nutrient intakes. The apparent ileal digestibility coefficients for Na and also N have discrete optima in terms of dietary Ca and av.P concentrations resulting in target patterns on the geometric surface (Figs. 5). In most instances ileal digestibility coefficients were maximized at 0.45-0.50% av.P and 0.9% total Ca, with divergence away from this in any direction resulting in a reduction in ileal digestibility coefficients. These response surfaces shed some light on the anecdotal effects of phytase on litter moisture content and on the efficacy of microbial phytase on ileal amino acid digestibility coefficients. For example, Ravindran et al. (2008) showed that increasing dietary Na concentrations resulted in a reduction in the efficacy of microbial phytase, especially for the extra-phosphoric nutrients and this was confirmed by Cowieson et al. (2011). Cowieson et al. (2004) also found that phytate ingestion by broilers was associated with an increase in endogenous Na flow from the intestine. So, though phytate and phytase have been implicated in Na balance in broilers the mechanisms are not clear. To the authors knowledge the data presented in Fig. 5 are the first to show a relationship between dietary Ca and av.P per se on ileal Na balance. It is possible therefore that the involvement of phytase in this area is mediated directly through changes in digestible Ca and av.P concentrations. As observed in Fig. 5, relatively modest changes to either (or both) dietary Ca and av.P concentrations would result in significant changes to ileal Na (and indeed N) digestibility, presumably altering the birds need for water, and perhaps altering Na dependent transport systems. It is also possible that Ca and av.P concentrations are directly involved in amino acid digestibility in broilers. The relevance of these geometric maps of Ca and av.P in broilers is that when a phytase is added to a poultry diet the diet will move from one point on the surface to another. For example, superdosing evolved E. coli-derived phytases is increasingly common and 750-1500 FTU/kg may be added with modest matrix value application. The starting point may then be e.g. 0.90% Ca and 0.45% av.P but the end point, once 0.20% Ca and 0.20% av.P is added by the phytase (and perhaps only 50% of these effects accommodated by dietary re-formulation), are 1.0% Ca and 0.55% av.P. Considering Fig. 5,
9 this may result in a change in the ileal digestibility coefficient of Na from around -0.5 to 0.0, altering electrolyte balance and water retention. Similar effects may occur with N (and other nutrients), not due to phytase per se but rather due to phytase-induced changes in Ca and av.P ratios and concentrations. Formulation to a digestible Ca system would undoubtedly help overcome these obscurities as well as an appreciation for the starting point prior to phytase addition and/or prior to increasing the dose of an existing phytase. Xylanases new hypotheses The mode of action of xylanase has been described many times as being predominately associated with hydrolysis of high molecular weight non-starch polysaccharides in cereals and, to a lesser extent, in vegetable protein meals, with a reduction in viscosity in the lumen and an improved access for endogenous enzymes to cell contents (Bedford & Cowieson, 2012). The involvement of the GI tract microbial flora has been suggested previously though the mechanisms of this involvement (and whether it is ubiquitously positive) are not clear. Recently, mechanisms such as the ileal brake, peptide YY (PYY), gastric residency of feed etc have re-emerged as being involved in exogenous xylanase effects (Bedford & Cowieson, 2012; Masey ONeill, et al. 2012). The theory goes that xylanases partially hydrolyse arabinoxylan in the feed leading to increased fermentative activity in the caecum. This fermentation results in the production of VFAs and in turn their absorption stimulates a PYY feedback mechanisms that delay gastric emptying. In a sense xylanase fools the ileal brake mechanism into perceiving that nutrient recovery is not complete and increased gastric digestion is required. Recently our laboratory found that the addition of xylanase to a wheat-soy based diet for broilers to d49 resulted in a 1.4C (approx. 4%) increase in caecal temperature (Fig. 6; Cowieson & Masey-ONeill, 2012). This increase in caecal heat production associated with xylanase addition (especially when added from d1 with no corresponding energy dilution) is interesting and is likely associated either with an increase in fermentation activity per se or with a qualitative change in fermentation patterns. Cooney et al., (1968) measured fermentation heat production in vitro using various substrates and microbial species. They determined firstly that fermentation heat production is directly correlated to microbial O2 consumption but secondly that fermentation of different substrates by the same microbial species generates more or less heat. For example, the fermentation of glucose, molasses and soybean meal by Bacillus subtilis generated 26, 27 and 38 kcal/litre of fermentation fluid respectively (Cooney et al, 1968). Russell (1986) assessed heat production by two species of rumen bacteria (Selenomonas ruminantium HD4 and Bacteroides ruminicola) maintained in vitro under continuous culture conditions. It was observed that heat production was directly related to the concentration of substrate (in this case glucose) that was present. As fermentation substrate concentrations declined, heat production for both organisms declined proportionately. This is evidence that greater heat production in the caecum of broilers is likely to be associated with greater substrate concentrations in the distal GI tract or substantial changes in the nature of the substrate,
