Effect of graded doses and a high dose of microbial phytase on the digestibility

of various minerals in weaner pigs 1

A.K.Kies,*?2P.A.Kemme,?L.

B.J.S

ˇebek,§J.Th.M.van Diepen,?and A.W.Jongbloed?*DSM Food Specialties,R&D—FTD,2600MA Delft;?Animal Nutrition Group,

Wageningen University &Research Center,6700AH Wageningen;?Division Nutrition and Food;and §Division Applied Research,Animal Sciences Group,Wageningen University &Research Center,

8200AB,Lelystad,The Netherlands

ABSTRACT:An experiment with 224weaner pigs (initial BW of 7.8kg)was conducted to determine the effect of dose of dietary phytase supplementation on apparent fecal digestibility of minerals (P,Ca,Mg,Na,K,and Cu)and on performance.Four blocks,each with 8pens of 7pigs,were formed.Eight dietary treatments were applied to each block in the 43-d experiment:sup-plementation of 0(basal diet),100,250,500,750,1,500,or 15,000phytase units (FTU)or of 1.5g of digestible P (dP;monocalcium phosphate;positive control)per kilogram of feed.The basal diet,with corn,barley,soy-bean meal,and sun?ower seed meal as the main compo-nents,contained 1.2g of dP per kilogram of feed.Fresh fecal grab samples were collected in wk 4and 5of the experiment.Average daily feed intake,ADG,G:F,and digestibility of all of the minerals increased (P <0.001)Key words:digestibility,dose-response,minerals,phytase,weaner pig

J.Anim.Sci.2006.84:1169–1175

INTRODUCTION

Phytase improves dietary P digestibility in nonrumi-nant animals,such as pigs,allowing decreasing P excre-tion in manure.The main storage form of P in seeds is as phytate,the salt of phytic acid (myo-inositol hexakis-phosphate),a poorly used P source for nonruminant animals.Under normal physiological conditions,phy-tate is a negatively charged ion that is able to bind cations such as Ca,Mg,and Zn,and also proteins (Ravi-ndran et al.,1995;Bebot-Brigaud et al.,1999).Phytase hydrolyzes the ortho-phosphate groups from phytate.Phytate-bound nutrients are liberated as well.The re-sult is not only a greater digestibility of P but also of protein (Kies et al.,2001)and minerals.Dietary micro-

This work was funded by DSM Food Specialties,Agri Ingredients,Delft,The Netherlands and BASF AG,Ludwigshafen,Germany.2

Corresponding author:arie.kies@https://www.360docs.net/doc/9618546377.html, Received May 27,2005.

Accepted December 13,2005.

1169

with increasing phytase dose.Digestibility of P in-creased from 34%in the basal diet to a maximum of 84%in the diet supplemented with 15,000FTU,generating 1.76g of dP per kilogram of feed.At this level,85%of the phytate phosphorus was digested,compared with 15%in the basal https://www.360docs.net/doc/9618546377.html,pared with the basal diet,digestibility of the monovalent minerals increased max-imally at 15,000FTU,from 81to 92%(Na)and from 76to 86%(K).In conclusion,phytase supplementation up to a level of 15,000FTU/kg of a dP-de?cient diet improved performance of weaner pigs and digestibility of minerals,including monovalent minerals.Up to 85%of the phytate-P was digested.Thus,dietary phytase supplementation beyond present day standards (500FTU/kg)could further improve mineral use and conse-quently reduce mineral output to the environment.

bial phytase was reported to increase digestibility of Ca,Mg,Mn,Zn,Cu,and Fe in pigs at a dose level of 500to 1,500phytase units (FTU )/kg of feed (Pallauf et al.,1992;Adeola,1995;Jongbloed et al.,1995).

