Evaluation of Brewer's Waste as Partial Replacement

JOURNAL OF THE WORLD AQUACULTURE SOCIETY

Vol. 39, No. 4 August, 2008

Evaluation of Brewer’s Waste as Partial Replacement of Fish Meal Protein in Nile Tilapia, Oreochromis niloticus, Diets DESALE B. ZERAI, KEVIN M. FITZSIMMONS1, ROBERT J. COLLIER,

AND

GLENN C. DUFF

Department of Soil, Water and Environmental Science, University of Arizona, Tucson, Arizona 85721 USA

Abstract.—Brewer’s waste is one of the promising protein source by-products for fish diets. A 10-wk feeding trial experiment involving five different diets with increasing levels of brewer’s waste (32% crude protein) was carried out to evaluate the use of brewer’s waste in tilapia diets in place of fish meal. Growth performance was compared against a control diet formulated to have similar composition to a typical commercial diet. Four experimental diets replaced successively 25, 50, 75, and 100% of the fish meal protein with brewer’s waste. The diets were isonitrogenous and isocaloric. Results indicated that weight gain did not differ significantly (P . 0.05) with up to 50% replacement. Feed intake and utilization were depressed at high levels of brewer’s waste. In addition, methionine of high replacement level diets was low. The results of the digestibility trial demonstrated that the brewer’s waste used in this study has an apparent digestibility coefficient for protein of 70%. It was concluded that 50% of the fish meal protein in a typical commercial diet could be replaced with brewer’s waste with no adverse effect on growth and feed utilization for tilapia.

Fish meal is a major conventional ingredient in many aquafeeds (El-Sayed 2004). Besides the rising cost of fish meal, its supply is also a limiting factor for the expansion of aquaculture industry (Wu et al. 1999). It is believed that world annual fish meal production is going to stagnate at 6.5 million metric tons over this decade (Hasan 2000). Consequently, the need for finding alternative protein sources, especially those that are by-products and those that are not good for human consumption, is important (Hoffman et al. 1997). In addition, replacing a portion of the fish meal in aquafeeds is crucial for the expansion of aquaculture beyond the level at which fish meal supply restricts further growth (Stickney 1997; Hardy 2000; New 2002). The most commonly used plant protein sources like soybean meal and cottonseed meal are either not readily available or are expensive in the tropics. Under these situations, alterna1

Corresponding author.

tive protein sources like brewer’s waste may be important (Alceste 2000). Brewer’s waste, which has good nutritive value, is widely available in many parts of the world. However, the utilization of this by-product for animal feed, particularly for fish, is poorly understood in some parts of the world. Plant by-product protein sources, unlike animal protein sources, do not generally achieve the desired level of growth. This is because of presence of growth inhibitors and inherent essential amino acid deficiencies (Hasan 2000). Despite this, opportunities for use of plant protein by-products exist as partial substitutes, with or without supplementation by commercially available free amino acids. Wu et al. (1999) showed that growth of tilapia fed a totally plant-based diet of corn gluten meal and soy grits supplemented with lysine, tryptophan, and threonine did not differ from those fed fish meal–based diet. A protein called vitellogenin produced by recombinant yeast SMD1168H, Pichia pastoris, was found to bolster larvae growth in tilapia (Lim et al. 2005). Dersjant-Li (2003) reported that partial replacement of fish meal by soy protein concentrate improved survival, growth rate, and feed utilization in fish and shrimp. The reason for this is not clear. According to Dersjant-Li (2003), it might be because of the complementing effect of fish meal and soy protein concentrate. In addition, brewer’s yeast products were reported to be capable of enhancing innate immunity and disease resistance of certain fishes (Siwicki et al. 1994; Li and Gatlin 2003), although results from various studies may be inconsistent (Li et al. 2005). Soybean meal can replace, partially or totally, fish meal in tilapia diets depending on species, size, and soybean processing (El-Sayed 2004). However, the growing aquaculture industry

