Effect of sources of maize and maize
particle sizes on the utilization of
phytate phosphorus in broiler chicks
A.B. Kasim, H.M. Edwards Jr.
*Department of Poultry Science, The University of Georgia, Athens, GA 30602-2772, USA
Received 8 November 1999; received in revised form 30 March 2000; accepted 25 May 2000
Abstract
Two experiments were conducted to test the effects of sources of maize and maize particle sizes on the utilization of phytate P in broiler chicks fed on maize±soyabean meal diets de®cient in P. In Experiment 1, maize from four consignments was ground separately using the same milling facility. Four maize±soyabean diets representing each maize source, and two control diets made of maize-starch±soyabean meal (maize starch control diet) and maize±soyabean meal-casein (casein control diet) were tested. The maize starch control diet was used to evaluate the ability of chicks to utilize phytate P when they were provided solely by soyabean meal. The casein control diet had similar amounts of total P as the diets containing the four maize sources, but part of the maize and soyabean meal was substituted with casein so that the diet contained greater amounts of available P and less phytate P. Experiment 2 was conducted on a 32 factorial arrangement, consisting of three diets each containing maize differing in geometric mean diameter (GMD), and two levels of dietary phytase at 0 and 600 FTU/kg. Sources of maize signi®cantly (p<0.05) in¯uenced Ca, total P, and phytate P utilization, and the amount of ME derived from the diets. Only 0.248 of soyabean meal phytate P was utilized by the chicks fed on the maize starch control diet. Substituting part of the maize and soyabean meal with casein did not affect the total P utilization but reduced Ca utilization, possibly due to the binding of Ca in the gut with phytate P not utilized by the chicks. Maize particle sizes signi®cantly (p<0.05) affected utilization of Ca, total P, and phytate P. The coarser the GMD (894 vs 573 and 484mm), the greater the utilization of these nutrients. There were no two-way interaction effects of particle sizes and phytase.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Maize sources; Particle size; Phytate; Maize±soyabean meal; Chick 86 (2000) 15±26
*Corresponding author. Tel.:1-706-542-1351; fax:1-706-542-1827.
E-mail address: hedwards@arches.uga.edu (H.M. Edwards Jr.)
0377-8401/00/$ ± see front matter#2000 Elsevier Science B.V. All rights reserved.
1. Introduction
A practical maize and soyabean meal diet for chicks containing all the levels of nutrients recommended by NRC (1994), has 1.2 and 1.4 g/kg phytate P coming from maize and soyabean meal, respectively. Dietary and non-dietary factors in¯uencing phytate P utilization in chicks have been identi®ed. Dietary factors include Ca content of the diet (Waldroup et al., 1964; Edwards and Veltmann, 1983; Ballam et al., 1984; Simons et al., 1990; Mohammed et al., 1991; Mitchell and Edwards, 1996a,b), and cholecalciferol supplementation (Edwards, 1993; Roberson and Edwards, 1994; Biehl et al., 1995; Mitchell and Edwards, 1996a,b). Non-dietary factors identi®ed include sex of chicks (Edwards et al., 1989; Carlos, 1997), and genotype and age of dams (Carlos, 1997). Cossa et al. (1997) showed that phytate P content varies between maize varieties. The 27 varieties of maize they analyzed contained from 1.9 to 3.5 g/kg phytate P. Thornton et al. (1969) and Leeson and Summers (1976) reported that the nutrient composition of maize varies with adverse growing conditions and the stage of maturity, and these changes may affect its feeding value for poultry. Edwards (1985) reported that sources of soyabean meal in¯uenced the incidence of tibial dyschondroplasia in chicks. Carlos et al. (1996) showed that two sources of soyabean meal differing in the content of trypsin inhibitor affected body weight (BW), gain-to-feed ratio, and plasma dialyzable P (dP) of chicks.
