Abstract
ABSTRACT Two experiments were conducted to study the effect of corn extrusion and feed form on feed quality and growth performance of broilers. In experiment 1, whole corn was extruded at a water flow rate of 16, 19, and 22 L/h. The experimental design was a completely randomized design with 3 replications for each treatment. The extrusion process significantly decreased (P < 0.01) ether extract, crude fiber, and moisture content of corn. Increased water flow significantly decreased enzyme susceptibility from 56 to 47% (P < 0.01), but increased the bulk density of extrudate from 134 to 224 g/L (P < 0.01). However, there was a trend for lower total energy consumption (P < 0.01) and extruding production rate (P < 0.05) when corn was processed at the water flow of 22 L/h. In Exp. 2, 1,600 broilers (initially BW of 41 g) were
INTRODUCTION
The nutritional improvement of corn is of interest to animal producers because corn is one of the most important sources of dietary energy, in addition to other nutrients. The extrusion process may be a method to improve starch utilization of corn, such as enzyme susceptibility and digestion. Bjö rk et al. (1985) and Hancock and Behnke (2001) suggested that extrusion improved energy utilization and palatability. Hongtrakul et al. (1998) reported that nursery pigs fed extruded corn showed inconsistent results in growth performance and feed efficiency. Van der Poel et al. (1989 , 1990 determined starch availability in extruded corn compared with infrared corn and showed that in vitro starch availability, ileal digestibility coefficient of DM and nitrogenfree extract was higher for extruded corn compared with To whom correspondence should be addressed: brian48@example.net. 3 Model PHG 135, Wuhan Machinery, China. 1640 used to determine the effect of extrudate and feed form on the growth performance in poultry during a 42-d growth assay. There were 50 chicks per pen and 8 pens per treatment. Treatments were arranged in a 2 × 2 factorial arrangement with the main effect of feed form and the main effect of 25% processed corn inclusion. Treatments were 1) conventional corn mash, 2) conventional corn pelleted feed, 3) +25% extruded corn mash, and 4) +25% extruded corn pelleted feed. Chicks fed pelleted feed, regardless of processing of corn, had a greater BW and feed conversion than those fed mash feed diet (P < 0.001). Extruded corn mash decreased the growth performance. Results suggested that optimal growth rate and feed conversion of birds raised in a tropical environment given extruded corn might be achieved by steam pelleting. infrared corn. However, young pigs fed extruded corn had no improved performance 1 wk after weaning. Herkelman et al. (1990) suggested extruded corn improved energy utilization. Whereas several researchers (Van der Poel et al., 1989 , 1990 Herkelman et al., 1990; Hongtrakul et al., 1998) have studied the effect of feed extrusion in young pig, little research has been done in poultry, especially under tropical conditions, such as in Thailand. The purpose of this study was, therefore, to determine the effect of water flow in the extrusion process on nutritional properties of corn and the effect of feed form on the growth performance of broilers raised in a tropical environment. truder 3 with a barrel length of 0.81 m, 24 die openings, and a die diameter of 0.006 m. Three water flow rates were studied (16, 19, 22 L/h) . Extrusion throughput was adjusted for each water rate so that the process was constant to manufacture an acceptable quality of extrudate. Ampere current was measured on the main drive motor for the extruder with a recording power clamp meter, 4 and the production time for each water rate was measured with a digital clock to calculate the electrical energy consumption. Temperature of extrudate was immediately recorded with an insulated container equipped with a probe thermometer 5 after the extruder die. Barrel temperature was measured in the same manner at the second part of the barrel. Production rate was determined from the total weight of extrudate and the production time. Hot extrudate was taken after the extruder die to analyze for moisture content.
INTRODUCTION
Extrudate was air-dried overnight, and the bulk density was measured before grinding through a 15-horsepower locally built hammermill 6 equipped with a 1.7-mm screen. Samples were taken and analyzed for proximate analysis, enzyme susceptibility (Gibson et al., 1993) , degree of gelatinization (Sriroth et al., 1999) , total starch (McCleary et al., 1997) , geometric mean diameter of particles (d gw ), and geometric standard deviation of particles (s gw ), according to ASAE Standard S319 (ASAE, 1983) .
INTRODUCTION
Data were analyzed as a complete block design with 3 replications in each treatment, using the GLM procedure of SAS (2003) with the main effect of 3 water flow rates of 16, 19, and 22 L/h in the extrusion process.