Ó Copyright by the World Aquaculture Society 2008

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needs to look at other sources too. This is because soybean production is not pacing with aquaculture (Kubaryk 1996). In addition, soybean is not available in many countries, and also the fact that mixing of different ingredients can complement and yield a better result. The inclusion of unconventional protein sources has to be viewed not only from the influence on growth but also from the economics and local availability as well. The results of research on replacement of fish meal by unconventional plant protein sources are often encouraging (El-Sayed and Teshima 1991). This study is intended to evaluate the possibility of replacing fish meal by brewer’s waste in a typical commercial tilapia diet. The brewer’s waste was obtained from a local microbrewery. Microbreweries lack the ability of segmenting by-products unlike larger breweries (Westendorf and Wohlt 2002). The brewer’s waste used in this study has a protein content of 32% and it was obtained from the following process. Malted grain and hot water were mixed in a large tank for mashing. Mashing broke down starch into simple sugars ready for fermentation. The resulting sugars were transferred into a fermentation tank whereby yeast was added and left for about 2 wk to ferment. Once the beer was drained, the leftover was the brewer’s waste used in this experiment. Although these fish were reared in an intensive recirculating system, results of this study will contribute to the knowledge of utilization of brewer’s waste in extensive and semi-intensive culture practices in developing countries in the tropics, where there are few protein sources like soybean meal. Materials and Methods Diet Preparation The brewer’s waste was supplied by a small local microbrewing company in Tucson, Arizona (Table 1). The study was partitioned into a feeding trial experiment and a digestibility experiment. Four diets containing progressively increasing levels of brewer’s waste were formulated for the feeding trial experiment. The brewer’s waste replaced 25% of the fish meal protein

TABLE 1. Chemical composition of basal ingredients used in the formulation of the diets. Ingredients

Nutrient

Fish meal (%)

Soybean meal (%)

Wheat flour (%)

Brewer’s waste (%)

Protein Fat Ash Fiber

65 6 151 2

50 — 31 4

13 1 11 21

32 41 9 101

1

Estimated value.

in the first experimental diet (D2). Thereafter, it replaced 50, 75, and 100% of the fish meal protein in the subsequent experimental diets (D3, D4, and D5). A control diet, D1, was formulated to have no brewer’s waste (Table 2). The control diet has similar ingredients and formulation as most commercial diets in the fish feed industry. Therefore, it contains soybean meal as a protein source as well. Shiau et al. (1990) showed that soybean meal could replace 30% of fish meal protein in tilapia diets. Chemical analysis of the diets was conducted (Table 2). Amino acid analysis was performed by Midwest Laboratories, Inc., in Nebraska. The diets have similar crude protein level of around 40%. For each diet, the dry ingredients were mixed thoroughly using kitchen mixer and then the wet ingredients were added while mixing in such a way that the overall moisture was about 35%. Hand mixing was also employed to break up clumps. The feed was made into strands or pellets by passing through a meat mincer with 3.5-mm die and then sun dried. After drying, the pellets or strands were made to a uniform size by breaking them using a small hand-driven crumbling machine. Finally, the diets were stored in a refrigerated cold room until use. For measuring the digestibility of the brewer’s waste, two diets, a reference diet (DR) and a test diet (DT) were formulated according to the procedures set forth in Bureau et al. (1999). The reference diet has similar formulation to the control diet in the feeding trial experiment but included chromic oxide as a digestion indicator (Table 2). The test diet is formulated in a proportion of 70–30% of the reference diet mix and the test ingredient (brewer’s waste),

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TABLE 2. Compositions (%) and chemical analyses of the five feeding trial diets and two digestibility trial diets on dry matter basis (DMB). Diets Ingredients (%) Fish meal Brewer’s waste Soybean meal Wheat flour Fish oil Lecithin Vitamin & mineral mix Reference diet mix Chromic oxide (65%) Analysis (% DMB) Dry matter Crude protein Crude fat Ash Fiber NFE calculated GE (Kcal/100 g)1 P:E (mg/Kcal GE)2 EAA (% DMB) Arginine Histidine Isoleucine Leucine Total lysine Methionine Phenylalanine Threonine Tryptophan Valine

D1

D2

D3

D4

D5

DR

25.0 0.0 25.0 44.0 2.0 2.0 2.0

18.8 12.5 25.0 37.7 2.0 2.0 2.0

12.5 25.0 30.0 26.5 2.0 2.0 2.0

6.3 37.5 30.0 20.2 2.0 2.0 2.0

0.0 45.0 35.0 14.0 2.0 2.0 2.0

25.0 0.0 25.0 43.0 2.0 2.0 2.0

88.7 39.2 6.7 9.0 9.2 36.0 423.8 92.5 1.82 0.79 1.18 2.43 1.89 0.68 1.39 1.14 0.24 1.44

88.7 41.1 6.5 8.2 3.1 41.0 452.4 90.8 2.03 0.80 1.34 2.46 2.12 0.71 1.48 1.59 0.41 1.59

