Processed cereals in diets for early-weaned piglets

Processed cereals in diets for early-weaned piglets

Animal Feed Science and Technology 82 (1999) 145±156 Processed cereals in diets for early-weaned piglets P. Medel, S. Salado, J.C. de Blas, G.G. Mate...

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Animal Feed Science and Technology 82 (1999) 145±156

Processed cereals in diets for early-weaned piglets P. Medel, S. Salado, J.C. de Blas, G.G. Mateos* Departamento de ProduccioÂn Animal, ETSI AgroÂnomos, Universidad PoliteÂcnica de Madrid, 28040 Madrid, Spain Received 5 May 1999; received in revised form 16 August 1999; accepted 23 September 1999

Abstract Two trials were carried out to study the influence of cereal processing in diets for early-weaned piglets. Six diets were formulated on an alternate base of two cereals: barley or maize, and three types of processing: control (raw), extrusion and micronization according to a factorial arrangement. In Trial 1, 120 male piglets weaned at 23 days and weighing an average of 6.4 kg, were randomly affected by litter in successive blocks and used in a 25-days performance trial. In Trial 2, 18 piglets, three animals per treatment, were used to measure apparent faecal digestibility of dry matter (DMD), organic matter (OMD), energy (ED), crude protein (CPD) and neutral detergent fibre (NDFD). Ileal viscosity, and pH of stomach, caecum and colon were also performed. In Trial 1, piglets fed barley-based diets grew faster than piglets fed diets based on maize (423 versus 404 g/ d; P ˆ 0.04). Heat processing of the cereal improved daily gain (423 versus 396 g/d, P ˆ 0.01) and feed conversion ratio (1.18 versus 1.25 g/g, P ˆ 0.02) with respect to diets based on raw cereal. An interaction between type of cereal and heat processing was observed for the first 2 weeks of the experiment: the improvement in daily gain associated with heat treatment was higher for barley than for maize (13.7 versus ÿ4.3%; P ˆ 0.08). The same occurred for feed efficiency (ÿ14.5 versus 3%; P ˆ 0.06). No differences were detected between types of processing for growth or feed efficiency. In Trial 2, maize-based diets had higher DMD (0.908 versus 0.871; P < 0.001), OMD (0.921 versus 0.880; P < 0.001), ED (0.905 versus 0.863; P < 0.001), CPD (0.886 versus 0.860; P ˆ 0.04) and NDFD (0.795 versus 0.708; P ˆ 0.03), than barley-based diets. Processing of cereals increased digestibility of nutrients but only OMD was improved significantly (0.908 versus 0.886; P ˆ 0.05). Neither ileal viscosity nor pH of stomach and caecum contents were affected by the experimental treatment. Maize-based diets reduced colon pH compared to barley-based diets (6.18 versus 7.00; P < 0.01). It is concluded that processing of cereals improves performance and OMD of diets for piglets and that this improvement is higher for barley than for maize. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Barley; Maize; Extrusion; Micronization; Piglet nutrition *

Corresponding author. Tel.: ‡34-915-497-978; fax: ‡34-915-499-763 E-mail address: [email protected] (G.G. Mateos) 0377-8401/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 8 4 0 1 ( 9 9 ) 0 0 1 1 1 - X