Maize particle size in¯uenced retention time in the upper digestive tract of the chicken. Heuser (1945) demonstrated that the length of time that samples of maize remained in the crop decreased from whole maize, cracked maize to maize meal, and the contraction cycle of the gizzard of the chicken was lengthened when it contained whole grain instead of mash feed. Carlos and Edwards (1997) reported that grinding part of the soyabean meal in the diet reduced the utilization of phytate P by broiler chicks. The purpose of these studies was to determine whether sources of maize and maize particle sizes in¯uence phytate P utilization by broiler chicks.
2. Materials and methods
2.1. General procedure
Two experiments were conducted for a period of 16 days using Peterson x Arbor Acres day-old broiler chicks of mixed sexes from a commercial hatchery (Harrison and Feeds, Bethlehem, GA). All the basal diets were P de®cient but were adequate in other nutrients, as shown in Table 1. The soyabean meal used for both experiments was the same. Diets were given to chicks in mash form. Experiments were conducted in electrically heated wire mesh-¯oored battery brooders (Petersime Incubator, Gettysburg, OH) with feed and water always available. Ultraviolet irradiation was eliminated from the chick room by completely covering glass windows with opaque plastic sheets and by ®tting Arm-a-Lite1
sleeves (Thermoplastics Processes, Sterling, NJ) to all the ¯uorescent ®xtures in the room and battery brooders (Edwards et al., 1994). The ¯uorescent lights were put on throughout the day. The temperature of the room was maintained at 228C.
At 14th day of the experiment, excreta samples were collected for 24 h and a sample of droppings oven-dried. Feed and excreta were analyzed for Ca (Hill, 1955), total P (O'Neill and Webb, 1970), phytate P (Latta and Eskin, 1980), chromic oxide (Brisson, 1956), nitrogen (Dumas combustion procedure, Etheridge et al., 1998), and gross energy (instructions for 1241 and 1242 Adiabatic Colorimeters, Parr Instrument, Moline, IL). Calcium, P, and phytate P retentions were calculated using the methods of Edwards and Gillis (1959). Metabolizable energy (ME) derived from the diets was calculated using the method of Potter (1972) which gives aN-corrected ME value that is not corrected for endogenous energy loss.
At the end of the experiments, one bird from each pen was selected randomly and a blood sample is obtained by heart puncture for plasma total Ca (Section N-31 Technicon Autoanalyzer Methodology, Technicon, Tarrytown, NY) and dP (Section N-46 Technicon Table 1
Composition of the experimental diets (g/kg)
Ingredients Maize±soyabean
Ground yellow maize 532.4 ± 400.0
Soyabean meal (dehulled) 380.9 474.0 110.7
Maize starch ± 437.6 280.0
Casein ± ± 72.6
Cellulose ± ± 47.6
Poultry fat (stabilized) 50.0 50.0 50.0
Dicalcium phosphate 6.1 6.1 5.9
Limestone 19.9 19.9 21.5
Iodized sodium chloride 4.5 4.5 4.5
DL-methionine (98%) 1.9 1.9 1.9
Vitamin premixa 2.5 2.5 2.5
Trace mineral premixb 0.8 0.8 0.8
NaH2PO4H2O ± 1.1 1.0
Chromic oxide 1.0 1.0 1.0
Calculated compositionc
Crude protein 231.1 231.3 231.3
Metabolizable energy (MJ) 13.10 13.10 13.14
Ca 10.0 10.2 10.3
Total P 5.0 4.3 4.4
aVitamin premix provided in milligrams per kilogram diet (except as noted): vitamin A (as all-trans-retinyl
acetate), 5500 IU; vitamin E (all-rac-/-tocopheryl acetate); 1.1 IU; ribo¯avin, 4.4; calcium panthotenate, 12;
nicotinic acid, 44; choline Cl, 220; vitamin B12, 9mg; vitamin B6, 3.0; menadione (as menadione sodium
bisul®te), 1.1; thiamin (as thiamin mononitrate), 2.2; folic acid, 3.0; biotin, 0.3; and ethoxyquin, 125.