88.5 39.9 7.2 7.6 9.6 35.7 431.3 92.5 1.75 0.71 1.23 2.40 1.91 0.61 1.40 1.18 0.34 1.68

87.4 40.9 5.7 6.7 5.1 41.6 446.9 91.5 2.12 0.81 1.35 2.35 2.00 0.54 1.30 1.21 0.47 1.90

87.7 41.0 5.9 5.9 5.3 41.9 450.6 91.0

DT

30.0

1.0

69.7 0.3

95.6 38.2 8.1 7.7 7.1 38.9 443.3 86.2

91.1 38.4 5.9 8.2 6.5 41.0 433.0 88.7

1.96 0.82 1.31 2.31 2.12 0.54 1.51 1.27 0.38 1.61

NFE 5 nitrogen-free extract; EAA 5 essential amino acids; GE 5 gross energy. Calculated using the factors 5.5, 9.1, and 4.1 for protein, fat and carbohydrate, respectively; (Jauncey and Ross 1982). 2 Protein to energy ratio. 1

respectively (Table 2). The test diet also included chromic oxide. These diets were also prepared in the same way as that of the feeding trial experimental diets and then stored in refrigerated cold room until use. Chemical analysis was also performed for these diets (Table 2). Experimental Setup and Animals One hundred and fifty juvenile Nile tilapia, Oreochromis niloticus, average weight of 24.8 6 5.0 g (mean 6 SD), were obtained from the Environmental Research Laboratory (ERL) for the feeding trial experiment. The fish were tagged individually by a Passive Integrated Transponder in their body cavity by injection following the

procedures of Biomark, Inc. The electronic tag enabled each fish to be assessed for weight gain throughout the experiment. A mini portable reader from Destron Fearing Corp., St. Paul, Minnesota, was used to read the tags. The feeding trial experiment was carried out in 15 cylindrical 210-L-capacity fiberglass tanks, with a diameter of 60.5 cm (filled with 138 L of water). The system is a closed recirculation system with mechanical and biological filters. For the digestibility experiment, fish of average size 35.0 6 9.9 g (mean 6 SD) in two tanks each with 20 fish were used. The tanks are the same size as in the feeding trial experiment and are also on the same closed recirculation system.


Experimental Procedure and Feeding The feeding trial experiment was conducted from February 5 through April 16, 2003 (10 wk) at the ERL’s green house in Tucson, Arizona. After tagging, the fish were introduced randomly into the 15 tanks at a stocking density of 10 fish per tank. They were allowed to recover from the stress of tagging injection for a week and fed with commercial tilapia diet of 32% protein (Star Milling Co., Perris, CA, USA). The fish were weighed individually. The five diets were assigned at random to the tanks in such a way that each diet had three replicate tanks. All the tanks had common water supply and were handled the same way throughout the experiment except for the diet treatment. Therefore, each diet has three bulk tank weight replicates and 30 data points or replicates of individual fish. Feed conversion ratio (FCR), protein efficiency ratio (PER), and feed intake were based on three replicates. The fish were subject to natural photoperiod throughout the experiment (about 32°N latitude). Aeration was constantly provided by air-stone diffuser in each tank. An average of 1.4 L/min flow rate was maintained for the duration of the experiment for each tank. Dissolved oxygen, temperature, pH, ammonia, nitrite, and nitrate were measured every week and they were 5.85 6 0.56 mg/L, 25.8 6 0.4 C, 8.44 6 0.17, 0.19 6 0.11 mg/L, 0.250 6 0.070 mg/L, 17.9 6 11.6 mg/L, respectively (mean 6 SD). The fish were individually weighed every 2 wk to the nearest 0.1 g and were fed at a rate of 5% biomass per day divided into three rations (morning, noon, and late afternoon). The fish were not fed on the day of weighing. Diet performance was measured by weight gain (g), FCR (dry feed intake (g)/live weight gain (g)), and PER (live weight gain (g)/protein intake (g)). Because the fish were not eating all the feed provided, it was decided that uneaten feed has to be collected for a period of 2 wk to determine net feed intake. After 6 wk from start of experiment (beginning on March 19), uneaten feed was periodically collected, dried, and weighed. A screen of 1.5 mm was set in the drainage pipe to block uneaten feed, and uneaten feed was collected as quickly as possible to reduce leaching.