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1. Introduction Weaning of piglets is associated with an extreme change in their physical and nutritional environment. In addition, there has been a trend to reduce weaning age to 21 days, to increase sow productivity and decrease the risk of piglet infection through the sow (Alexander et al., 1980). However, at these early ages the digestive system is immature (Lindemann et al., 1986; Aumaitre et al., 1995; Jensen et al., 1997). At birth the digestive tract of piglets is well adapted to digest sow milk, which is very rich in highly digestible fat and lactose (Partridge and Gill, 1993). At weaning, milk is replaced by diets containing starch as the main energy source but an insufficient secretion of a-amylase (Owsley et al., 1986) and the fact that starch is stored in plants in a crystalline complex structure, might impair its digestion in piglets (Cunningham, 1959). The structure of starch can be modified by heat or by physical treatment, inducing a change in the crystalline structure or degree of gelatinization (Atwell et al., 1988), thereby facilitating its enzymatic degradation (Holm and BjoÈrck, 1988; Osman et al., 1990). The modification of starch granules depends on the treatment (Holm et al., 1988) and the starch source (Farber and Gallant, 1976; Faulks and Bailey, 1990). Improvements of performance or digestibility due to cereal processing in piglets has been reported by some workers (Aumaitre, 1976; Skoch et al., 1983), whereas slight or no benefits at all were found by others (Van der Poel et al., 1989; Vestergaard et al., 1990). Heat treatment of cereals also partly solubilizes the non-starch polysaccharides fraction (Fadel et al., 1988), and might increase digesta viscosity. A high viscosity of feed reduced the digestibility of nutrients in broilers (Petterson et al., 1991), but the effects on pigs are not well understood. The aim of this study was to evaluate the effect of processing cereals (barley or maize) by extrusion or micronization on performance and apparent faecal digestibility of diets for early-weaned pigs. 2. Material and methods 2.1. Diets A maize batch was split into three fractions. The first (raw maize) was ground through a hammer mill (2.5 mm screen). The second fraction was wet extruded (WENGER X185, Sabetha, KA, USA) after being ground by a hammer mill (2.0 mm screen). Temperatures inside the extruder were 80, 145, 134, 140 and 1288C for the five consecutive sections. The highest pressure reached was 31 bars. Maize kernels of the third fraction were macerated for 24 h until reaching a moisture of 175 g/kg and then micronized, in a Microred 20 micronizer (Micronizing Company, Framlingham, UK) at a final temperature of 70.78C and a moisture content of 138 g/kg for kernels in the last section of the micronizer. The same procedure was used with a barley batch to obtain raw, extruded and micronized barley. The temperatures and pressure reached in the extruder during processing were 94, 147, 156, 149 and 1288C with a highest pressure of 29 bars. The micronized fraction reached 193 g/kg moisture before, and 112 g/kg moisture after micronizing at 748C. Both barley and maize micronized fractions were flaked through

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Table 1 Composition and estimated nutrient value of diets (on an air dry basis) Type of cereal

Barley

Maize

Ingredient (g/kg) Barley Maize Lard Full fat soybean (extruded) Fish meal LT Spray dried skim milk Dried whey Wheat bran Calcium carbonate Dicalcium phosphate Sodium chloride DL-Methioninea L-Lysine hydrochloridea Citric acid Vitamin & mineral premixb

500 0 34 145 90 67 94 23 4.5 3.5 1.0 6.2 13.3 15.0 3.5

0 500 5 192 90 67 94 6 4.4 5.0 1.0 5.1 12.0 15.0 3.5

Estimated nutrient valuec Metabolizable energy (MJ/kg) Net energy (MJ/kg) Crude protein (g/kg) Lysine (g/kg) Crude fibre (g/kg) NDF (g/kg) Starch (g/kg) Calcium (g/kg) Available phosphorus (g/kg) Sodium (g/kg)

13.9 10.5 213 16.2 33.3 112 261 7.7 4.6 2.5

14.2 10.6 210 16.0 24.1 71.6 319 7.9 4.5 2.5

a

200 and 250 g/kg of active product for DL-Methionine and l-lysine, respectively. Mineral and vitamin composition for 1 kg of complete diet: Vitamin A: 15.000 IU; Vitamin D3: 1.900 IU; Vitamin E: 30 IU; Vitamin K: 1.6 mg; Thiamine: 1.1 mg; Riboflavin: 5 mg; Pantothenic acid: 14 mg; Niacin: 25 mg; Pyridoxine: 2.5 mg; Biotin: 150 mg; Folic acid: 200 mg; Cyanocobalamin: 25 mg; Choline: 250 mg; Fe: 75 mg; Cu: 160 mg; Zn: 110 mg; Mn: 50 mg; Co: 100 mg; Se: 300 mg; I: 1 mg. Carbadox: 50 ppm. c According to FEDNA (1999) tables of composition of raw materials. b