There is a dearth of information about the effect of phytase doses greater than 1,500FTU/kg on mineral digestibility.Du ¨ngelhoef and Rodehutscord (1995)and Kornegay (2001)estimated that only a small additional effect on P digestibility in pigs would be obtained at dose levels exceeding 1,500FTU/kg.The effect of high phytase doses on P and Ca digestibility was studied in broiler chicks (Shirley and Edwards,2003;Augspurger and Baker,2004)and in pigs (Harper et al.,1999).The authors concluded that phytase continued to improve performance,bone characteristics,and P and Ca digest-ibility up to a dose of 10,000(Harper et al.,1999;Aug-spurger and Baker,2004)or 12,000FTU/kg (Shirley and Edwards,2003).

The present experiment was performed to obtain more information about the effect of graded phytase doses up to a high level (100,250,500,750,1,500,and

Kies et al. 1170

15,000FTU/kg of feed)on the digestibility of P,Ca,Mg, Na,K,and Cu in weaner pigs.

MATERIALS AND METHODS Animals and Housing

The ethics committee of DLO-Institute for Animal Science and Health approved the experimental proto-col.Two hundred and twenty-four crossbred[(York-shire×Dutch Landrace)×Yorkshire]female and cas-trated male weaner pigs were used from weaning(at about28d of age;mean initial BW of7.8kg).Each of the8treatment groups consisted of4replicates(pens) with7pigs each.The7pigs in each pen(4females and 3castrates,or3females and4castrates)were selected from7pairs of sows and were chosen randomly.No littermates were used in any one pen.Selection criteria of the weaner pigs were health status,weight,and sex. Pigs remained in the farrowing house during the43-d experiment.Four farrowing houses were used,each one being considered a block and each block containing8 pens.The8treatments were randomly assigned to pens in each of the4blocks.Pen size was1.6×1.8m.Temper-ature at weaning was kept at25°C and was lowered by <1°C every week thereafter.Ventilation was thermo-statically controlled.

Treatments,Diets,and Feeding

Eight treatments were the basal diet containing0 FTU of added phytase(basal diet),the basal diet to which100,250,500,750,1,500,or15,000FTU of phy-tase was added per kilogram of feed(as-fed basis)and the basal diet to which monocalcium phosphate was added(positive control).The basal diet contained nutri-ents at or above the levels recommended by the CVB (2002),except for Ca and P.Digestible P(dP)content of the basal diet was estimated at1.25g/kg;its composi-tion and proximate analyses are presented in Table1. As the positive control,the basal diet was supplemented with1.5g of dP/kg in the form of monocalcium phos-phate monohydrate(MCP),with an assumed P digest-ibility of83%(CVB,2000).One FTU is de?ned as the phytase activity that liberates1micromole of ortho-phosphate per minute from5.1m M sodium phytate at 37°C and at pH5.5(Engelen et al.,1994).The phytase source was Natuphos Granulate(DSM Food Special-ties,Delft,The Netherlands)and was added based on its analyzed activity.

The feed was produced by?rst making a mix of the ingredients(without mineral mix,microbial phytase, or MCP).This mix was split into8equal parts,to which a starch-based premix,containing mineral mix,micro-bial phytase,or MCP,was added.After mixing,the feeds were pelleted(diameter3.2mm)without the addi-tion of steam to prevent possible inactivation of phy-tase.Temperature during the pelleting process was be-low60°C.It was assumed that100,250,500,750,1,500,Table1.Feed composition and nutrient contents of the basal diet(g/kg,as-fed)1

Ingredient Content Corn321 Barley300 Soybean meal108 Sun?ower seed meal65 Soybeans,toasted40 Potato protein38 Acid casein33 Cane molasses35 Soy oil13.5 Fumaric acid10 Salt 2.5 L-Lysine?HCl0.95 DL-Methionine0.25

Chromic oxide premix2 1.0 Choline chloride(40%)0.30 Premix3 2.0 Limestone 6.1 Cornstarch23.4 Analyzed composition4

DM868 CP202 Ash38 Ileal digestible Lys59.9 NE,5MJ/kg9.6 Ca 3.5 P 3.7 Digestible P5 1.25 Phytate P 2.6 1Limestone was added to obtain a Ca:digestible P ratio of2.8.To diet8(positive control),7.96g of monocalcium phosphate was added per kilogram of feed.Cornstarch was used to balance to1,000g/kg. 2Contained0.25g of chromium oxide per kilogram of feed.