10 biasing toward substrates that have an inherently higher heat production on their fermentation. Further, fermentation heat production is calculable based on various inputs such as substrate concentrations, ambient temperature and microbial species involved as it must be removed during large scale fermentation processes in order to keep the yeast or bacteria under optimum thermal conditions. If these equations were applied to heat production changes in the caecum of broilers it may be possible to estimate which substrates or indeed which bacteria, are dominantly involved. Thus, it is possible that the microbial flora is not only involved in the mechanism of action of xylanase but rather is central to its effect. If xylanases increase caecal temperature this will create a new microclimate that will presumably advantage some species of bacteria and disadvantage others. The implications of this require further exploration but may extend to protection against some species of pathogen if the correct substrate (presumably soluble arabinoxylan, though this is poorly defined) and xylanase are fed simultaneously. Increased gastric residency, as a side-effect of these mechanisms may favour protein and phosphorus digestibility and possibly enhance the efficacy of phytases. Conclusions The enormous gains in genetic efficiency in broilers in the past 30 years and the significant improvements in understanding nutrient requirements and husbandry have resulted in modern broilers that are exceptionally capable of converting largely vegetable-based matter into breast and leg meat. These efficiency improvements have, over time, reduced the opportunity for intervention with microingredients such as feed enzymes. Furthermore, the micro-ingredient/digestibility enhancer market is a competitive environment with vast numbers of products, many of which have overlapping mode of action. Thus, the relatively consistent beneficial effects of feed enzymes noted in the 1980s and early 1990s are no longer observed today and the magnitude of the response has decreased. Whilst this is true, commodity prices and the global financial markets are applying unprecedented pressure to margins across the globe and even relatively moderate FCR reductions are enormously valuable to producers of chicken meat and eggs. Before embarking on a new nutritional regime involving one or more feed enzymes an appreciation for the starting point (ground zero) is extremely important. A question such as what is the quantity of undigested nutrients leaving the ileum? must be addressed before any reformulation with an exogenous enzyme matrix value is applied. As each new additive is added to the diet, an acknowledgement of a likely reduction in the value of the incumbents (and the concentration of substrate) is required. Today, feed enzymes are much less costly than at any time in their 20-30 year commercial history. Nevertheless, end users should first pick the low hanging fruit e.g. higher doses of phytase, before considering less well understood enzyme classes such as pectinases, galactosidases, proteases etc. Establishing a new baseline following addition of higher phytase doses will allow the potential for further enzymes to be calculated more accurately. Finally, it is likely that there is a limit to nutritional input cost savings
11 that may be made when using feed enzymes. Speculatively, this cost saving may be in the region of $20-25/tonne of feed. There may be several paths to achieve this e.g. xylanase/phytase combinations, higher phytase doses, use of protease with phytase etc but even if enzymes were entirely free of charge it is unlikely that simply adding every conceivable product available would result in savings greater than the above. In conclusion, the feed enzyme market has a bright future and a substantial role to play in the sustainability of the global poultry industry. However, as existing enzymes are iteratively improved, as costs per unit come down, and as more enzymes are launched, nutritionists are chasing diminishing returns on each subsequent investment.
12 Fig.1 Graphical representation of the effect of dietary Ca and av.P on (left to right) d7-28 bodyweight gain (g/b) and FCR (g:g) of broiler chicks. Total Ca (%) Av.P
13 Fig.3 Graphical representation of the effect of dietary Ca and av.P on (left to right) latency to lie (mobility score, mins to sit) and tibia ash (d28 % DM) in broilers. Total Ca (%) Av.P P
14 Fig.5 Graphical representation of the effect of dietary Ca and av.P on (left to right) apparent ileal Na and N digestibility coefficients in broilers (d28). Total Ca (%) Av.P
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