riffled rolls and ground through a hammer mill (2.5 mm) before being included in the feed. Two isonutritive diets based on 500 g/kg of raw barley or maize were least-cost formulated on net energy basis keeping constant the proportions of high quality protein concentrates (Table 1). Raw cereal was substituted (w/w) by extruded or micronized cereal to formulate the other four diets. After being mixed, all diets were steam-pelleted (60±658C) by using a Rosal press (Barcelona, Spain) with a 2.5 mm-diameter die. The same experimental diets were used in both trials. 2.2. Feeding trial One hundred and twenty crossbred (Landrace  Large White) male piglets from 30 litters (four males per litter) weaned at 23 days of age and weighing 6.4 kg on average,

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were used. Piglets were blocked by litter in a randomised incomplete block design with four animals per block and six treatments (Milliken and Johnson, 1984). Animals were alloted in groups of five per cage, involving four repetitions (cages) per treatment. The animals were housed in 24 flat-deck pens (1  1 m) provided with individual feeder (5 spaces) and nipple drinkers, and having ad libitum access to feed for 25 days. Final average body weight was 16.8 kg. Environmental temperature was maintained at 308C for the first week and decreased weekly until 258C for the last week. Average feed intake (ADFI), average daily gain (ADG) and feed conversion ratio (FCR) were recorded weekly and at the end of the trial (25 days post weaning). Incidence of diarrhoea, evaluated as number of animals that were injected with antibiotics, and mortality were recorded throughout the trial. Only one pig, fed the micronized barley-based diet, died during the trial. 2.3. Digestibility trial Eighteen male piglets of the same origin and age of those used in the feeding trial were used. Piglets were allocated randomly and individually in digestibility cages (0.3  50.75  0.4 m) with ad libitum access to feed and water for 9 days. After a 4day adaptation period, faeces were collected during 5 days, according to Den Hartog et al. (1987). Feed consumption was recorded daily. After the end of the collection period, the piglets were made to fast overnight, fed ad libitum for 3 h and then slaughtered. Afterwards, the portion of the digestive system between cardia and rectum was removed, and the contents of the stomach, caecum and colon were placed in clean plastic test tubes, and the pH was measured with a temperature-corrected pH-meter (CRISON Micro-pH 2001, Crison Instruments S.A., Barcelona, Spain). Ileal viscosity was measured in samples taken from the last 100 cm of the ileum. After homogenisation, two Eppendorf tubes with 1.6 g of sample were centrifuged (3 min, 12 000 rev/min) and the supernatant was used to determine viscosity with a digital viscometer (Brookfield digital rheometer, model DVIII, Brookfield Engineering Laboratories, Stoughton, MA, USA). Total tract apparent digestibility of nutrients was calculated from feed intake and total faeces excreted. 2.4. Analytical methods Feeds and faeces were heat dried (608C, 48 h), ground (1 mm screen) and analysed following the methods of Van Soest et al. (1991) for NDF, and of the Association of Official Analytical Chemist (1990) for DM, ash, and CP. Gross energy of feed and faeces was determined by using an adiabatic bomb calorimeter. Starch gelatinization, as a proportion of total starch, was determined by enzymatic hydrolysis. A sample of 10 g was split into two equal fractions. The first fraction (Sample A) was mixed with 35 ml of distillated water and maintained in a silicone bath at 1038C for 1 h, in order to gelatinize all the starch. The second fraction (Sample B), was mixed with 35 ml of distillated water. Afterwards, an enzymatic digestion was made in both samples: 5 ml of phosphate buffer 1 M (pH ˆ 4.8) plus 5 ml of amyloglucosidase

