www.elsevier.nlrlocateraqua-online
Effects of grinding, steam conditioning and
extrusion of a practical diet on digestibility and
weight gain of silver perch, Bidyanus bidyanus
Mark A. Booth, Geoff L. Allan
), Rebecca Warner-Smith
NSW Fisheries, Port Stephens Research Centre, Taylors Beach, Salamander Bay, NSW 2316, AustraliaAccepted 7 July 1999
Abstract
As processing can affect the digestibility and utilisation of diets by fish, we examined the
Ž .
effects of grinding, steam conditioning and extrusion of a commercially available diet SP35 on
Ž .
weight gain and performance of silver perch. SP35 35% protein, 18 MJrkg gross energy with
Ž .
approximately 80% of particles between 710 and 1000mm was either left unground or finely
Ž .
ground to 500mm ground . Both unground and ground fractions were made into sinking pellets
Ž .
in a commercial steam pelleting mill with or without the addition of steam 908C A fifth diet was
Ž
processed by pelleting finely ground material in a single-screw extruder after the addition of
.
approximately 5% fish oil at a temperature of 1208C. The extruded diet floated or sank slowly.
Ž .
Each diet was fed to 50 juvenile silver perch mean initial weight 17.8 g in each of three replicate 10 000-l tanks for 113 days. Fish gained between 55 and 71 grfish during the experiment, and
Ž .
feed conversion ratio FCR ranged from 1.5:1 to 2.0:1. Steam conditioning significantly improved weight gain and FCRs while neither grinding nor the interaction between grinding and steam conditioning had any effect. Fish were reluctant to consume the extruded diet and grew less on this diet than on the steam-conditioned diets, although FCR was better than for all other diets. Ground diets, uncooked and steamed, and the extruded diet were subsequently reground and 1% chromic oxide was added as an inert indicator. Each of these three diets was fed to juvenile silver
Ž .
perch mean initial weight 2.5 g in 170-l cylindroconical tanks from which faeces were collected by settlement to determine digestibility coefficients for dry matter, energy and nitrogen. Digestibil-ity coefficients for dry matter and energy were higher for the extruded diet but similar for the unsteamed and steamed diets. Protein digestibility was unaffected by processing. These results indicate that for silver perch fed diets similar to SP35, diets should be steam-conditioned, but the additional expense associated with fine grinding is unwarranted with respect to gains in either fish
)
C o rre sp o n d in g au th o r. T e l.: q6 1 -2 4 9 -8 2 1 2 3 2 ; fa x : q6 1 -2 4 9 -8 2 1 1 0 7 ; E -m a il: allang@fisheries.nsw.gov.au
0044-8486r00r$ - see front matterq2000 Elsevier Science B.V. All rights reserved. Ž .
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M.A. Booth et al.rAquaculture 182 2000 287–299
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performance or improvements in pellet stability. Extrusion significantly improved digestibility and FCR but consumption of floating extruded pellets was reduced in our facility. Sinking, extruded diets deserve evaluation.q2000 Elsevier Science B.V. All rights reserved.
Keywords: Nutrition; Bidyanus bidyanus; Digestibility; Growth; Diet; Processing
1. Introduction
In recent years, the focus on processing technologies used to produce commercial diets for use in aquaculture has increased. This focus has been primarily driven by the rapid expansion in aquaculture globally, which has resulted in an increased demand by
Ž
producers for high quality diets designed to meet their specific requirements Lovell,
.
1992 . This demand for nutritionally adequate, cost effective diets has also driven the search for high quality ingredients to either wholly or partially replace the fish meal component in aquaculture diets. In many instances, these alternative ingredients and diets often require some form of processing to improve their nutritional or physical qualities.
Three processing techniques currently dominate the production of pelleted diets in aquaculture and their use often, but not always, facilitates improvement in the raw product; grinding, steam conditioning and extrusion. These techniques inevitably affect
Ž .
both the physical and chemical characteristics of a feed Hilton et al., 1981 . They
Ž .
include water stability and durability, pellet hardness, nutrient availability Tan, 1991
Ž .
and digestibility Hardy, 1989; Hui-Meng, 1989 . Other factors influenced by processing such as the palatability and organoleptic properties of a diet may affect the amount of
Ž .
feed consumed by a target species Mackie and Mitchell, 1985 .