3The vitamin-mineral premix provided per kilogram of the diet: vitamin A,10,000IU;vitamin D3,2,000IU;vitamin E,20IU;vitamin K3,1.5mg;thiamine,1mg;ribo?avin,4mg;D-pantothenic acid,15 mg;niacin,25mg;vitamin B12,20?g;folic acid,0.2mg;pyridoxine, 1.5mg;Fe(FeSO4?7H2O),150mg;Cu(CuSO4?5H2O),125mg;Zn (ZnSO4?7H2O),80mg;Mn(MnO),30mg;Co(CoSO4?7H2O),0.15mg; I(KI),0.5mg;and Se(Na2SeO3?5H2O),0.3mg.

4Variation between diets was small.The Mg,Na,and K contents of the basal diet were1.6,1.1,and8.1g/kg,respectively.The Cu content was113mg/kg.

5Calculated values(CVB,2000).

and15,000FTU of phytase generated0.15,0.4,0.8,1.0, 1.1,and1.1g of dP/kg of feed,respectively.To obtain a?xed Ca:dP ratio of2.8,Ca content of the diets was increased by the addition of limestone.Analyzed Ca and P contents and phytase activity,and calculated and measured dP contents,are presented in Table2.The analyzed and calculated values agreed well.Weaner pigs had ad libitum access to feed and normal tap water. Contents of Ca,P,Mg,Na,and K in the water were approximately64,0.01,4.5,19.8,and1.0mg/L,respec-tively.The Cu level was negligible.The impact of these minerals was ignored in the digestibility calculations. Observations,Collection,

and Analytical Procedures

Body weights and intake were recorded on d0,8,29, and43.Health status of the pigs was monitored twice

Phytase and mineral digestibility1171 Table2.Analyzed Ca,P,and phytase activity of the diets,calculated and measured

digestible P(dP)levels(as-fed basis),and calculated digestibility coef?cient(DC)of

phytate-P

Ca,P,1Phytase,2Calculated dP,Measured dP,Calculated DC

FTU added/kg g/kg g/kg FTU/kg g/kg g/kg Phytate-P3

0 3.5 3.7350 1.25 1.2213.9

100 3.9 3.7470 1.40 1.4623.2

250 4.6 3.6600 1.65 1.6932.5

500 5.8 3.6810 2.05 2.0144.9

750 6.3 3.61,190 2.25 2.1952.1

1,500 6.5 3.61,740 2.35 2.6067.9

15,000 6.7 3.613,900 2.35 3.0485.4

Positive control7.6 5.4340 2.75 2.5812.2

1Calculation of P digestion is based on the mean P level of these diets(3.63g/kg,as-fed basis).

2One FTU is de?ned as the phytase activity that liberates1?mol of orthophosphate per minute from5.1

m M sodium phytate at37°C and at pH5.5.

3Digestibility coef?cient of phytate-P,assuming80%digestibility of nonphytate-P.

daily throughout the experiment.Fresh fecal grab sam-ples were collected in wk4and5;care was taken to collect feces from each of the different weaner pigs. Sampling was on Tuesdays and Thursdays at0800to 0830,1000to1030,1300to1330,and1500to1530. The samples were pooled per pen per week and frozen at?18°C pending analysis.Feed was sampled during its production.