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solution were added and incubated 15 min at 378C. The digestion was stopped by adding 5 ml of Carrez solutions 1 and 2. The concentration of released glucose in the samples was obtained by an enzymatic-colorimetric kit (Sigma, Trinder cat no. 315±100 h) measuring absorbancy at 505 nm. Starch gelatinization rate was determined as the percentage of glucose concentration of Sample B in comparison with Sample A. 2.5. Statistical analysis Performance, digestibility, pH and viscosity data were analysed using the GLM procedures of Statistical Analysis Systems Institute (1990). Preplanned orthogonal comparisons were used to determine the effect of cereal (maize versus barley), processing of cereals (raw versus processed), and processing technique (extrusion versus micronization) by the contrast method. The interactions type of cereal by processing and type of cereal by processing technique were also tested statistically. Initial body weight was used as a linear covariate for performance traits. Data in tables are presented as least square means. Repeated measures procedure of SAS was used to analyse interactions between type of diet and age on growth of animals. Diarrhoea incidence was analysed using the CATMOD procedure of SAS. 3. Results 3.1. Composition of diets and starch gelatinization Chemical composition and degree of starch gelatinization of the cereals used are shown in Table 2. Analysed chemical composition of the six experimental diets is shown in Table 3. Degree of starch gelatinization was higher for maize than for barley (0.36 versus 0.30, P ˆ 0.001). Both extrusion and micronization improved the degree of starch gelatinization compared to raw cereals (0.47 versus 0.055, P ˆ 0.001, respectively), but this improvement was higher for extrusion than for micronization (0.71 versus 0.23, P ˆ 0.001, respectively). Table 2 Determined chemical composition of raw and processed cereals (g/kg, on an air dry basis) Type of cereal Treatment

Barley Raw

Extruded

Micronized

Raw

Extruded

Micronized

Dry matter Crude protein Crude fibre Ash Starch Starch gelatinizationa

877 100 56 22 528 0.05

914 105 49 23 507 0.66

891 104 47 21 507 0.19

883 82 21 13 650 0.06

915 83 24 12 650 0.76

877 81 21 9 650 0.27

a

As a proportion of total starch.

Maize

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Table 3 Analyzed chemical composition of experimental diets (g/kg, on air dry basis) Type of cereal Treatment

Barley

Maize

Raw

Extruded

Micronized

Raw

Extruded

Micronized

Dry matter Crude protein NDF Ash Starch

910 211 171 66 326

901 208 176 61 304

910 211 164 64 304

905 207 122 56 362

909 213 122 57 370

904 207 123 56 380

Table 4 Effect of dietary treatment on performance of piglets according to the period Period (d) 0±14

0±25

ADGa ADFIb

FCRc

ADG AFI

FCR

ADG AFI

FCR

272 314 305

1.30 1.10 1.17

564 592 596

1.26 1.23 1.13

401 436 433

1.27 1.17 1.14

5 10 5

1.16 1.20 1.19 0.05 0.96 0.21 0.64 0.06 0.50

526 672 584 675 588 721 14 27 0.17 0.56 0.004 0.71 0.82 0.80 0.28 0.50 0.97 0.08

1.28 1.14 1.24 0.05 0.78 0.10 0.95 0.91 0.12

391 481 419 493 402 488 10 11 0.04 0.13 0.01 0.85 0.36 0.39 0.44 0.54 0.49 0.59

1.23 1.17 1.22 0.03 0.61 0.02 0.72 0.20 0.26

10 15 15 0.22 0.49 0.72 0.82 0.72

Barley diets Raw Extruded Micronized Maize diets Raw Extruded Micronized SEM (n ˆ 4) Significance of contrastd