While processing techniques such as those described can improve the nutritional and physical qualities of diet ingredients, heating processes can also have detrimental effects.
Ž
Heat labile vitamins and nutrients can be lost at elevated temperatures Slinger et al.,
.
1979; Kiang, 1989; Springate, 1991 . For example, ascorbic acid has been shown to be
Ž
unstable during heat treatments such as steam conditioning and extrusion Slinger et al.,
.
1979 , and overheating can reduce the available levels of essential amino acids such as
Ž
lysine and cystine Evans and Butts, 1951; Carpenter and Booth, 1973; Viola et al.,
.
1983 .
In Australia, aquaculture of silver perch, a native freshwater finfish, is expanding rapidly. This expansion has been aided by the development and production of a
Ž .
commercial diet SP35 based in part on published requirements for other omnivorous
Ž .
species such as the channel catfish NRC, 1983; Lovell, 1989; Robinson, 1989 , but
Ž .
formulated specifically for silver perch Allan and Rowland, 1992 . SP35 is commer-cially available and is capable of supporting rapid growth of silver perch in ponds with
Ž .
production outputs approaching 10 trharyr Rowland et al., 1995 .
2. Materials and methods
2.1. Experimental fish
Ž .
Silver perch Bidyanus bidyanus were bred and reared at NSW Fisheries, Grafton
Ž .
Research Centre following techniques described by Thurstan and Rowland 1994 . Afterwards, they were moved to NSW Fisheries, Port Stephens Research Centre where they were held in 10 000-l indoor tanks and fed SP35 until transferred to experimental facilities and diets.
Ž
During all stocking procedures, fish were anaesthetised 25 mgrl
ethyl-p-amino-.
benzoate and weighed individually, or in small groups, before being systematically dispersed to experimental tanks.
2.2. Diets
Ž .
SP35 Table 1 was subjected to three commercial processing techniques for the purpose of the growth trial; grinding, steam conditioning and extrusion. All processing of SP35 was carried out by Ridley Aquafeeds, Narangba, Queensland, Australia. The
Ž
complete diet was initially supplied as a cold-pressed, 3-mm pellet Janos Hoey, Forbes,
.
NSW, Australia and was subsequently broken down through a commercial hammermill
Ž .
fitted with a 3-mm screen. The bulk of this coarse or ‘unground’ diet f80% consisted
of particles ranging between 710 and 1000mm. Half of this material was then subjected
to fine grinding using a pulverizerrair classification system similar to that described in
Ž .
Tan 1991 , which ensured an accurate grind size of 500 mm with low variance. Both
the coarse and finely ground diets were then separately re-pelleted in a commercial
Ž .
steam pellet mill with or without the addition of steam 908C . This produced four diets for evaluation.
A fifth diet was produced to evaluate the effects of extrusion by pelleting the finely
ground material in a commercial, single-screw extruder at a temperature of 1208C.
Extrusion of this diet after grinding proved difficult and in order to produce a practical pellet the addition of fish oil was required which resulted in a 5% increase in fat content. All diets apart from the extruded diet were isonitrogenous and isoenergetic after
Ž . Ž .
processing Table 2 . Physical characteristics for each of the five diets Table 3 were
Ž .
determined following the methods described by Evans et al. 1998 .
Results obtained from the growth trial indicated that steam conditioning improved
Ž .
performance and extruding the diet improved feed conversion ratio FCR . To assess these improvements in terms of digestibility, diets were reground through a 1.5-mm
Ž .
hammermill Raymond Laboratory Mill, Transfield Technologies, Rydalmere, Australia
Ž .
and 1% dry basis chromic oxide was added. This reprocessing ensured pellets from the extruded diets sank at a similar rate to other diets and major physical differences were eliminated. After grinding, greater than 92% of the particles in each diet was less than
710mm. Diet and marker were then thoroughly dry mixed before the addition of f600
Ž .
ml distilled water Hobart Mixer: Troy, OH, USA . The mixture was then pelleted
Ž
through a meat mincer fitted with a 1.5-mm pellet die Barnco Australia, Leichhardt,
.