Experiment 2
Broiler Experiment. A total of 1,600 commercial broilers (Ross × Ross) were used to determine the effects of extruded corn and feed form on growth performance and mortality under tropical environment. One-day-old broilers (average initial BW of 41 g) were allotted to dietary treatments based on sex. The method to identify sex was a feather sexing done at the hatchery. Chicks were housed in a 0.6 × 1.2 m pen in an open-air facility. There were 50 birds per pen and 8 pens per treatment. All diets were corn-soybean-based, with approximately 9% full-fat soybeans. Treatments were 1) conventional corn mash, 2) conventional corn pelleted feed, 3) +25% extruded corn mash, 4) +25% extruded corn pelleted feed. Chicks were fed a common starter corn-soybeanbased diet formulated to meet NRC (1994) requirement for all nutrients (3,100 kcal of ME/kg; 22.0% CP; 1.3% lysine) from 1 to 21d of age, a grower diet (3,150 kcal of ME/kg; 20.0% CP; 1. age (Table 1) . Chick weight and feed consumption were measured on d 0, 21, and 42.
Experiment 2
Feed Processing. The same extrusion machine from experiment 1 was used to produce the extruded corn, with a water flow rate of 19 L/h, considered to be optimal for nutritional, processing, and economic conditions. The extruded corn was reground through a 15-horsepower locally built hammermill 6 equipped with a 1.7-mm screen before being mixed into the diet.
Experiment 2
Balanced feed with or without 25% extruded corn was mixed and manufactured as a mash feed or steamconditioned to approximately 57 to 72°C and then pelleted with a 125-horsepower pellet mill 7 using 4-mm pellet die openings. Pellet mill throughput was adjusted for each diet so that the pellet mill motor was constant based on the motor load required to produce an acceptable quality of pellets. Pellets were cooled and crumbled. Hot pellet samples were collected immediately after the pellet die in an insulated container equipped with a probe thermometer to determine hot pellet temperature. Cooled pellets were collected after cooling and measured for pellet quality. Pellet durability index (PDI) was measured with a tumbling box according to ASAE Standard S269.3 (ASAE, 1987) for standard PDI and by modifying the procedure with the addition of 5 12.7-mm hexagonal nuts prior to tumbling for modified PDI. Ampere current was measured on the main drive with a recording power clamp meter to calculate electrical energy consumption. Production rate of pelleted feed was determined from the total weight of feed and the production time from each batch of the diet.
Experiment 2
Feed samples were taken and analyzed for proximate analysis, enzyme susceptibility (Gibson et al., 1993) ; degree of gelatinization (Sriroth et al., 1999) ; total starch (McCleary et al., 1997) , d gw , and s gw according to ASAE Standard S319 (ASAE, 1983) .
Experiment 2
All growth data were analyzed as a randomized complete block design using the GLM procedure of SAS (2003), with pen as the experimental unit. Treatments were randomly assigned. Sex was defined as the block factors. Treatment comparisons were made using orthogonal contrasts: 1) conventional corn vs. extruded corn, 2) mash vs. pelleted feed, and 3) the interaction of processed corn (conventional corn and extruded corn) vs. feed form (mash and pelleted feed).
RESULTS

Experiment 1
Extrusion Process. As depicted on Table 2 , extrusion processing significantly affected moisture, crude fiber, and ether extract content of corn (P < 0.01). Moisture content was significantly decreased in extruded corn compared to native corn (P < 0.01). As the water flow increased from 16 to 22 L/h, moisture content in extruded corn also increased from 6.01 to 7.14%. Rungnapa and Boonyasirikool (1997) also have reported that This premix provided the following microelements (g/kg): vitamin A, 4,500,000 IU; vitamin D, 750,000 IU; vitamin E, 10,000, vitamin K3, 750; vitamin B1, 1,100; vitamin B2, 3,000; vitamin B3, 10,000; vitamin B6, 2,000; vitamin B12, 12.5; pantotenic acid, 7,000; folic acid, 425; biotin, 100; Cu, 5,000; Fe, 4,800; I, 500; Mn, 30,000; Se, 100; Zn, 50,000. increased water flow in the extrusion process induced the increased moisture content in the extruded corn. Additionally, there was a significant decrease of ether extract in extruded corn compared with native corn (P < 0.05). Water flow rate had no effect in ether extract content of extruded corn. Crude fiber of extruded corn decreased (P < 0.01) from 1.84% in native corn to 0.91% in extruded corn with the water flow of 16 L/h. The most pronounced decrease in crude fiber content was observed in the lowest water flow rate (Table 2) .
Experiment 1
Water flow in the process significantly affected temperature and moisture in the extruded corn (P < 0.01) ( Table 3 ). Barrel temperature of corn was not significantly different regardless of water flow. In contrast to the exit temperature, there was an increase in extruded corn from 64.8°C to 87.9°C as the water flow increased Mean within the same row with different superscripts differ highly significantly (P < 0.01).
