Computer Formulation of Fish Feeds
To determine minimum-cost feed formulations, the following information must be available:
1. cost of feed ingredients;
2. nutrient content of feed ingredients;
3. nutrient requirements of the animal;
4. availability of nutrients to the fish from various feed materials; and
5. minimum-maximum restrictions on levels of various ingredients.
Items 1 and 2 are readily available for most commercial feedstuffs. Enough information is available on nutrient requirements (item 3) for several fish and shellfish species to formulate satisvactory production diets for most commercially cultured fish. The availability of nutrients to fish from various types of feed materials must be known in order to make computerized substitutions among ingredients. Digestible or metabolizable energy and nutrient values for commercial ingredients are presented in Appendix A. These values are more limited and variable for fish than for livestock. For example: digestibility of energy from carbohydrates is much less for coldwater fish than for warmwater species: digestibility of phosphorus is less for fish than for livestock, especially for fish without gastric sections in the digestive tract; and the lysine in cottonseed meal is only 75% as digestible as the lysine in soybean meal.
Table 5.2 presents nutrient and ingredient restrictions that have been used for least-cost formulation of catfish feeds. Theoretically, protein level dose not have to be restricted if essential amino acid requirements are well known or unless protein supplies another essential nutritional need, such as energy. Experience has shown that for channel catfish, if the minimum requirements for lysine and thae sulfur amino acids (methionine and cystine) are met, the requirements for the other eight essential amino acids will also be met. Other nutitional restrictions are minimum available phosphorus and digestible energy. Only maximum calcium is considered because dissolved calcium in the water will usually make a deficiency unlikely, but a mineral imbalance can result from high levels of bone ash (indicated by high calcium) in the feed. Whole fish or other animal protein sources have been found to be beneficial in catfish feeds for reasons not explained on the basis of meeting amino acid requirements. Cottonseed meal is restricted, because of free-gossypol toxicity to channel catfish. Carotenoid (xanthophylls) level is restricted, because it imparts undesirable yellow pigmentation to catfish flesh.
Limitations to computer formulation of fish feeds should be recognized. Some considerations may not be programmable because of milling characteristics. For example, sorghum does not extrude as well as corn in processing floating catfish feeds. Maximum advantage of least-cost formulation is realized when the processor has access to and is able to work with a variety of ingredients. In many cases logistics and handling facilities limit the number of ingredients with which the processor can work. As discussed previously, availability of nutrients among feedstuffs varies, but knowledge of the substitutability of one feedstuff for another is essential. Also, when formulating least-cost feeds, considerations other than nutritional must be made, such as the physical, palatability, and toxicological properties of the feed.
Use of quadratic growth responses to select protein or amino acid concentrations for production feeds is used to formulate poultry feeds. Arraes (1983) showed that feeds derived from quadratic programming constraints produced market-size broiler chickens for 5% to 15% less cost than linear programming that used NRC nutrient requirements. The NRC requirements are for a fixed, usually maximum, rate of growth. Quadratic programming takes into account diminishing productivity with increasing nutrient inputs. It also considers changes in value of growth produced as well as changes in cost of production.
In order to use quadratic programming, biological response functions from actual feeding trials are needed. With known growth gain on nutrient concentration in the diet, the most profitable concentration of the nutrients for give cost-price conditions can be derived. Santiago (1985) presented response function (quadratic regressions) for the ten essential amino acids for Nile tilapia, which is discussed in chapter 6. Much of the growth reponse data from fedding trials where fish have been fed various concentrations of protein, amino acids, or energy can be fitted with regression equations that can be used in quadratic programming. This approach to the formulation of maximum profit feeds is not presently used with fish, but it has as much merit for fish as it dose with poultry.