NSW . After pelleting, diets were dried in a convection drier at -358C for
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Table 1
Ž .
Composition of commercial silver perch diet SP35
U UU
Ž .
Ingredient Amount in SP35 Vitamin premix A IU mgrkg Mineral premix grkg
Ž% dry basis.
Ž .
Fish meal 26.20 Retinol A 8000 Calcium carbonate 7.5
Ž .
Soybean meal 20.19 Cholecalciferol D3 1000 Manganese sulphate monohydrate 0.3
Ž .
Blood meal 2.04 DL-a-Tocopherol acetate E 125 Zinc sulphate monohydrate 0.7
Ž .
Corn gluten meal 3.87 Menadione sodium bisulphite K3 16.5 Copper sulphate pentahydrate 0.06
Ž .
Wheat 27.47 Thiamine hydrochloride B1 10.0 Ferrous sulphate heptahydrate 0.5
Ž .
Sorghum 11.21 Riboflavin B2 25.2 Sodium chloride 7.5
Ž .
Millrun 2.01 Pyridoxine hydrochloride B6 15.0 Potassium iodate 0.002
Cod liver oil 0.90 Folic acid 4
U
Ž .
Vitamin premix 0.97 Ascorbic acid C 1000
UU
Mineral premix 2.81 CalciumD-pantothenate 55
Di-calcium phosphate 1.79 Myo-inositol 600
Ž . Ž .
DL-Methionine 0.13 D-Biotin H 2% 1
Choline chloride 1500
Nicotinamide 200
Ž .
Cyanocobalamin B12 0.02
Ž .
Ethoxyquin anti-oxidant 150
Ž .
Table 2
Ž .
Analysed composition dry basis of process modified SP35 diets
Ž . Ž . Ž .
Processing method Protein % Energy MJrkg Fat %
Unground, no steam conditioning 33.67 18.94 5.38
Unground, steam-conditioned 37.11 18.66 4.29
Ground, no steam conditioning 35.31 18.32 3.03
Ground, steam-conditioned 35.57 18.70 4.36
Ground, extruded 33.66 20.02 9.56
2.3. Growth trial
The growth experiment was undertaken in a hot-house facility which housed 15
Ž . Ž
circular 10 000-l tanks diameter 3.4 m; height 1.2 m . Fresh water -500 mgrl
.
salinity was circulated through each tank at approximately 17 lrmin then returned to a
Ž .
common sump 3000 l containing a submerged biofilter. Water was pumped from the sump via two rapid sand filters before returning to experimental tanks. Each tank was provided with two large air stone diffusers and covered with black shade cloth to reduce the growth of algae. Tanks were siphoned once a week to remove accumulated faeces.
Ž .
Each of the five diets was allocated to three randomly selected tanks ns3 . Tanks
Ž .
were stocked with 50 fish mean 17.8 g, range 17.2–18.3 g which were hand fed twice
Ž .
daily 0830 and 1500 h to apparent satiation for a period of 113 days. Observation of the feeding response in individual tanks was aided by the clarity of water in our experimental units, ensuring delivery of excess pellets was minimised. Feed intake was recorded daily. Biomass of each tank was weighed monthly over the course of the experiment with fish starved 1 day prior to weighing. At the end of the growth trial individual weight gain, FCRs and average daily feed consumption were determined for each tank to allow comparisons among different processing techniques.
