Types of processed feeds - Aquaculture: Artificial feeds

Types of processed feeds

The main purpose of using processed feeds is to ensure that the animals under culture receive a balanced diet that meets their nutritional requirements. The use of processed feeds also reduces uncertainties in the quality and availability of food, characteristic problems with unprocessed feedstuffs. As compound feeds are formulated according to nutrient specifications, the product quality can be kept uniform, even when ingredient substitution has to be made at times of shortages. As mentioned earlier, processed feeds may be formulated to serve as complete feeds to meet all the nutritional needs of the animal or as supplementary feeds to augment the major nutritional elements. Supplementary diets are usually less expensive than the complete diets.

From the point of view of composition, three types of processed feeds can be recognized: purified, semi-purified and practical. Purified diets are made with synthetic amino acids, fatty acids, carbohydrates of precisely known com-position and chemically pure vitamins and minerals. Naturally, such diets are comparatively expensive and are used only for research purposes. Semi-purified diets contain natural ingredients in as pure a form as is available, as for example casein, corn oil, fish oil, etc. These are commonly used as test diets in nutritional studies to determine the efficiency of different levels of dietary components in terms of food conversion or growth. Lovell (1980) quotes examples of such test diets (Table 7.20). The third category of diets, practical diets, comprise

formulations of available ingredients and would consist to a large extent of natural products such as fish meals, oil seed meals, cereal grains, etc. They are aimed at fulfilling the nutritional needs of the animal under culture, at the minimum cost.

Another classification of artificial compound feeds into moist and dry feeds is based on the consistency or water content. Moist or wet feeds are preferred as practical or test diets, because they can be prepared without exposure to heat while hard-pelleting and drying processes result in nutrient losses. Grinding and mixing machines only may be needed. Many species prefer moist diets and show better feed conversion ratios and growth on such diets. These diets are, however, susceptible to spoilage if not used immediately after preparation. Freezing will be required to prevent spoilage in storage by the action of micro-organisms and deterioration of oxygen-sensitive nutrients, such as ascorbic acid. Dry feeds, on the other hand, have the advantage of easy storage and transport. They contain only about 8–11 per cent water and are relatively more water-stable. By the use of appropriate binders and processing techniques, the water stability can be adjusted to suit the feeding habits of the species. Similarly, the density of the pellets can be adjusted to make them float or sink in water, by using extrusion techniques in processing.

The sizes and shapes of pelletted dry feeds are easily adjusted by the use of suitable dies. Different sizes of pellets, with different nutrient composition, are needed to feed successive growth stages of a species. Sizes vary from finely ground granules and encapsulated forms for first feeding, through crumbles and small pellets up to large pellets of about 1 cm diameter and 1.5 cm length for adults. The composition is related to the nutrient requirements of each life stage. Starter and fry feeds have a relatively higher protein content and in many cases contain animal proteins. For example, reasonably good trout feeds have a protein content of about 50 per cent, of which 75 per cent is recommended to be of animal origin. Grower feed has a lower protein content of 45 per cent (with 70 per cent animal protein) and finishing feed has 40–45 per cent (with 60 per cent animal protein). Brood fish are fed on special high-protein diets.

Other types of practical feeds that need to be considered are medicated feeds incorporat-ing antibiotics and curative, prophylactic or growth-promoting drugs, and pigment-fortified feeds containing pigment compounds such as carotenoids. When the cultured species does not have access to sufficient natural pigment sources in aquatic flora and fauna, it is often necessary to include a suitable source in their diets to obtain the natural coloration to make it acceptable to the consumer. Meals from various crustaceans such as shrimps and lob-sters are commonly used as pigment supplements. Astaxanthin or canthaxanthin is often added in salmonid diets to impart an attractive red colour to trout and salmon grown in cages.