bTrace mineral premix provided in milligrams per kilogram diet: Mn (MnSO
4), 36; Zn (ZnSO4), 33; Fe
(FeSO4), 20; Cu (CuSO45H2O), 0.76 and I [Ca(IO3)2], 0.75; Mg (MgO), 12; Se (Na2SeO3), 0.14; Ca (CaCO3),
210.
cAnalyzed values of Ca, total P, and phytate P as grams per kilogram of diets: Experiment 1, maize±
soyabean meal diet: Maize A: 10.5, 5.0, 2.7; Maize B: 10.1, 5.0, 2.8; Maize C: 10.4, 4.9, 2.7; Maize D: 10.0, 5.1, 3.0; maize starch control diet: 9.3, 3.7, 3.0; casein control diet: 9.9, 4.9, 2.2. Experiment 2, maize±soyabean meal diet: 9.4, 5.3, 3.0.
Autoanalyzer Methodology, Technicon, Tarrytown, NY) content. The birds were weighed by pens and their feed consumption recorded. They were then killed by carbon dioxide asphyxiation and examined at random for P type rickets. A longitudinal cut was made across the right tibia and diagnosis of P type rickets was made based on a lengthened primary spongiosa but with normal proliferating zones. In phosphorus de®ciency, the metaphysis is dark and increased in length. When the ratio of the darkened lengthened metaphysis to the diameter of the bone at the epiphysis is greater than 0.5 the phosphorus de®ciency is scored as severe. Bone ash on fat-free dry basis (Association of Of®cial Analytical Chemists, 1995) was determined on the left tibia.
Analysis of variance and regression analysis were computed using the General Linear Model procedure of SAS1
(SAS Institute, 1990). The means, where applicable, were separated by Duncan's multiple range test (Duncan, 1955).
2.2. Experiment 1
This experiment was conducted to determine the effect of sources of maize on phytate P utilization in broiler chicks. Whole maize from four consignments (Maize A, B, C, and D) delivered to the University of Georgia Poultry Research Farm's feedmill over a period of two months was ground separately. The grindings were done using the same hammermill without changing the screen size to ensure uniformity among the maize consignments. The nutrient contents of the maize were analyzed and presented in Table 2. Of the six dietary treatments, four consisted of practical maize±soyabean meal diets, each containing maize from one of the four consignments; these diets were formulated to provide similar ME and CP levels, and contained 5.0 g/kg total P and 2.4 g/kg nonphytate P. Of two control diets, the ®rst was prepared using soyabean meal and maize starch as the main ingredients and will be referred to as the maize starch control diet. It was calculated to contain 4.3 g/kg total P and 2.4 g/kg nonphytate P, and was designed to determine the phytate P utilization in soyabean meal since maize starch did not contain phytate. The second control diet was prepared by substituting part of the maize and soyabean meal with casein and contained 4.4 g/kg total P and 2.4 g/kg nonphytate P, and
Table 2
Gross energy, moisture, crude protein, ether extract, ash, total P, and phytate P analyzed values from four maize sources (g/kg)
Nutrients analyzeda Maize A Maize B Maize C Maize D
Moisture 110 122 123 112
Crude protein 76 79 72 80
Ether extract 42 38 37 40
Ash 12.9 12.8 13.1 14.0
Total P 2.5 2.4 2.3 2.8
Phytate P 2.1 1.9 1.8 2.5
MJ ME per kilogram diet
Gross energy (MJ/kg) 17.03 16.66 16.51 19.46
aAnalysis on a fresh weight basis (not corrected for moisture); calcium contents were negligible, thus not
reported.
will be referred to as the casein control diet. It was designed to determine phytate P utilization at a nonphytate P level of 2.4 g/kg equal to the nonphytate P content of the diets containing the four maize sources. The analyzed levels of Ca, total P, and phytate P in all the diets are presented as footnotes in Table 1. For Experiment 1, 240 chicks were divided among the six treatment groups, each containing four replicates with 10 chicks per pen.