2.4. Digestibility trial
The digestibility of ground diets was evaluated using similar methods, facilities and
Ž .
experimental units 170-l cylindroconical digestibility tanks to those described in the
Ž .
work of Allan et al. 1999 . Digestibility tanks consisted of an upper tank and lower settlement chamber separated by a mesh screen which prevented migration of fish to the settlement chamber. The settlement chamber terminated in a 250 mm length of silicone tubing which collected faecal pellets. Each digestibility tank was stocked with 12 silver
Ž .
perch mean 2.5 g, range 2.3–2.8 g and they were acclimated on experimental diets for 7 days prior to collection of faeces. Spare fish to replace any mortalities were stocked
Ž .
into separate holding tanks 100 l and fed the appropriate test diets. Fish were fed in
Ž .
excess of their daily requirements )10% of biomass for 3 h once daily between 0830
Ž
and 1130 h using clockwork feeders Fischtechnik Fredelsloh, Moringen, West
Ger-.
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Table 3
Ž .
Physical characteristics of commercial silver perch diet SP35 after processing. Values are means"SEM for ns2 replicates
Physical characteristic Processing method
Unground, no steam Unground, steam Ground, no steam Ground, steam Ground, extruded
Ž .
Pellets retained after 0.5 h % 48.6"3.0 73.1"0.4 38.4"0.7 56.5"12.0 94.5"0.5
Ž .
Particulates after 0.5 h % 39.2"2.3 17.2"0.1 51.0"1.1 33.4"10.4 1.4"0.1
Ž .
Soluble material after 0.5 h % 12.2"0.7 9.7"0.3 10.6"0.3 10.1"1.6 4.2"0.5
Ž .
Water absorption after 0.5 h % 50.7"5.3 46.4"5.6 47.5"3.3 48.9"4.3 52.0"0.3
Ž .
Breaking shear force N 13.0"0.5 9.6"0.5 15.4"0.9 12.2"1.0 20.4"0.6
2
Ž .
Breaking shear force Nrmm 1.5"0.1 1.2"0.1 1.8"0.1 1.5"0.1 1.5"0.1
Hardness index 1.2"0.1 0.9"0 1.6"0.1 1.2"0.1 1.4"0.1
Ž .
Bulk density grl 810.5"0 784.9"8.3 841.2"0.1 789.9"0 579.5"6.1
Ž .
Durability % 97.7"0.1 98.3"0 97.2"0.2 97.9"0.1 98.5"0.2
Ž .
Sinking rate cmrs 11.0"0 11.0"0 11.0"0 11.0"0 F2.0
Ž .
Degree of gelatinisation % 16.65 20.1 20.7 14.9 82.1
Ž .
Ž .
18 h Allan et al., 1999 . Feacal samples were removed each morning prior to feeding and dried over silica gel in vacuum desiccators. Individual tank samples from daily collections were pooled to provide sufficient sample for chemical analyses. The experi-ment was run for 24 days.
2.5. Water quality analyses
Temperature, dissolved oxygen and pH for each experiment were monitored weekly
Ž
using a Model 611 electronic water quality analyser Yeo-Kal Electronics, Brookvale,
.
NSW, Australia . Colourimetric methods were used to measure total ammonia nitrogen
ŽDal Pont et al., 1973 and nitrite Major et al., 1972 . Over the course of the growth. Ž .
trial temperature ranged between 23.8 and 28.38C, dissolved oxygen 6.2–7.7 mgrl and
pH 6.1–8.5, NO -N 20–60 mgrl and total ammonia-N 20–100 mgrl. As ambient
2
Ž .
temperatures dropped approaching April 1996 heaters were installed in the sump to maintain water temperatures above 238C.
For the digestibility experiment, temperature ranged between 25.0 and 26.58C,
dissolved oxygen 6.9–8.8 mgrl, pH 7.75–8.32, NO -N -57mgrl and total ammonia-N
2 -233mgrl.
2.6. Chemical analyses
All analyses were carried out in duplicate on samples of feed and faecal material by NSW Agriculture, Wollongbar Agricultural Institute. Values for dry matter, fat and
Ž .
energy bomb calorimetry were determined following procedures described in AOAC
Ž1990 . Nitrogen was determined using the Kjeldahl method. ŽAOAC, 1990. and
multiplied by 6.25 to establish the content of crude protein. Determination of chromic
Ž .
oxide was by the method described in Scott 1978 . Amino acids were analysed using
Ž .