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The digestibility experiment was carried out from April 7 to 30. The two diets, reference diet and test diet, were assigned randomly to two tanks each with 20 fish. The fish were fed 5% of biomass per day in three rations. After 5 d (beginning on April 12), feces from each tank were collected daily with scoop net. The feces were collected several times a day in an effort to limit leaching into water. After collection, they were dried and then stored frozen for analysis of protein and chromium. The digestibility of brewer’s waste was determined according to the method and formulae presented in Bureau et al. (1999). Protein and chromium of the fish feces fed the respective diets were analyzed. The apparent digestibility coefficients (ADC) for protein of the reference and test diets were determined according to the following formula: ADC ¼ 1 ðF=D 3 Di =Fi Þ where D 5 % nutrient of diet, F 5 % nutrient of feces, Di 5 % digestion indicator of diet, and Fi 5 % digestion indicator of feces. Then, the ADC of the brewer’s waste was calculated as: ADCI ¼ ADCT 1 ðð1 sÞDR =sDI Þ 3ðADCT ADCR Þ where ADCI 5 apparent digestibility coefficient of test ingredient, ADCT 5 apparent digestibility coefficient of test diet, ADCR 5 apparent digestibility coefficient of reference diet, DR 5 % nutrient of reference diet, DI 5 % nutrient of test ingredient, s 5 proportion of test ingredient in test diet (i.e., 0.3), and 1s 5 proportion of reference diet in test diet (i.e. 0.7). Data Analysis Feed performance was assessed by weight gain, FCR, PER, and feed intake. These data were subjected to one-way ANOVA with repeated measures. Least square means were used to compare treatment means. The data were analyzed using the Statistical Analysis System (SAS release 8.02) software Proc Mixed Procedure (SAS Institute Inc).

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Results Growth Performance The growth response and feed utilization data of tilapia fed the five diets are presented in Table 3. There were no significant differences between the average final weights of the fish fed D1 or control diet, D2 (25% fish meal protein replaced diet) and D3 (50% fish meal protein replaced diet) (P . 0.05) (Table 3). D4 and D5 (75 and 100% fish meal protein replaced diets) fed fish exhibited significantly less average final weights compared to the control diet fed fish (P , 0.05) (Table 3). No significant differences of average final weights were noted between D4 and D5 fed fish (P . 0.05) (Table 3). Generally, weight gain decreased as the replacement level of the fish meal protein by brewer’s waste increased (Table 3). Weight gain of fish fed with 25 and 50% replacement (D2 and D3) did not differ significantly (P . 0.05) from the control diet (D1) fed fish (Table 3). The fish fed with 75 and 100% replacement (D4 and D5) experienced significantly lower growth compared to the control diet (D1) and to diets D2 and D3 (P , 0.05) (Table 3). The fish fed with 100% replacement experienced lowest growth parameters numerically (Table 3). Survival was not affected by the increased replacement of brewer’s waste. Feed Intake and Utilization Feed intake of fish fed 100% replacement (D5) was significantly different from the control diet (D1) fed fish (P , 0.05) (Table 3). Simi-

larly, feed intake for D5 was significantly different from D2 and D3 (P , 0.05). On the other hand, feed intake of fish fed diets D2, D3, and D4 did not differ significantly from D1 fed fish (P . 0.05). Mean dry feed intake measured in g/kg fish weight/d varied from 30.0 for Diet 3 to 25.8 for Diet 5 (Table 3). The FCR of D1 is not significantly different from D2, D3, and D4 (P . 0.05) but is significantly different from D5 (P , 0.05) (Table 3). PER was highest for the control diet and lowest for the 100% fish meal protein replacement diet (D5). The PER was noticeably different among the treatments than it is for the FCR or feed intake (Table 3). PER of fish fed with 25 and 50% replacement (D2 and D3) did not differ significantly (P . 0.05) from the control diet (D1) fed fish. The fish fed with 75 and 100% replacement (D4 and D5) had exhibited a significantly lower PER compared to the control diet (D1) (P , 0.05). On the other hand, PER of the fish fed D4 and D5 did not differ significantly from the PER of fish fed D2 (P . 0.05) but it is significantly different from PER of the fish fed D3 (P , 0.05) (Table 3). In the digestibility trial, the ADC for protein of the reference and test diets were found to be 95.9 and 89.1%, respectively. And the ADC of the brewer’s waste was 70%. Pellet stability data is presented in Table 4. The weekly water quality data is presented in Table 5. Discussion The search for alternative protein sources to fish meal is pursued by setting up feeding trial

TABLE 3. Growth parameters and feed utilization values of tilapia fed increasing levels of brewer’s waste.1 Diets Parameters Initial weight (g) Final weight (g) Biweekly weight gain (g) Feed intake (g/kg BW/d) FCR PER