2.3. Experiment 2
This experiment was conducted to determine the effect of maize particle size on phytate P utilization in broiler chicks. Whole maize from a consignment received by the University of Georgia Poultry Research Farm's feedmill was divided into three portions. One portion was ground at the Research Farm using a hammermill with 6.7 mm openings in the screen. Two other portions were ground separately with a comminuting machine (The W.J. Fitzpatrick, Elmhurst, IL) but using two screens of different opening sizes (3 and 7 mm). This machine grinds corn in a way similar to a hammermill except the projections on the rapidly turning rotor are ®xed, therefore, they do not swing and hammer. Thus, three ground products with particle sizes classi®ed as coarse, medium, and ®ne were produced. Ground samples representative of their particle size group were analyzed for their geometric mean diameter (GMD) and geometric standard deviation (GSD) using the American Society of Agricultural Engineers Standard S319 (ASAE, 1983). The calculated GMD values for the coarse, medium, and ®ne ground maize were 894, 573 and 484mm, respectively. The calculated GSD values were 2.41, 2.44, and 2.42mm for the coarse, medium, and ®ne ground maize, respectively. The three maize batches provided the maize needed to make three maize±soyabean meal diets.
The design was a 32 factorial arrangement, containing three maize±soyabean meal diets made from three different maize particle sizes, two dietary levels of phytase, at 0 and 600 FTU/kg. The microbial phytase product (Natuphos1
, obtained from BASF, Parsippany, NJ) contained 5000 U of phytase activity per gram with 1 FTU of phytase activity de®ned as the quantity of enzyme required to produce 1mmol of inorganic P per minute from 5.1 mmol/l of sodium phytate at a pH of 5.5 and a water bath temperature of 378C. For Experiment 2, 240 chicks were divided among the six treatment groups, with four replicates, containing 10 chicks per replicated pen.
3. Results
3.1. Experiment 1
The responses of broiler chicks fed on maize±soyabean meal diets representing the four sources of maize, and two control diets are presented in Table 3. The 16-day BW responses among chicks fed on the diets containing the four maize sources were not signi®cantly different (p<0.05). The 16-day BW responses by the chicks fed on maize A, B, C, and D diets were not signi®cantly different from those of chicks fed on the casein control diet, but were signi®cantly different from chicks fed on the maize starch control
Table 3
Effects of sources of maize fed to chicks on body weight (BW), gain:feed, plasma Ca, plasma dialyzable P (dP), bone ash, rickets incidence and severe, retention of Ca,
P, phytate P (PP), and metabolizable energy (ME) derived from dieta
Diets description and analyzed
10.5 5.0 2.7 322a 0.713ab 14.0a 1.8a 27.0bc 65bc 37b 0.302a 0.489ab 0.283a 13.94
Maize B soyabean meal
10.1 5.0 2.8 325a 0.729a 13.1a 1.4a 27.4b 67bc 48ab 0.285a 0.476ab 0.292a 14.91
Maize C soyabean meal
10.4 4.9 2.7 314a 0.735a 15.3a 1.3a 26.9bc 85ab 70a 0.210ab 0.444bc 0.211abc 14.55
Maize D soyabean meal
10.0 5.1 3.0 314a 0.718ab 14.1a 1.3a 26.1c 75abc 48ab 0.144ab 0.397c 0.138bc 14.32
Maize starch control
9.3 3.7 3.0 256b 0.667c 13.2a 1.3a 24.9d 88a 68a 0.261a 0.531a 0.248ab 14.26
Casein control
9.9 4.9 2.2 308a 0.690bc 14.3a 1.6a 29.9a 65bc 32b 0.102b 0.488ab 0.104c 14.06
SEM 10.4 0.0116 0.71 0.21 0.35 6.7 9.7 0.0484 0.0174 0.0373 0.196
aValues are means of 4 pens per treatment. Each pen initially contained 10 chicks that were used to obtain all of the above means except plasma Ca and dP, for which
one chick per pen was used.