HPLC and Water Pico-Tag Waters, Lane Cove, NSW, Australia after being subjected to acid hydrolysis. Tryptophan was not determined. Sulphur amino acids were deter-mined separately following performic acid digestion.
2.7. Calculation of digestibility coefficients
Apparent digestibility coefficients for dry matter, energy and nitrogen for experimen-tal diets were calculated following indirect procedures similar to those outlined in Cho
Ž .
and Kaushik 1990 . All values are presented on a dry basis.
ADCs 1y
Ž
FrD=DCrFC.
=100r r
where F is the percent of nutrient or energy in faeces, D is the percent of nutrient or
energy in diet, DC is the percent of chromic oxide in diet and FC is the percent ofr r
chromic oxide in faeces.
2.8. Statistical analyses
Data from four diets used in the growth trial were subjected to a two-factor ANOVA
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M.A. Booth et al.rAquaculture 182 2000 287–299
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gate interaction between level of grind and presence or absence of steam conditioning. Data from the extruded treatment were excluded from statistical examination of results for the growth trial as extruded pellets floated or sank slowly, and this clearly reduced feed consumption. Digestibility coefficients for dry matter, energy and nitrogen deter-mined from the digestibility trial were compared with one way ANOVA after tests for
Ž .
homogeneity of variance were satisfied using Cochran’s test Winer, 1971 . Differences
Ž .
between means were determined by Student–Newman–Keuls SNK multiple range test. Retrospective tests for power, where reported, were calculated from equations presented
Ž .
in Searcy-Bernal 1994 and the significance level for all tests was as0.05.
3. Results
3.1. Pellet characteristics
Ž .
Wet stability tests 0.5 h indicate the ground extruded diet exhibited superior water stability in comparison to all other diets, followed by those subject to steam condition-ing. The ground-unsteamed treatment proved to be the most unstable pellet in water with greater than 50% of material collected as particulate matter. Values for other character-istics such as water absorption and dry pellet durability were similar, however the extruded diet showed the greatest increase in gelatinisation of starch with approximately 82% of raw starch modified. All diets with the exception of the extruded diet sank
Ž .
immediately they were administered Table 3 .
Ž .
Table 4
Weight gain, FCR and feed consumption for individual juvenile silver perch fed on SP35 subjected to processing. Values are means"SEM for ns3 replicates. Experimental period ts113 days
a b
Processing method Weight gain FCR Average daily
Žgrfish. feed consumption
Ž% biomassrday.
Unground, no steam conditioning 54.5"5.5 2.01"0.04 1.91"0.08 Unground, steam-conditioned 71.3"3.2 1.75"0.04 1.91"0.05 Ground, no steam conditioning 61.3"2.1 1.91"0.09 1.90"0.07 Ground, steam-conditioned 70.7"1.5 1.76"0.02 1.93"0.06 Ground, extruded 58.2"4.9 1.50"0.01 1.49"0.04
a
FCRsdry weight feedrwet weight fish.
b
Ž . Ž . Ž .
Average daily feed consumption % biomassrdays100=total feed intakertraverage biomass . 3.2. Effect of processing on weight gain
Differences in mean fish weight between diets became more pronounced toward the
Ž .
end of the study Fig. 1 . Mean values for individual weight gain ranged between 54.5 and 71.3 g for the unground, unsteamed treatment and unground steam-conditioned
Table 5
Ž .
Apparent digestibility coefficients for dry matter, energy, nitrogen and amino acids of ground diets 500mm fed to silver perch. Values are means"SEM for ns3 replicates. Row means with similar letters in superscript
Ž .
are not significantly different P)0.05, ANOVA, SNK Processing method
Steam-conditioned No steam conditioning Extruded Digestibility coefficient
a a b
Ž .
Dry matter % 67.00"1.39 64.76"0.97 71.40"0.32
a a b
Ž .
Gross energy Mjrkg 77.95"1.10 76.47"0.73 83.22"0.22
a a a
Ž .