All analyses were performed on freeze-dried feed and fecal samples.Feeds were analyzed for DM,ash,Ca, P,Mg,Na,K,Cu,Cr,and phytase activity.Phytate-P and nitrogen were analyzed in the basal diet only.Feces were analyzed for DM,ash,Ca,P,Mg,Na,K,Cu,and Cr.Dry matter,ash,and nitrogen(Kjeldahl)were as-sayed using AOAC procedures(1984).Mineral levels (except Cr)were determined using inductively coupled plasma,atomic emission spectrometry,according to NEN-ISO(1998).The method of Williams et al.(1962) was used to analyze chromium.Phytate-P was mea-sured by the enzymatic method(Bos et al.,1993)and phytase activity by the method of Engelen et al.(1994). Digestibility coef?cients of DM,ash,and the minerals under investigation were calculated using Cr as an indi-gestible marker according to standard procedures. Statistical Analysis

Data were analyzed by ANOVA as a randomized com-plete block design,with pens as the experimental units, using SAS Version6.12(SAS Inst.,Inc.,Cary,NC). Because no treatment×week interaction was observed for the digestibility coef?cients,results for the2-wk collection period were averaged and analyzed as such. The effect of phytase addition was tested using orthogo-nal polynomial contrasts and differences between se-lected treatments as single-df contrasts,at a signi?-cance level of P<0.05.Negative exponential dose-re-sponse equations were?tted using the NLIN-procedure of SAS for performance and mineral digestibilities for all treatments excluding the positive control.

RESULTS

General Observations and Performance

Few health problems occurred in this experiment.A limited number of animals were treated for lameness. Eight weaner pigs died,mainly due to edema disease. Mortality was not related to treatment.Average?nal weight of the weaner pigs was27.5kg.

Performance of pigs fed the positive control diet was signi?cantly better than for those fed the basal diet(P <0.001;Table3).These results indicate that the basal diet was clearly de?cient in dP.Dietary phytase supple-mentation affected ADFI,ADG,and G:F in a dose-de-pendent manner.The calculated exponential relation-ships were:

ADFI(g/d)=751?167×e?0.0021×FTU

(SE=10.8;P<0.001);

ADG(g/d)=512?141×e?0.0018×FTU

(SE=9.4;P<0.001);

G:F=687?49×e?0.0008×FTU

(SE=6.9;P=0.0015),

in which FTU is the added microbial phytase activity (FTU/kg).

Digestibility Values

Treatment affected the digestibility of all measured response criteria(P<0.01;Table4).Dry matter digest-ibility was slightly lower in the positive control diet than in the basal diet(P=0.09)and the diet with500 FTU/kg(P<0.05),but differences between diets were small.Ash,P,Mg,Na,K,and Cu digestibility increased (P<0.001)in a dose-dependent(linear and nonlinear) manner up to a level of15,000FTU/kg.Also,Ca digest-

Kies et al.

1172Table 3.Average daily feed intake,ADG,and G:F of pigs receiving diets with different levels of phytase,or a diet containing 1.5g of digestible P from monocalcium phos-phate monohydrate (positive control)1

FTU added/kg,as-fed basis 2ADFI,g ADG,g G:F,g/kg 0(basal diet)5883706291006033886452506664366545006974586587507084676591,50074749966715,000

752517689Positive control 699468671SEM 18.411.4 6.5P -value

<0.0013

<0.0014

<0.0015

Data are means of 4replications of 7weaner pigs over a 43-d feeding period;mean initial BW was 7.8kg.2

One FTU is de?ned as the phytase activity that liberates 1?mol of orthophosphate per minute from 5.1m M sodium phytate at 37°C and at pH 5.5.3

Linear (P <0.001),quadratic (P <0.001),and cubic (P =0.018)phytase effects.4

Linear (P <0.001),quadratic (P <0.001),and cubic (P =0.003)phytase effects.5

Linear (P <0.001),quadratic (P =0.003),and cubic (P =0.078)phytase effects.

ibility increased in a dose-dependent manner,but this effect may be biased due to different limestone levels in the diets.Copper digestibility was negative at phytase doses below 750FTU/kg.An exponential model was ?tted to the dP levels,resulting in the following equation:

Digestible P (g/kg)=3.02?1.76×e ?0.00102×FTU

(SE =0.045;P <0.001).