Experiment 1
1 NFE = nitrogen-free extract and was calculated based on the proximate analysis. from 16 to 22 L/h. The same trend was observed in both exit moisture and final moisture and cooling for 12 h, with the value ranging from 8.2 to 10.7%, and 6.01 to 7.14%, respectively (P < 0.01). This suggested that increased water flow affected the moisture content and the temperature of the final product. The extrusion process induced a 3 to 5% loss of initial moisture content in the extruded material due to a sudden change of pressure and temperature at the exit head. This induced water to evaporate out of the cell structure (Tables 2 and 3) . Total electrical energy consumption consisting of extruding and fine grinding energy significantly depended on water flow (P < 0.01), as shown on Table 3 . As the water flow increased from 16 to 22 L/h, there was a decrease of 16.4% in the extruding energy consumption, whereas fine grinding energy consumption had a 69% increase. The increase in fine grinding energy consumption resulted from a hard and rigid texture of extruded corn being ground. Total electrical energy consumption in extrusion of whole corn, however, significantly decreased by 14.5% and had a 13.4% decrease in price as the water flow increased from 16 to 22 L/h. Production rate, on the other hand, decreased by 6%, as the water flow rate increased (Table 3) .
Experiment 1
Bulk density of extrudate before grinding increased by 40.2% from 134 to 224 g/L as the water flow rate increased (Table 4) . Enzyme susceptibility and the degree of starch gelatinization in extruded corn were analyzed to determine the starch utilization. Enzyme susceptibility was decreased from 56.0% to 46.7% as the water flow increased from 16 to 22 L/h. There was a similar trend observed in the degree of starch gelatiniza- Mean within the same row with different superscripts differ highly significantly (P < 0.01).
Experiment 1
a,b
Experiment 1
Mean within the same row with different superscripts differ significantly (P < 0.05).
1
Temperature was recorded immediately after extrusion.
1
2 kWh/t = kilowatt-hour/t.
3
Electrical energy charge rate and energy adjustment charge based on the price of 1.7 and 0.2 baht/unit, respectively. tion, with the value ranging from 87.6 to 95.0% (P < 0.01). The differences in enzyme susceptibility and the degree of starch gelatinization were qualitatively showed in the microscopic structure of the extruded cornstarch granule (Figure 1 ). With the different water flow, granules started to distort in shape and eventually, birefringence mostly disappeared. After fine grinding, the d gw was between 342 to 440 m and the uniformity of particles was 1.7 to 1.8. K. Behnke (Kansas State University, Manhattan, KS, personal communication) suggested that a s gw value of less than 2.0 enhances the mixing efficiency of mash feed diet.
Experiment 2
Broiler Experiment. Chemical composition of the experimental diets was analyzed as shown in Table 5 . The results were close to the nutrient calculations in the formulated diets shown in Table 1 . Processing variables Mean within the same row with different superscripts differ significantly (P < 0.05).
Experiment 2
in manufacturing of the experimental diets in each phase are shown in Table 6 , but they were not statistically analyzed because they were different in composition and processed in at a different time period. The mash feed diets required electrical energy consumption only in the mixing process within the range of 0.11 to 0.22 kilowatt-hour/t and had numerical less enzyme susceptibility compared with the pelleted feed. Energy consumption required to pellet a ton of feed largely increased with the pellet mill capacity of 3 to 4 t/h. Pellet durability index was in the range of 50 to 80% for standard PDI but was extremely small in modified PDI from 21 to 55% depending on each diet. The low quality of pellets is likely due to the use of full-fat soybean in the experimental diets.
Experiment 2
Under the tropical environment, with an average temperature of 31°C and 70% relative humidity, the performance of chicks from d 1 to 21 indicated that pelleted feed significantly improved BW gain (P < 0.001) compared with mash feed (Table 7) . There was an improvement of 11.2% in BW gain in the extruded pelleted corn diet and only 5.7% in the pelleted normal corn diet (P < 0.05). Chicks fed pelleted feed consumed more feed than those fed mash feed (P < 0.001). Feed conversion in pelleted diets improved by 4.9% (P < 0.001). Pelleting the normal corn diets showed an increase of 3.6% for feed conversion and further improved 6.0% in the extruded corn diets (P < 0.05). Regardless of feed form, there was an improvement of 2.0% for feed conversion in chicks fed extruded corn diets compared with those fed normal corn diets (P < 0.01).
Experiment 2
Growth performance of chicks from d 22 to 42 showed a pronounced effect in pelleted feed. Chicks fed pelleted feed regardless of normal or extruded corn had a greater BW gain (P < 0.001) and feed conversion (P < 0.05) compared with those fed mash feed. Extruded pelleted diet greatly improved 10.7% BW gain and 6.6% feed conversion. On the other hand, normal pelleted diet improved 7.3% BW gain but there was a slightly improvement in feed conversion (P < 0.05). In addition, pelleted feed significantly increased bird mortality (P < 0.01).