Crude protein % 90.16"0.39 88.97"0.49 89.28"1.62 ( )
AÕailability %
Aspartic acid 94.1"0.5 93.9"0.3 94.9"0.1
Glutamic acid 95.2"0.1 94.7"0.2 95.8"0.1
Serine 92.0"0.4 91.4"0.3 91.5"0.5
Glycine 86.3"0.4 83.9"0.5 87.4"0.4
Histidine 95.3"0.4 93.8"0.3 91.6"0.4
Arginine 94.0"0.2 92.8"0.2 94.1"0.2
Threonine 94.8"0.4 94.2"0.4 94.0"0.4
Alanine 91.5"0.3 90.5"0.2 91.3"0.3
Proline 86.8"0.7 84.6"0.3 87.7"0.4
Tyrosine 94.6"0.4 93.7"0.3 94.6"0.3
Valine 92.4"0.3 90.9"0.3 91.4"0.2
Isoleucine 93.4"0.3 92.1"0.3 94.5"0.2
Leucine 93.7"0.4 92.8"0.3 92.6"0.2
Phenylalanine 94.2"0.6 92.7"0.3 92.5"0.1
Lysine 94.5"0.4 93.8"0.2 93.7"0.2
Cystine 94.4"2.8 90.4"0.8 89.4"1.4
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treatments, respectively. FCRs were poorest in fish fed both unsteamed diets and
Ž .
markedly improved for fish fed both the steamed and extruded diets Table 4 .
Improvements in FCR for fish fed the extruded diet was probably related to their feeding behaviour. The extruded diet floated, or sank slowly and fish were reluctant to feed at the surface during the trial. As a consequence, average daily feed consumption was lower for this treatment. Despite this, individual weight gains for the extruded treatment
Ž .
matched those of the unground, unsteamed treatment Table 4 . Average daily feed
Ž .
consumption was similar in silver perch for all other diets Table 4 .
Two-factor ANOVA indicated steam conditioning significantly improved both the
Ž
weight gain and FCR of silver perch in this study Fs14.36, dfs1r8, Ps0.005 for
.
weight gain; Fs14.99, dfs1r8, Ps0.005 for FCR . Grinding SP35 to 500 mm had
no effect on weight gain or FCR and there was no interaction between these factors in either test.
3.3. Effect of processing on digestibility
The ground extruded treatment had significantly higher digestibility coefficients for both dry matter and energy than the alternative dietary treatments. No statistical
Ž
difference was found between digestibility coefficients for nitrogen Fs2.69, dfs2r6,
. Ž
Ps0.15, power 0.48 and availability of amino acids were not statistically tested Table
.
5 .
4. Discussion
Different processing treatments of SP35 affected weight gain and performance of silver perch reared under our experimental conditions. Differences may have been due to processing effects on feed consumption, digestibility or utilisation.
Feed consumption was clearly affected by diet processing. Fish fed the extruded diet consumed significantly less on a biomass basis than fish fed the other diets evaluated in this study. This difference is attributed to differences in feeding behaviour of fish fed the
Ž .
extruded pellets. Extruded pellets had a low bulk density 579.5 grl and either floated
Ž .
or sank very slowly F2.0 cmrs . Silver perch were reluctant to consume these floating pellets in our clear water tanks. As a consequence, satiation feeding was difficult to achieve, and fish probably stopped feeding before they were satiated. This overly restricted or sub-satiation feeding regime was dissimilar to that for other diets and may
Ž .
account for the lower FCR associated with the extruded diet Pfeffer et al., 1991 . Secondly, the 5% addition of fish oil required to manufacture an acceptable extruded
pellet may have affected the attractiveness andror palatability of the diet and the
Ž
increased energy content may have increased FCR Hilton et al., 1981; Pfeffer et al.,
.
1991 . In addition, the extruded diet had a much higher degree of starch gelatinisation
Ž82% than all other diets and this may have accounted for the significant improvement.
in digestibility coefficients for dry matter and energy in comparison to the steamed and
Ž .
Ž .
plant costs associated with producing an extruded pellet Springate, 1991 and also lead
Ž
to significant reductions in the pollution associated with intensive aquaculture Cho,
.