Table 4.Effect of dietary supplementation with different levels of phytase on apparent fecal digestibility of DM,ash,and some minerals in pigs 1

FTU/kg,as-fed basis 2DM Ash Ca P Mg Na K Cu 0(basal diet)84.452.357.233.519.881.176.1?5.810084.755.264.040.124.683.473.5?2.325085.258.566.946.624.281.076.0?3.050084.560.470.655.325.483.077.2?1.575084.461.669.260.424.486.078.50.41,50084.666.271.871.527.788.881.5 2.815,000

85.370.275.883.833.892.185.7 6.0Positive control 83.955.758.147.820.884.475.3?2.2SEM 0.220.610.95 1.06 1.17 1.050.90 1.05P -value

<0.0033

<0.0014

<0.0015

<0.0016

Data are means of 4replications of 7weaner pigs.Digestibility was measured in wk 4and 5of the experiment;these data were averaged per treatment.2

One FTU is de?ned as the phytase activity that liberates 1?mol of orthophosphate per minute from 5.1m M sodium phytate at 37°C and at pH 5.5.3

Linear phytase effect (P =0.013).4

Linear (P <0.001),quadratic (P <0.001),and cubic (P <0.001)phytase effects.5

Linear (P <0.001)and quadratic (P =0.006)phytase effects.6

Linear (P <0.001)and quadratic (P <0.001)phytase effects.

The estimated dP level of the basal diet according to this model was 1.26g/kg (measured was 1.22g/kg;Ta-ble 2).The plateau level was 3.02g of dP/kg,which was reached at about 5,000FTU/kg (3.04g measured at 15,000FTU/kg).Consequently,the maximal amount of dP generated by phytase in this diet was 1.76g/kg.Similar exponential models were ?tted for the other minerals.Digestible DM gave no good ?t,and digestible ash and Ca are biased due to the different inclusion levels of limestone.These equations are,therefore,not presented.The equations for Mg,Na,K,and Cu are as follows:

digestible Mg (mg/kg)=547?188×e ?0.00045×FTU

(SE =28.0;P =0.021);

digestible Na (mg/kg)=960?117×e ?0.00067×FTU

(SE =14.7;P =0.011);

digestible K (g/kg)=6.8?0.89×e ?0.00059×FTU

(SE =0.09;P =0.006);

digestible Cu (mg/kg)=7.22?12.96×e ?0.00083×FTU

(SE =1.20;P =0.003).

The amounts of these minerals digested due to phy-tase can easily be calculated from these equations.

DISCUSSION

Addition of microbial phytase to the basal diet,with a measured dP content of 1.2g/kg,improved weaner pig performance.The recommended dP level for weaner pigs is 3.7g/kg (CVB,2002).The animals in this experi-ment will not show their maximal performance because

Phytase and mineral digestibility1173

dP contents of all diets were below their requirement (Jongbloed,1987).Phytase increased P digestibility and improved performance,which is in agreement with pre-vious studies(Beers and Jongbloed,1992;Cromwell et al.,1993;Kornegay,2001).In the current experiment, performance improved up to a phytase inclusion level of15,000FTU/kg.The maximal effect of phytase sup-plementation on performance has been estimated to be in the range of500to1,500FTU/kg in basal diets with similar dP contents to the one in the present experiment (Beers and Jongbloed,1992;Gentile et al.,2003).In a review,Kornegay(2001)came to a similar conclusion. At a phytase dose of1,500FTU/kg,dP level was similar to that of the positive control diet(2.6g/kg).At this phytase level,feed intake and ADG tended to be im-prove compared with the positive control diet by6.9 (P=0.076)and6.6%(P=0.074),respectively.These tendencies may indicate some effect of phytase on per-formance beyond the dP-related effect(Kies et al., 2001).

