The feeding value and protein quality in high-fibre and fibre-reduced sunflower cakes and Kenya’s “omena” fishmeal for tilapia (Oreochromis niloticus)

J G Maina, R M Beames, D Higgs, P N Mbugua, G Iwama and S M Kisia

Department of Animal Production, College of Agriculture and Veterinary Sciences, University of Nairobi,
P. O. Box 29053, (0065), Nairobi, Kenya
jmaina@clubinternetk.com

Abstract

This study was undertaken to assess the nutritive values of some locally available protein sources in Kenya, as replacements for anchovy fishmeal in tilapia diets.  The test protein sources were omena fishmeal made from Rastrineobola argentea, anchovy fishmeal, as well as fibre-reduced and high-fibre sunflower cakes.  The four protein sources were each tested at two protein concentrations.  Oreochromis niloticus fingerlings with an initial weight of 16 g. were used for the study. Eight experimental diets, four based on fishmeal and four on sunflower cake were formulated. Each diet contained one of two levels of protein, viz., approximately 20% (low-protein) and 30% (high-protein). Further, each diet was fed to triplicate groups of fish for 78 days.

Diets based on the fibre-reduced cake had higher levels of all amino acids than the ones based on the high-fibre cake. Lysine and threonine concentrations were lower in diets based on the sunflower cakes than the ones based on the fishmeals.  Fish fed diets with 20% protein gained less weight and had higher feed:gain ratios than those fed diets with 30% protein. Fish fed diets based on anchovy fishmeal had higher weight gains than those fed diets based on the high-fibre sunflower cake. Reducing the fibre content of sunflower cake improved growth rate and weight gain. Growth rates and weight gains of fish fed diets based on the two fish meals were not significantly different.

Key words: Amino acids, fibre, hulls, productive protein value, protein efficiency ratio

Introduction and objectives

Tilapia (Oreochromis niloticus) are herbivorous fish that possess morphological and physiological adaptations for utilization of diets high in fibre content. This aspect of its feeding habits has not been fully exploited in commercial aquaculture.  Most formulated feeds for tilapia resemble those for omnivorous fish in that they contain significant levels of animal proteins (Hughes and Handwerker 1993).  Much research has been done to evaluate new protein sources to partially or wholly replace fishmeal in diets for fish.  Among the plant protein sources, soybean meal has been used widely because of its good amino acid profile. Used as the main protein source, soybean meal supports the growth of most fish species (Tacon et al 1984; Wilson and Poe 1985; Shiau et al 1987; Viola and Arieli 1983). Soybeans, however, are not suitable for growing in many countries; hence the need to evaluate other plant proteins sources.

Sunflower is cultivated extensively due to its adaptability to a wide range of climatic and soil conditions (Ravindran and Blair 1992).  Its seeds are inexpensive to process, and the cake remaining after oil extraction is used as a protein supplement in animal diets (Daghir et al 1980).  The crude protein content of the cake ranges from 25 to 45% (air-dry basis) depending on the extent of dehulling and the efficiency of the oil extraction process.  The crude fibre level in the cake generally varies between 14% and 39% (air-dry basis) (Villamide and San Juan 1998).  Protein concentration in sunflower cake is inversely proportional to the fibre content. 

The potential use of sunflower cake in fish diets is limited by its high fibre content.  Crude fibre not only has no known dietary value for fish, but it also dilutes digestible nutrient densities, thus increasing the release of polluting wastes into the environment.  In view of the above, a fibre-reduced, high-fat sunflower cake was tested as a replacement for fishmeal in tilapia feeds. In addition to the sunflower cakes, Kenya’s omena fishmeal was also evaluated as a source of protein. 

The objectives of the study were to compare the nutritional values and protein qualities of diets based on high-fibre and low-fibre sunflower cakes, and omena and anchovy fishmeals when fed to tilapia (Oreochromis niloticus) at each of two levels of dietary protein.

Materials and methods

Experimental diets and design

The fibre-reduced sunflower cake was made from a hybrid sunflower seeds (Kenya Fedha) purchased from a commercial trader (Rift Valley Products, Nakuru, Kenya).  The seeds were partly dehulled using a manually-operated Cecoco dehuller (Ibaraki, Osaka 567 Japan) which incorporated a dehuller and a sorting machine. All seeds were dried to less than 10% moisture before dehulling.  The oil content of the partly dehulled seeds was reduced by a commercial screw press oil extractor (Gold Feeds, Nairobi, Kenya). 

The high-fibre sunflower cake was processed from the same variety of sunflower as the low-fibre sunflower cake.