bMeans in a column with no common letters (a, b, c) are signi®cantly different (p<0.05).
cME values are expressed on a dry matter basis.
diet. Maize A, B, C, and D diets each produced a gain-to-feed ratio which does not differ signi®cantly between the diets. The maize starch control diet, but not the casein control, signi®cantly lowered the gain-to-feed ratio when compared to chicks fed on the maize A, B, C, and D diets. The responses in plasma Ca and dP by the chicks were not signi®cantly different. However, all the plasma Ca levels recorded were slightly elevated, and the plasma dP across the treatments was very low when compared to values of chickens from previous studies conducted in this laboratory that were fed on maize±soyabean meal diets adequate in dietary phosphorus (Edwards et al., 1994; Kasim, 1998).
The bone ash content of chicks fed on the maize D diet was signi®cantly lower than that of chicks fed on maize B diet. Chicks fed on the maize starch control diet had the lowest bone ash content, and chicks fed on the casein control diet had the highest. The incidence of P rickets in chicks fed on maize A, B, C, and D was not signi®cantly different, but maize A gave a signi®cantly lower severe rickets score than maize C. The maize starch control diet caused a higher incidence of P type rickets than the casein control diet.
The only signi®cant difference in Ca retention was between chicks eating maize A or B and those eating the casein control diet. Chicks consuming maize A and B retained over twice as much Ca as chicks consuming maize D, but this difference was not signi®cant. The total P retention for chicks receiving maize D diet was signi®cantly lower than that for chicks receiving maize A, maize B, the maize starch control, and the casein control diets. The chicks eating maize D had signi®cantly lower phytate P retention than chicks receiving maize A and maize B, but was not signi®cantly different from that of chicks eating the maize starch or casein control diets.
The ME derived from maize B diet was signi®cantly more than the ME derived from maize A diet, but was not different from maize C and D diets. The ME values derived from all diets were greater than the formulated ME.
3.2. Experiment 2
The responses of chicks fed on maize±soyabean meal diets containing maize of three different particle sizes, and the effect of adding phytase to these diets are presented in Table 4. Maize particle sizes did not in¯uence 16-day BW. When phytase was added to the diets, the 16-day BW of chicks was signi®cantly (p<0.05) increased across all maize particle sizes. Gain-to-feed ratio was not affected by maize particle size. However, gain-to-feed ratio was increased by the addition of phytase to the diets, and an interaction between maize particle size and phytase was signi®cant. Chicks fed on the diet containing maize of medium particle size had the greatest increase in gain-to-feed ratio when the phytase was added to the diet.
Plasma Ca was not affected by any of the treatments. Plasma dP was signi®cantly increased by adding phytase to the chicks diets, but not by particle size. Regression analysis indicates that bone ash signi®cantly increased as maize particle size increased in both the absence and presence of phytase. Adding phytase to all diets signi®cantly increased bone ash at all particle sizes. Neither the incidence or severity of phosphorus type rickets was affected by maize particle sizes of the diets, but was signi®cantly reduced by adding phytase to diets. Without phytase supplementation, chicks fed on the
Table 4
Effects of maize particle sizes and phytase fed to chicks on BW, gain:feed, plasma Ca, plasma dialyzable P, bone ash, rickets incidence and severe, retention of Ca, P, and
phytate P (PP), and metabolizable energy (ME) derived from dieta
Category/mm Phytase levels
P rickets (%) Mineral coefficient of
retention
ME derived from
diet (MJ/kg)b
Ca P Incidence Severe Ca P PP
Coarse/894 0 354 0.705 14.3 2.0 30.1 50 5 0.355 0.506 0.475 15.02
Coarse/894 600 411 0.725 13.9 3.0 35.7 3 3 0.420 0.609 0.628 14.99
Medium/573 0 344 0.682 13.0 1.7 27.9 63 12 0.263 0.476 0.426 14.89
Medium/573 600 427 0.750 13.3 2.3 34.1 15 0 0.312 0.587 0.585 15.02
Fine/484 0 370 0.723 13.3 1.2 28.4 70 19 0.203 0.444 0.389 14.91
Fine/484 600 414 0.730 13.3 3.1 34.0 10 3 0.277 0.567 0.558 15.12
SEM 10.3 0.0090 0.77 0.26 0.42 7.75 4.8 0.0295 0.0120 0.0285 0.134
Regression on particle sizes
Probabilities
0 0.47 0.76 0.28 0.08 0.01 0.17 0.16 <0.01 <0.01 0.02 0.46
600 0.67 0.35 0.48 0.65 <0.01 0.14 0.76 <0.01 0.04 0.16 0.67
ANOVA
Source of variation d.f.