The exponential models used showed an excellent?t to the https://www.360docs.net/doc/9618546377.html,paring the curve for dP with results calculated from Kornegay(2001;equation1,500)shows a similar dose-effect of phytase up to a level of about 750FTU/kg.After that,the curve calculated from Kor-negay’s equation plateaus rapidly.The level of the cal-culated plateau(after correction for difference in P di-gestibility at0FTU/kg and for a diet containing3.63 g P/kg)is about0.8g of dP/kg feed lower than the plateau of the curve calculated from the current ex-periment.

Supplementation of microbial phytase to low-P diets improved P digestibility.This con?rms previous results (Jongbloed et al.,1992;Adeola et al.,1995;Kornegay and Qian,1996).Generation of digestible P by phytase was,however,much greater than expected,especially at the greatest dose(Table2).The assumed dP genera-tion with the addition of15,000FTU/kg was1.1g/kg, but the measured value was1.83g/kg.In earlier dose-response experiments,maximum dP generation was found at much lower phytase doses.Beers and Jong-bloed(1992)found a maximum at around1,000FTU/ kg,both for corn-soybean meal and for by-product-based diets.Also,the curves calculated from the experiments of Kornegay and Qian(1996)and Yi et al.(1996)showed no further increase in P digestibility at doses greater than1,000to1,500FTU/kg of feed.Based on a litera-ture review,Du¨ngelhoef and Rodehutscord(1995)cal-culated a dose-response curve and concluded that little improvement in P digestibility at doses greater than 750FTU/kg could be expected.The additional effect of phytase on P digestibility at a dose>1,500FTU/kg was unexpected.The inclusion level of15,000FTU/kg was chosen to improve the estimation of the plateau level of the exponential curve.Because this model seems the most appropriate to describe the effect of phytase,all data points were included in estimation of the equation. For P digestibility,results similar to those in current experiment were obtained in earlier experiments in-vestigating very high doses of microbial phytase. Harper et al.(1999)concluded that a dose of10,000 FTU/kg continued to improve performance,bone miner-alization,and mineral digestibility of grower pigs.Aug-spurger and Baker(2004)reported improvements of performance and bone characteristics in broilers with phytase inclusion up to10,000FTU/kg,although they observed some differences between phytase sources. Shirley and Edwards(2003)conducted a broiler trial with phytase doses up to12,000FTU/kg.They observed that performance,bone characteristics,and P-retention improved with increasing phytase dose.In their study, phytate-P disappearance increased up to85and95% at supplementations of6,000and12,000FTU/kg,re-spectively.These?ndings are comparable to our results in weaner pigs.Assuming a digestibility of80%for nonphytate P(Jongbloed,1987),phytate-P digestibility was85%at15,000FTU/kg,compared with14%in the basal diet(Table2).

The continuing improvement of digestibility up to very high phytase doses was unexpected,given the ear-lier assumptions that the maximum effect would be realized at a dose of1,000to1,500FTU/kg.A possible explanation may be the following:Due to a number of factors,such as the residence time of feed and phytase, pH value,grade of phytate accessibility,and grade of phytase degradation in the stomach,and the phytase and phytate characteristics,the main site of phytase activity is the stomach(Jongbloed et al.,1992;Yi and Kornegay,1996).In the case of a very high phytase dose(e.g.,15,000FTU/kg),soluble phytate is the lim-iting factor in the biochemical reaction(Kemme,1998). Phytate may be degraded faster or to a greater extent at such a high dose than is the case with normal doses of about500FTU/kg,in which soluble phytate is avail-able in excess.One could imagine that the faster degra-dation of phytate creates space for additional phytate molecules to dissolve but that these molecules would not dissolve when a phytase dose of500FTU/kg was applied.The extra dissolved phytate molecules will be degraded by phytase.