D1 25.3 81.7 11.5 29.6 1.97 1.30

± ± ± ± ± ±

0.9 2.1a 0.3a 1.2a 0.02a 0.01a

D2 24.5 79.5 11.1 29.1 1.99 1.23

± ± ± ± ± ±

1.0 3.1a 0.5a 2.7a 0.08ab 0.05ab

D3 23.8 76.9 10.8 30.0 1.93 1.29

± ± ± ± ± ±

0.9 2.3a 0.4a 1.8a 0.02a 0.02a

D4 24.9 66.7 8.5 27.8 2.06 1.19

± ± ± ± ± ±

0.8 1.5b 0.3b 1.4ab 0.03ab 0.02b

D5 25.7 62.4 7.6 25.8 2.11 1.17

± ± ± ± ± ±

1.1 2.2b 0.4b 2.1b 0.07b 0.03b

1 Values are means 6 SE. Means in a row followed by different letters are significantly different (P , 0.05). Initial weight, final weight, and biweekly weight gain (n 5 30). Feed intake 5 dry feed intake (g)/fish body weight (kg)/d (n 5 3). FCR 5 dry feed intake (g)/live weight gain (g) (n 5 3). PER 5 live weight gain (g)/protein intake (g) (n 5 3).

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TABLE 4. Percentage recovery of the feeding trial diets after immersion for 10 min in unperturbed water.1 Diets Pellet stability

D1

D2

D3

D4

D5

% dry matter recovery after immersion for 10 min

95.4 ± 0.4a

94.6 ± 0.3a

90.1 ± 1.4b

89.8 ± 0.7b

85.6 ± 2.0c

1

Values are means 6 SE (n 5 3). Means in a row followed by different letters are significantly different (P , 0.05).

experiments which compare the alternative protein-based diets at different replacement levels against the fish meal–based diet. In this experiment, the control diet was chosen to be a typical commercial diet formulation in which the protein source is not only fish meal but also soybean meal. This approach is aimed at maximizing the amount of overall fish meal being replaced by complementing effects of the alternative protein sources. Brewer’s waste has been fed to livestock since the advent of beer production (Westendorf and Wohlt 2002). Its wide spread availability makes it a good candidate in the search for alternative protein sources for aquafeeds, especially in the developing regions of tropics. This is primarily because of the lack of sufficient plant protein sources like soybean in these parts of the world. The results of this study show that brewer’s waste can effectively substitute up to 50% of the fish meal protein of a typical commercial feed with no adverse effect on growth of tilapia. Olvera-Novoa et al. (2002) in a study of Mozambique tilapia, Oreochromis mossambicus, found out that a 15% fish meal and a combination of four other protein sources (meat and bone meal [5%], alfalfa protein concentrate [15%], soybean meal [20%], and torula yeast [yeast produced

from wood sugars, 45%]) yielded a growth not significantly different from a totally fish meal– based diet for protein source. Comparably, the 50% replacement level (D3) of this experiment contains 12.5% fish meal, 25% brewer’s waste, and 30% soybean meal of the entire formulation. These results agree with the results of Rumsey et al. (1991a) on rainbow trout with brewer’s dried yeast. They found that 25% brewer’s dried yeast fed fish grew better. This demonstrates that the typical commercial diet of about 25% fish meal and about 25% soybean meal could be reduced by half the amount of fish meal with the incorporation of brewer’s waste. In addition, Oliva-Teles and Goncalves (2001) found that 50% of the fish meal in a totally fish meal protein–based diet can be replaced with brewer’s yeast in sea bass, Dicentrarchus labrax, diet with no adverse effect on growth. Schneider et al. (2004) found that about one-third of the fish meal can be replaced with single cell protein (SCP) primarily composed of brewer’s yeast in Nile tilapia diets. Inclusion of brewer’s waste above 50% fish meal protein replacement resulted in a significant decline in the growth of tilapia. Weight gain of tilapia fed 75% fish meal replacement diet were significantly lower than the control diet and diets

TABLE 5. Weekly water quality parameters throughout duration of the experiment. Week

Dissolved Oxygen (mg/L)

pH

T(C)

NH3 (mg/L)

NO2 (mg/L)

NO3 (mg/L)