Omena fishmeal was purchased from Tamfeeds, (Nairobi, Kenya).  It was made from the cyprinid fish, Rastrineobola argentea. The anchovy meal was a high quality Chilean LT. meal. The chemical compositions of the main ingredients used are shown in Table 1.

Eight diets whose compositions are shown in Table 2 were formulated.  LT anchovy fishmeal, omena fishmeal, high-fibre and fibre-reduced sunflower cakes were used as sources of protein. 

In the diets based on anchovy and omena fishmeals, the fishmeals provided most of the dietary protein, while in the diets based on sunflower cakes, only 50% of the dietary protein was provided by the cake, while the remaining 50% was provided by anchovy fishmeal, as shown in Table 2.  Each diet contained one of two levels of protein, approximately 20% or 30%.  At each protein level, the diets were formulated to contain similar levels of digestible energy by varying the level of corn oil.

Each diet was randomly assigned to triplicate groups of 25 fish, and all groups were fed their prescribed diets by hand to satiation three times daily.

Fish sampling

Male Oreochromis niloticus fingerlings weighing 16g were purchased from Baobab Fish Farm (Bamburi Nature Trail, Mombasa) and transferred to the University of Nairobi for this experiment. They were acclimated to laboratory conditions for a period of two weeks. 

They were later weighed in groups of 25 fish selected at random and allocated to the experimental circular tanks. Twenty four tanks with a diameter of one metre and filled with water to a depth of 0.50 metres were used for this experiment.  The water level was adjusted as the fish biomass increased to maintain the stocking density below 0.1 kilograms of fish per litre of water.

A Sweetwater TM Regenerative blower was used for aeration.  Each tank was fitted with an AS8-1 (3 inches) diffuser.  Water temperatures and dissolved oxygen concentration in the tanks was maintained at 26ºC + 2ºC and above 5.5 mg/liter, respectively.  Water was completely exchanged in each tank every 48 hours. The fish were kept under a natural photoperiod (Nairobi, Kenya, 1º 16' S, 36º 48' E). The duration of the experiment was 78 days.  

The fish were starved for 24 hours before weighing, and each fish was weighed individually.  Sampling of the fish for determination of whole body compositions was done at the end of the experiment and in this regard, five fish representative of each group (tank) were selected for this purpose.

They were killed with an overdose of MS222, and frozen at -20°C in plastic bags until analyzed. During analysis, all five fish from each tank were chopped into small pieces and thoroughly minced in a blender.

Data collection and analytical procedures

Fish growth and performance were assessed by calculating the following parameters: initial and final absolute weights, weight gain and specific growth rates (SGR, % day^-1) which were calculated as follows: 100 [(ln final wt (g) – ln initial wt (g))/number of experimental days], feed consumption (g/fish), and feed conversion ratio (feed consumption, g/wet weight gain, g).  The protein quality parameters that were assessed included: protein efficiency ratio (PER: wet weight gain, g/protein consumption, g), and productive protein value (PPV: 100*(gain in body protein/protein intake)).

Chemical analyses

AOAC (1984) procedures were used to determine the various proximate fractions of raw materials and diets.  All analyses were carried out in duplicate.  Calcium was determined by atomic absorption spectroscopy (Perkin-Elmer, model 2380), while phosphorus was determined colorimetrically using a Beckman Model Du-8B spectrophotometer at 450 nm wavelength. Samples for the analyses of calcium and phosphorous were digested by wet ashing.  Amino acid analyses of the feed samples were conducted at the University of Alberta in Canada according to standard procedures (AOAC 1998).  Individual amino acids were quantified using a HPLC. 

Statistical analyses

The data were analyzed using PROC GLM of the SAS Statistical Package (1985). The means were compared using Tukey’s test with level of significance set at P < 0.05.  All parameters were analyzed as a 4 x 2 factorial design (4 protein sources at 2 levels of protein intake). 
 

Results and discussion

Chemical composition of the diets

The chemical compositions of the diets used in this experiment are presented in Table 1, while the compositions of the ingredients used are shown in Table 2.  The low protein diets were formulated to contain less DE concentration than the high-protein diets in order to minimize differences in energy:protein ratios between the diets at the two protein levels.  The stipulated DE requirement for tilapia (Oreochromis niloticus) is 3000 kcal/kg DM (NRC 1993). The calculated DE concentrations in most of the low protein diets were just slightly below this level. 

In the diets where fishmeal was partially replaced by the sunflower cakes, phosphorus was balanced by the addition of dicalcium phosphate. 

The amino acid profiles of the diets are shown in Table 3.