Particle size 2 0.66 0.37 0.42 0.15 <0.01 0.18 0.36 <0.01 <0.01 <0.05 0.62
Phytase 1 <0.01 <0.01 0.96 <0.01 <0.01 <0.01 0.02 <0.01 <0.01 <0.01 0.31
Particlesizephytase 2 0.21 <0.01 0.89 0.06 0.77 0.69 0.36 0.91 0.72 0.96 0.87
aValues are means of 4 pens per treatment. Each pen initially contained 10 chicks that were used to obtain all of the above means except plasma Ca and dialyzable P,
where one chick per pen was used.
bME values are expressed on a dry matter basis.
diets containing maize of coarse particle size had the lowest incidence and severity of P type rickets.
The retentions of Ca, P, and phytate P by chicks were signi®cantly in¯uenced by maize particle sizes of the diet. The retention of these minerals was the greatest when chicks were fed on diets containing maize of coarse particle size. Retention of these minerals decreased with medium particle size maize in the diets and was lowest with ®ne particle size maize. Adding phytase to the diets signi®cantly increased retention of these minerals. With phytase in the diet, chicks given maize of coarse particle size gave the greatest mineral retention, and chicks fed on diets containing maize of ®ne particle sizes gave the lowest mineral retention. The ME derived from the diet by chicks was not affected by either maize particle size or the addition of phytase to the diet.
Regression of maize particle sizes on phytase levels indicate positive linear relationship of particle size on bone ash and mineral retention when diets did not contain phytase (Table 4). When phytase was present in the diet similar signi®cant linear responses for bone ash and retentions of Ca and total P, but not on phytate P retention resulted from decreasing particle size.
4. Discussion
The sources of maize causing the differences in bone ash content and the amounts of Ca, total P, and phytate P retained appears not to be related to the amount of phytate P provided by the diets. Responses of chicks given the two control diets provided additional explanations regarding phytate P utilization in these studies. Chicks consuming the maize starch control diet that contained a low level of total P had the highest proportional P retention (0.531). However, the chicks were only able to utilize 0.248 of the phytate phosphorus that was supplied entirely by soyabean meal in this diet even though they were very de®cient in phosphorus as indicated by the low bone ash and high incidence of phosphorus de®cient rickets. Even though chicks eating the casein control diet received similar amounts of total P as chicks eating maize A, B, C, and D diets, the greater amount of available P in the casein control diet caused by substituting part of the maize and soyabean meal with casein, did not favor the utilization of phytate P. Only 0.104 of phytate P was utilized. The phytate P that remained in the gut might have bound with Ca, making it unavailable to the chicks, as shown by a 0.102 Ca retention by chicks eating the casein control diet. On the other hand, with a similar level of total P, a larger amount of phytate P in the diet as in maize D diet will also reduce the retention of Ca, total P, and phytate P, which resulted in very low bone ash content. The signi®cant differences in P retention between chicks consuming maize D diet and chicks consuming maize A and B diets suggested that maize sources in¯uenced these parameters.