14±25

Diarrhoea, %

1 2 3 4 5

348 338 354

285 332 289 349 256 304 14 9 0.11 0.04 0.33 0.67 0.20 0.18 0.08 0.85 0.40 0.01

709 730 670

507 511 493

a

Average daily gain (ADG, g). Average daily feed intake (ADFI, g). c Feed conversion ratio (FCR, g feed intake /g weight gain). d 1 ˆ Barley versus maize; 2 ˆ raw versus (extruded ‡ micronized): 3 ˆ extruded versus micronized; 4 ˆ (barley versus maize)  (raw versus (extruded ‡ micronized)); 5 ˆ (barley versus maize)  (extruded versus micronized). b

3.2. Feeding trial Data on the influence of diet on performance of piglets are shown in Table 4. Feeding barley-based diets resulted in a higher average daily gain (ADG) than feeding maizebased diets throughout the trial (423 versus 404 g/d; P ˆ 0.04). Average daily feed intake (ADFI) also tended to be higher for pigs fed barley (504 versus 487 g/d; P ˆ 0.13). These differences were due to a higher ADG and ADFI from 0±14 and 14±25 days of age, particularly in the first 2 weeks of the experiment. Neither feed efficiency nor diarrhoea incidence were affected by the type of cereal. The effect of diet on weight of animals is

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Table 5 Effect of type of diet on live weight (kg) at different periods. Treatment

Days after weaning 7

14

21

25

Barley diets Raw Extruded Micronized

8.32 8.75 8.83

10.2 10.8 10.7

13.8 14.5 14.6

16.4 17.3 17.3

Maize diets Raw Extruded Micronized SEM (n ˆ 4)

8.55 8.48 8.24 0.12

10.4 10.5 10.0 0.19

13.9 14.3 13.8 0.32

16.2 16.9 16.5 0.24

0.07 0.22 0.56 0.01 0.24

0.11 0.33 0.20 0.08 0.40

0.34 0.18 0.59 0.38 0.40

0.04 0.009 0.36 0.44 0.49

Significance of contrasta

1 2 3 4 5

a 1 ˆ Barley versus maize; 2 ˆ Raw versus (extruded ‡ micronized): 3 ˆ Extruded versus micronized; 4 ˆ (barley versus maize)  (raw versus (extruded ‡ micronized)); 5 ˆ (barley versus maize)  (extruded versus micronized).

shown in Table 5. Piglets fed barley-based diets were heavier than those fed maize-based diets. Processing of cereals did not affect feed intake, but improved both ADG and feed conversion rate (FCR) from 14 to 25 days, and for the whole experimental period. A significant interaction of cereal processing with time was observed for body weight, whereby processing only improved body weight of animals at 25 days (P ˆ 0.006). Diarrhoea incidence was not affected by processing of cereals. A significant interaction for ADG and FCR between type of cereal and processing was found during the first 2 weeks of the experimental period: heat treatment was more beneficial for barley than for maize-based diet. No differences were found between processing techniques for any of the traits studied. However, an interaction between type of cereal and processing (P ˆ 0.01) was found for feed intake during the first 2 weeks of experiment. Micronization increased feed intake of pigs fed barley-based diets but extrusion did not. The opposite occurred with maize-based diets. These effects were compensated from 14 to 25 days, and therefore, no interaction was detected at the end of the trial. 3.3. Digestibility trial The effects of treatment on apparent faecal digestibility of dry matter (DMD), organic matter (OMD), energy (ED), crude protein (CPD) and neutral detergent fibre (NDFD), pH of the stomach, caecum and colon contents, and ileal viscosity are shown in Table 6. Maize-based diets showed a higher total tract apparent digestibility (TTAD) for DM, OM,

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Table 6 Effect of diets on total tract apparent digestibility coefficients of dry matter, organic matter, energy and neutral detergent fiber (NDF), ileal viscosity (iVISC) and gut pH Digestibility coefficients

Barley diets Raw Extruded Micronized Maize diets Raw Extruded Micronized SEM (n ˆ 3) Significance of contrasta