Another speculation regarding the large effect at the high phytase dose is that a large part of the active phytase escapes the stomach to the small intestine. Phytase from Aspergillus niger has an optimal pH of

5.5but retains35to80%of its activity at pH values of

6.0to6.5(Engelen et al.,1994).These pH values are observed in the upper half of the small intestine at4.5 h after feeding(Van der Meulen and Bakker,1991), which means that phytase can still be active in the small intestine.When phytase is included at500FTU/ kg of feed,the activity in the small intestine is of limited magnitude,but in the case of a very high dose,the resulting phytase activity in the gut may be high,with consequently a greater level of phytate hydrolysis.An argument against this hypothesis could be that at greater pH phytate can precipitate with cations,e.g., Ca or Mg,depending on the mineral:phytate ratio (Cheryan,1980).The solubility of Ca-phytate decreases

Kies et al. 1174

rapidly at a pH above approximately6,but the Mg salt precipitates at a greater pH.This indicates that in the upper small intestine,phytate is probably still available in a soluble form and that it is available for hydrolysis by phytase.De Rham and Jost(1979)showed that phy-tate might be soluble at pH from5.5to11.Thus the hypothesized explanation for the additional effect at the high phytase dose on mineral digestibility,com-pared with the practical standard(500FTU/kg)seems feasible.It needs to be tested in experiments in which phytate degradation and phytase activity in the duode-num of weaner pigs fed diets with a high phytase activ-ity are measured.

Calcium digestibility increased with increasing levels of phytase addition,which is in agreement with earlier reports.O’Quinn et al.(1997)found a linear increase in Ca digestibility with increasing phytase supplemen-tation up to1,000FTU/kg.The effect is,however,not always signi?cant(Murry et al.,1997).In the current experiment,a constant Ca:dP ratio was realized in the diets by adding limestone to diets that contained micro-bial phytase.The observed increase in Ca digestibility was,therefore,confounded with limestone level.Jong-bloed et al.(1995)showed increased Ca digestibility with phytase addition to the diet of grower pigs,but the increase was smaller at a high level of dietary Ca than at a low level.Digestibility of Mg and Cu increased with phytase supplementation.This is in agreement with earlier observations in weaner pigs(Pallauf et al., 1992;Adeola,1995;Kies et al.,2005),in grower pigs (Jongbloed et al.,1995),and in sows(Jongbloed et al., 2004).The increased Ca digestibility due to phytase was not accounted for when formulating the diets.This may be a confounding factor on the estimated digestibil-ity of minerals,because greater Ca levels may reduce digestibility of cations(Jongbloed et al.,1995).This would mean digestibility of Mg and Cu might be under-estimated.Because diets in the current experiment did not contain excessive levels of minerals,this effect is likely of limited magnitude.

The improved digestibility of Na and K with phytase supplementation is surprising.Phytase addition in-creased fecal digestibility of these monovalent cations up to10%-units(P<0.001).This con?rms recent?nd-ings in sows(Jongbloed et al.,2004)and in weaner pigs (Kies et al.,2005).The effect of phytase on digestibility of monovalent cations was not reported before those experiments.Sodium and K-phytates are highly solu-ble:Na-phytate dissolves more than96%over the pH-range of0.3to11.2(Scheuermann et al.,1988).Proba-bly for this reason,inhibition of Na and K-absorption by phytate has not been studied in vivo.

To obtain the maximal effect of phytase on animal performance,the increased mineral digestibility,in-cluding digestibility of Na and K,needs to be considered in practical feed formulation.Dersjant-Li et al.(2002) showed that the anion-cation difference affects energy use in pigs,and Kies et al.(2005)calculated that in-creased absorption of minerals,and their subsequent excretion with urine,might increase energy expendi-ture.Even though this effect of minerals on energy use is small,it may have practical relevance.

IMPLICATIONS

The dose-dependent effect of microbial phytase sup-plementation of weaner pig diets on performance and mineral digestibility continued up to the very high in-clusion level of15,000phytase units per kilogram. Greater inclusion levels of phytase than the current industry standard of500phytase units per kilogram permit,therefore,to further reduce dietary mineral in-clusion levels and,consequently,the excretion of phos-phorus and other minerals into manure.The economi-cally most advantageous phytase level depends on the balance of these advantages and of costs and character-istics of the phytase product and needs to be evaluated per case.

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