In addition to variation in ash, total P, and phytate P contents between maize sources (Table 2), there is a possibility that differences in phytase activity between maize sources may have contributed to the differences in mineral retention observed in these studies. The analysis for phytase activity performed by Eeckhout and Paepe (1994) on 11 samples of maize indicated that they contained from 0 to 46 phytase units/kg. However, the effect of maize phytase on mineral retention might be small since the average phytase activity
of maize (15 phytase units/kg) when compared to wheat and barley (1193 and 582 phytase units/kg, respectively) was low in their analysis.
The observation that maize particle size did not affect 16-day BW and gain-to-feed ratio was contrary to the works of Reece et al. (1985) that indicated maize of coarser particle size increased BW and improved feed ef®ciency. More Ca, total P, and phytate P were utilized by chicks fed on diets containing maize of higher GMD, possibly because of a longer retention time in the gut when the GMD of the maize was large. Heuser (1945) indicated that the retention time of cracked maize in the chicken crop is longer than that of maize meal. Nir et al. (1994) showed that increased particle size of grain resulted in larger gizzard with increased acidic contents. Exactly how increased time in the crop and gizzard along with reduced pH of the contents might affect phytate P utilization will require further work.
The effect of phytase in improving the responses in 16-day BW, gain-to-feed ratio, plasma dP, bone ash, retention of Ca, total P, phytate P, and the reduction in P type rickets incidence and severity was in agreement with several other works (Mitchell and Edwards, 1996a,b). With the exception of the variable effect on gain-to-feed ratio, the effect of dietary phytase giving consistently greater bone ash, greater retention of Ca, total P, and phytate P with maize of higher GMD indicated that longer gut retention time by maize of higher GMD might have improved the digestion and absorption of the minerals.
The results from Experiment 2 also showed that the growth performance of broiler chicks was not affected by maize with GMD between 484 and 894mm when given in mash form. Reece et al. (1985) reported that broiler chicks fed on a mash diet containing maize with GMD of 814mm had lower BW and higher feed conversion ratio than birds fed on a mash containing maize with GMD of 1343mm. They also reported that broiler chicks fed on crumble diets performed better than chicks fed on mash diets for each GMD category. Deaton et al. (1995) reported that broilers fed on either crumbled or pelleted feeds containing either maize of GMD 679, 987, or 1289mm were similar in growth performance. Therefore, if diets are given to chicks in crumble or pelleted forms, increasing the GMD of maize could possibly improve the retention of phytate P when compared to feeding chicks on a mash diet. Our studies indicated that growth performance is not a determinative criterion in assessing phytate P utilization since it was not affected despite the distinct differences in phytate P utilization.
The ME values of the diets in the second experiment do not indicate that particle size or phytase supplementation in¯uenced the ef®ciency of utilization of the diets in this experiment. A previous report from this laboratory also indicates no effect of phytase supplementation on energy utilization (Edwards, 1993). Biehl and Baker (1997) presents evidence that phytase supplementation of a soyabean meal diet increased true amino acid digestibility by only a non-signi®cant 2%. In the present studies the average ME value of the unsupplemented diet was 14.94 MJ/kg and the phytase supplemented 15.04 MJ/kg with a probability of signi®cant difference of 0.31. If amino acid digestibility had been increased by 2% in the present studies, it would have increased the ME by less than 0.08 MJ/kg. If phytase does affect amino acid digestibility or ME of a nutritionally adequate maize±soyabean meal diet, very careful and precise measurements would be needed to show this.
5. Conclusions
Small signi®cant differences in bone ash, plasma Ca and dP as well as P and phytate P retention were detected when birds were fed with different sources of maize. However, the differences were small and the error terms often fairly large, making the differences appear to have little practical importance. Maize particle size had a large signi®cant effect on bone ash and Ca, P and phytate P retention by broilers fed with corn±soyabean meal diets low in P. Increasing maize particle size increased bone ash and Ca, P and phytate P retention in the absence or presence of phytase.
Acknowledgements
This study was supported in part by state and Hatch funds allocated to the Georgia Agricultural Experiment Stations of The University of Georgia.
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