1 2 3 4 5

Energy CP

pH Stomach

Caecum

Colon

DM

OM

0.862 0.880 0.870

0.855 0.853 0.895 0.875 0.889 0.861

0.846 0.684 0.877 0.709 0.858 0.730

2.2 3.3 4.7

4.19 3.88 2.44

6.05 6.47 5.87

7.05 6.72 7.23

0.904 0.909 0.911 0.007 0.0002 0.22 0.81 0.73 0.55

0.917 0.923 0.924 0.011 0.001 0.05 0.93 0.15 0.84

0.881 0.882 0.896 0.012 0.04 0.33 0.85 0.72 0.47

2.3 2.5 3.2 1.1 0.31 0.29 0.79 0.56 0.31

3.66 2.94 2.98 0.57 0.54 0.19 0.31 0.84 0.40

5.85 6.22 5.98 0.20 0.59 0.43 0.16 0.77 0.52

6.65 5.94 5.95 0.20 0.01 0.12 0.33 0.20 0.35

0.902 0.903 0.909 0.008 0.0002 0.26 0.81 0.57 0.41

NDF

iVISC (cp)

0.805 0.788 0.791 0.02 0.03 0.69 0.64 0.30 0.72

a 1 ˆ Barley versus maize; 2 ˆ Raw versus (extruded ‡ micronized): 3 ˆ extruded versus micronized; 4 ˆ (barley versus maize)  (raw versus (extruded ‡ micronized)); 5 ˆ (barley versus maize)  (extruded versus micronized).

energy, CP and NDF than barley-based diets (P < 0.04). Type of cereal did not affect ileal viscosity or pH of the digesta from stomach or caecum. However, pigs fed maize-based diets had lower colon pH (P ˆ 0.01) than pigs fed barley-based diets. Although only OMD was improved significantly (P ˆ 0.05) by cereal processing, most of the digestibility coefficients were higher for processed than for raw cereals-based diets. Furthermore, the improvement of OMD due to processing tended to be higher for barley than for maize-based diets (‡4.3 versus ‡0.7%; P ˆ 0.15). Processing of cereals (especially barley) increased viscosity values, but the low number of piglets used and the high variability of this parameter prevented the detection of any statistically significant difference between treatments. No effects of cereal processing were found for digesta pH in any of the segments studied. 4. Discussion Data indicate that both heat processing techniques improved in vitro starch availability, assessed as the percentage of gelatinization, and that extrusion caused greater rates of gelatinization than micronization. These results agree with those of Van der Poel et al. (1989), who compared extrusion and micronization of maize. Barley-based diets supported for the overall period a higher growth rate than maizebased diets, despite of a higher NDF content (112 versus 72 g/kg). Bardon and Fiaramonti (1983) reported that high levels of fibre decreased digesta transit time in pigs. This could allow a higher feed intake, and therefore, a higher growth rate, since efficiency of feed