Experiment 2
The overall performance from d 1 to 42 showed that pelleting improved the BW of chicks by 8.8% (P < 0.001). Chicks fed pelleted feed consumed more feed than those fed the mash diet (P < 0.001). Overall, feed conversion in chicks fed pelleted feed was better (P < 0.001). There was an interaction between feed form and corn type (P < 0.01) in feed conversion with a 6.2% improvement in chicks fed extruded pelleted diet compared with those fed the normal pelleted diet, that saw a smaller improvement of 1.5%.
DISCUSSION
Low-moisture extrusion provided the high temperature and mechanical shear rate. This beneficial effect likely improves starch utilization. The limited access of enzyme in native starch may reduce starch utilization (Hibberd et al., 1982) . Extruded corn, therefore, might enhance the digestibility of starch due to the greater enzyme susceptibility, resulting in greater chick growth performance.
DISCUSSION
Nutritional changes during the extrusion processing likely occurred and may be critical for extrudate quality. In this experiment, ether extract, crude fiber, and moisture content of extruded corn were lower than in native corn. This was likely due to friction force in the process. Less water flow applied more fiction force into the corn. Gomez and Aguilera (1983) reported extrusion of corn changes starch solubility. Martin-Cabrejas et al (1999) extruded common bean (Phaseolus vulgaris L.) with 2 different feed moisture contents of 25 and 30%. Results showed that extruded common bean at 25% feed moisture content had a greater percentage of soluble fiber than at 30% moisture. In our experiment, feed moisture was extremely low, thus enhancing more soluble fiber and less insoluble fiber in the corn extrudate. Additionally, adding water into the process did not result in increased water content in the extrudate. On the other hand, extruded corn with added water during the process had lower water content compared with native corn. Added water likely did not penetrate into the granule. This external water evaporates quickly when the temperature increases (Van der Poel et al., 1989) and may have induced internal water inside the granule to evaporate as well.
DISCUSSION
From the processing standpoint, the lower water flow increased electrical energy consumption and production capacity. Skoch et al. (1983) explained that water changed into steam, which had a lubricant effect, thus less mechanical friction resulted in decreasing motor loaded in the process. D. Forte (Dennis Forte and Assoc. Pty. Ltd., Birallee Park, Victoria, Australia, personal communication) also suggested that uncooked starch inside the extrusion barrel had high viscosity, as measured by the rapid viscosity analyzer, to obstruct the material flow out of the barrel. This retardation in prod- uct output, consequently, increased energy consumption.
DISCUSSION
Bulk density was one of the critical indexes to monitor the product quality and was affected by water flow into the process. In this experiment, we observed that as the water flow increased, the bulk density of extrudate also increased. There also was an inverse relationship between bulk density and enzyme susceptibility. Once bulk density increased, enzyme susceptibility decreased. These results were in agreement with Gomez and Aguilera (1983) and Hongtrakul et al. (1998) , which suggested that extrudate processed at a lower moisture content resulted in higher enzyme susceptibility and degree of gelatinization. Lower moisture content induced more heat mechanical shear and friction being applied into corn particles; distorting the starch granules made them more susceptible to enzyme hydrolysis and also destroyed the crystalline structure of corn granule as measured with a differential scanning calorimeter.
DISCUSSION
The optimal water flow was decided to manufacture extruded corn, based on the economic advantage and nutritional content. The completed feed with extruded corn in the diet was manufactured into 2 different forms either mash or pellet. The results in the improvement of BW and feed conversion of pelleted feed were generally in agreement with earlier reports (Hussar and Robblee, 1962; Runnels, 1976; Proudfoot and Hulan, 1982; Scheider, 1991) . The beneficial effects of pelleting attributed to decrease feed waste, may reduce selective feeding and decrease ingredient selection (Behnke, 1994) . However, once extruded corn was in the diet, mash extrude corn feed decreased growth rate, decreased feed consumption, and eventually was detrimental to feed conversion. Generally, feed consumption is a critical factor for optimal chick performance in tropical environments. Feed consumption may have been most critical in chicks fed extruded corn in the mash form. The possible explanation to the negative response was less bulk density in extruded corn, which was approximately 160 g/L (experiment 1). The bulk density of normal corn was approximately 750 g/L (unpublished data). Bulk density changes may have forced the chick to use more energy for prehension. The steam-conditioned pelleting, therefore, may be necessary to overcome less Contrasts were 1) mash vs pellet; 2) conventional corn vs. extruded corn and 3) feed form extruded corn interaction. ***P < 0.001; **P < 0.01; *P < 0.05; NS = not significant (P > 0.15).
DISCUSSION
bulk density of extruded corn, resulting in optimization of growth in chicks fed extruded corn.
DISCUSSION
In conclusion, water level in extrusion affected the physical and nutrional quality of extrudate. Inclusion of 25% extruded corn in pelleted feed was beneficial to growth performance of chicks raised under tropical conditions.

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