1 2 3 4 5 6 7 8 9 10

6.89 6.18 5.80 5.39 4.85 5.74 6.00 5.46 6.38 5.84

8.44 8.35 8.50 8.10 8.38 8.53 8.60 8.55 8.26 8.69

25.6 26.4 25.6 26.1 26.2 26.1 25.7 25.3 25.1 25.5

0.07 0.28 0.14 0.08 0.24 0.42 0.10 0.15 0.30 0.16

0.140 0.236 0.227 0.216 0.259 0.359 0.195 0.190 0.355 0.276

5.7 5.7 4.2 10.7 17.5 32.3 37.9 16.8 23.4 24.6

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D2 and D3. On the other hand, feed intake given per body weight of D5 is significantly lower than all the diets except D4. Rumsey et al. (1991a) also observed a similar decline in feed intake of rainbow trout fed with increasing levels of brewer’s dried yeast. Texture and palatability of diets change at high levels of plant protein, often lowering feed intake (Al-Hafedh and Siddiqui 1998). High nucleic acid content of SCP sources like brewer’s yeast can depress feed intake (Oliva-Teles and Goncalves 2001). The PERs of D2 and D3 were not significantly different from the PER of D1, but the PERs of D4 and D5 were significantly lower than the PER of D1. These trends suggest that the reduced growth rate of tilapia fed successively with increased levels of brewer’s waste could also be, in addition to reduced intake, because of low methionine in the high replacement diets. Brewer’s yeast is considerably lower in methionine (0.79 vs. 1.91) than menhaden fish meal in percent dry basis (Lovell 1998). This might be the limiting amino acid in the use of brewer’s waste as a replacement of fish meal. Blood meal and poultry by-product meal are fairly good in this amino acid, and hence they might complement brewer’s yeast well. Oribhabor and Ansa (2006), in extensive tilapia ponds, have demonstrated good tilapia growth with a diet composed of yellow maize, groundnut cake, fish meal, blood meal, and brewer’s waste. Webster et al. (1991) showed that a diet of 70% distillers’ grains with solubles (29% crude protein) in channel catfish had a negative effect on growth, but when supplemented with 0.4% lysine, there was no difference in growth to that of a control diet. Oliva-Teles and Goncalves (2001) found better feed efficiency and protein utilization when 30% of the fish meal of their control diet (total fish meal protein–based diet) was replaced with brewer’s yeast for sea bass, Dicentrarchus labrax. The lower performance of fish diets having high levels of brewer’s yeast may be because of intact yeast cells (Oliva-Teles and Goncalves 2001). The results of the digestibility trial demonstrated that brewer’s waste has an ADC value of 70% for protein. SCP sources have low digestibility as compared to other protein sources and

this could be because of the presence of cell wall and high nucleic acid (Schneider et al. 2004). Cheng et al. (2003) studied the digestibility of dried brewer’s yeast (42.3% crude protein) in rainbow trout, Oncorhynchus mykiss. They found that brewer’s dried yeast has an ADC of 57.1% for protein. Contrary to this, Oliva-Teles and Goncalves (2001) reported an ADC of protein as 88.3% for brewer’s yeast (43.8% crude protein) in sea bass, Dicentrarchus labrax. It is unlikely that these variations are only because of differences of fish species. It is possible that much of this variability might be explained by the different sources and different processing methods of brewer’s yeast used in each of these experiments. Rumsey et al. (1991b) demonstrated an improvement in the apparent digestibility of brewer’s dried yeast after disruption of intact cells. The breaking of cell wall facilitates digestion of intracellular nutrients. In addition, in catfish, an increase in ADC of protein is apparent with increase in fish size for high protein sources like fish meal, but a decrease in ADC of protein is apparent with increase in fish size for low protein sources like wheat bran and rice bran (Usmani and Jafri 2002). Conclusion The aquaculture industry needs to minimize the use of fish meal in aquafeeds to maintain its fast growth. Tilapia aquaculture needs to seek alternative protein sources that are readily and locally available for continued expansion. One such by-product is brewer’s waste. The results of this experiment show that 50% of the fish meal protein in a typical commercial fish diet could be replaced with brewer’s waste. Further reduction of the remaining fish meal could be achieved by looking for other ingredients having a complementary amino acid profile to that of brewer’s waste. Some ingredients that might be worth trying are blood meal and poultry byproduct meal. Literature Cited Alceste, C. C. 2000. Some fundamentals of tilapia nutrition. Aquaculture Magazine 26(3):74–78. Al-Hafedh, Y. S. and A. Q. Siddiqui. 1998. Evaluation of guar seed as a protein source in Nile tilapia,


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