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utilisation was not affected by type of cereal. Barley-based diets showed slightly but significant lower digestibilities than maize-based diets. This effect might be related to its higher NDF content (Just et al., 1983) and to a faster rate of feed passage through the gastrointestinal tract. The lower colon pH observed with maize-based diets could be related to a higher retention of the undigested feed in this fermentative area. From the results shown in Tables 4 and 5 it can be deduced that under our experimental conditions, processing of barley and maize improved the performance of 23-day old weaned piglets. Higher improvements were observed for barley than for maize, especially in the first 14 days of experiment. These results agree partially with those reported by Aumaitre (1976), who did not find any improvement in the performance of 21-day weaned piglets due to processing of maize (steam flaked or popped), but a significant improvement both in growth and feed efficiency when barley was processed. The higher improvement due to processing of the barley diets is not well understood. Delort-Laval and Mercier (1976) reported that processing (steaming, popping, steam-flaking or extrusion) of barley increased in vitro starch hydrolysis by a-amylase as compared with the raw cereal, and that the response of maize to the same processing techniques was lower and only significant for the extrusion process. Farber and Gallant (1976) also reported distinct histological modifications by popping or steam flaking barley and maize, finding more homogenous effects of treatments (starch granules swelled, protein matrice stretched) in barley than in maize. Aumaitre (1976) observed that processed maize caused more diarrhoea incidence than raw maize diets, and hypothesised that processing might cause higher rates of feed intake during the first days after weaning (21 days), a situation that could induce overloading of the digestive tract and consequently diarrhoea. This hypothesis is in agreement with our results, and might partially explain the lack of improvement in performance with processed maize-based diets in the first 2 weeks period of our experiment. We did not observe differences in diarrhoea incidence, but the number of animals treated against scours was higher for processed cereals, especially in maize-based diets. In addition, the digestibility trial showed that faeces of piglets fed maize-based diets (especially when processed) were wetter than faeces of pigs fed barley-based diets (data not shown), although the measurement was subjective and requires further evaluation. We also found that the favourable response of piglets to barley processing was obvious throughout the feeding trial, but for processed maize was only noticeable between 14 and 25 days. At this time, the digestive system is more developed, and the feed may be better digested, avoiding potential digesta overloads. Skoch et al. (1983) reported similar growth rate but improved feed efficiency in heavier pigs (15.5 kg) fed extruded maize and wheat middlings-based diets compared to those fed a control diet. Moreira et al. (1994, 1995) compared raw with extruded or precooked maize in different blends. They found no improvements in feed efficiency but a decrease in growth rate when precooked maize was used. On the contrary, Van der Poel et al. (1990) reported that processing of maize improved feed efficiency in 21-day old piglets between 14 and 35 days after weaning, but not during the first 14 days or during the whole trial. These data partially agree with ours, since improvements due to treatment of maize were obtained between 14 and 25 days of trial. Total tract apparent digestibility of OM was improved with processed cereal-based diets. Aumaitre (1976) also found significant improvements of 3±4% in TTAD of OM

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and CP due to processing of barley and maize. In addition Sauer et al. (1990) reported differences in TTAD of DM, energy and CP when a wheat and barley-based diet was extruded or extruded and pelleted. Den Hartog et al. (1988) reported that TTAD did not differ between diets containing either 60% extruded or 60% raw maize, whereas the ileal digestibility of dry matter and nitrogen free extract was significantly higher for the extruded maize-based diet. Carter and Leibholz (1991) found higher digestibility of DM for extruded than for raw wheat-based diets in the small intestine but not in the colon. Van der Poel et al. (1989) also found differences due to maize processing (extruded, micronized or pressure cooked) in nitrogen free extract TTAD. In a subsequent study, Van der Poel et al. (1990) only found differences in ileal digestibility of organic matter and nitrogen free extract. These results showed that improvements due to processing of cereals obtained in our experiment could be higher if the digestibility had been assessed at the ileum. In addition, the higher improvement in OM digestibility coefficients due to processing in barley-based diets than with maize-based diets, and the similar digestibility coefficients for both extrusion and micronization, were consistent with piglets' performance data. Unexpectedly, no correlation between starch gelatinization and digestibility traits was observed. Vestergaard et al. (1990) compared extrusion, steaming, steaming under pressure, roller heating, and micronization of barley in 21-day old weaned piglets. They used a wide range of starch gelatinization and concluded that this trait was not correlated with the performance of piglets. This observation is consistent with current data, in which we found larger rates of gelatinization in extruded than in micronized cereals, but no differences were detected in performance or digestibility values. Our data indicate that barley-based diets promoted a higher growth than maize-based diets without modifying feed efficiency. Heat processing of barley or maize improves daily gain and feed efficiency of early weaned pigs. Therefore, the use of processed cereals (especially barley) is recommended in these diets. Acknowledgements This research was supported by CICYT Project AGF96-1142. Thanks to A. Sanz and Y. Alegre for their collaboration and to Dr. J. Sell for his help in the preparation of the manuscript.

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