1991; Jirsa et al., 1997 . However, given that the effects of floating extruded pellets and feeding behaviour in our tanks are likely to be different for other culture facilities such as ponds, it is inadvisable to draw conclusions about the relative overall merits of floating extruded vs. sinking pellets for silver perch farmers.
Steam conditioning SP35 improved performance of silver perch. This is supported by significant improvements in individual weight gain and FCR compared with fish fed the unsteamed diets. All steamed and unsteamed diets had similar analysed composition. Steamed and unsteamed diets had similar overall physical characteristics and as average daily feed consumption was unaffected by thermal treatment, improvements in fish performance are unlikely to be due to the effects of diet processing on feed intake. Further, while steam-conditioned diets were more water stable, our feeding practices, particularly careful individual feeding of each tank and rapid consumption of all sinking diets during feeding discounts differences in wastage or the loss of nutrients from pellets as factors likely to have influenced performance.
Thermal treatments such as steam conditioning have the potential to gelatinise a
Ž .
percentage of dietary starch Hardy, 1989; Hui-Meng, 1989 which is generally
consid-Ž .
ered to improve digestibility Bergot, 1991; Wilson, 1994 . However, in this study, steam conditioning at 908C had a variable effect on degree of starch gelatinisation and did not account for the difference in fish performance. For example, despite a 5%
Ž .
difference ie absolute difference in percent gelatinisation between the two
steam-condi-Ž .
tioned diets, weight gain on these two diets was almost identical Tables 3 and 4 . Similar differences in percentage gelatinisation occurred between both ground diets
Ž .
studied in the digestibility trial Table 3 , yet neither diet varied in digestibility
Ž .
coefficients for dry matter, energy or nitrogen P)0.05 and availability of amino acids
Ž .
remained unchanged Table 5 .
Clearly steam-conditioned diets were better utilised by silver perch. This is evident from improved FCR and weight gain. This improvement may have been due to nutrients in the steamed diets being more available or to different effects on residual levels of anti-nutrients. Resolution of this effect requires further investigation.
Grinding SP35 pellets to 500mm had no effect on the performance of silver perch in this study. This finding is contrary to the generally accepted benefits of such a process. For example, fine grinding is generally thought to improve the overall physical
Ž .
characteristics of a pellet Hui-Meng, 1989; Botting, 1991 , improve digestibility
ŽHardy, 1989; Hui-Meng, 1989 and ensure the non-selective ingestion of ingredients.
ŽTan, 1991 . Grinding SP35 to 500. mm did not improve wet pellet stability. In fact,
grinding SP35 below 500 mm in the absence of steam resulted in an unstable pellet,
potentially prone to nutrient losses and likely to reduce water quality. The most stable
Ž
pellet was produced from coarsely ground SP35 80% of particles between 710 and
.
1000mm subject to steam conditioning.
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Acknowledgements
We thank the following for their technical input to experiments: Dr Stuart Rowland, Mr Charlie Mifsud and other staff at NSW Fisheries Grafton Research Centre for producing juvenile silver perch and support for nutrition research; Mr Scott Parkinson and Mr David Stone at Port Stephens Research Centre; Mr Ken O’Brien and Ms Wendy
Ž .
Peasley NSW Agriculture for their assistance with biochemical analysis; Mr David Overend, Mr Kevin Smyth and Mr Gary Hoey of Ridley Agriproducts for their assistance with processing the diets; Mr Tony Hoey and Ms Kerry Page at Janos Hoey for supplying the diet SP35; Dr Wayne O’Connor and Mr Stewart Fielder for critically reviewing the manuscript and Mrs Helena Heasman for assisting with the preparation of the manuscript. The research developed was part of the Australian Fisheries Research and Development Corporation Sub-Program on Replacing Fish Meal in Aquaculture
Ž .
Diets 93r120 . Financial support from the Australian Centre for International Agricul-tural Research, the Australian Grain Research and Development and the Meat Research is also gratefully acknowledged.
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