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Batterham, E.S., Andersen, L.M., Baigent, D.R. & Darnell, R.E. 1990b. A comparison of the availability and ileal digestibility of lysine in cottonseed and soya-bean meals for grower/finisher pigs. , 64: 663-677.
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The legume proteins contain protease inhibitors, lectins, tannins, phytates, antigenic proteins flatulence factors (oligosaccharides), and oestrogens (Huisman and Jansman, 1991). To this list can be added high fibre (non-starch polysaccharides) levels (which limit the inclusion levels in many situations) and contamination with mycotoxins. The brassicas contain glucosinolates, tannins, phytate and have high fibre levels. The relevance of the different factors varies with animal species. Processing is available to deal with several of these problems - dehulling, heating, solvent extraction and addition of enzymes as appropriate for the target animal species. Plant breeding, as in production of double zero rapeseed or canola meal, is another avenue. Reference has been made previously to the different susceptibility of calves, fish and early-weaned piglets to antigenic proteins. For both calves and fish, the general principle is that the greater the degree of processing of vegetable proteins (with an increase in protein content from meal, to protein concentrate, to protein isolate), the better the performance but also the greater the feed cost. The improvement may be due to removal of a number of the factors, but the exact reason is not known. Even using soya protein concentrate with 68 percent protein content of high digestibility, growth of turbot and salmonids is significantly reduced when more than 50 percent of the fish meal protein is replaced (Day & Plascencia Gonzalez, 2000; Sveier ., 2001). Studies of digestibility of canola meal for trout also suggest that high levels of fibre, either alone or with phytate, result in poorer digestibility of protein (Mwachireya ., 1999). Insoluble fibre increases the rate of passage through the intestinal tract, while soluble fibre increases the viscosity of the digesta and reduces the diffusion of nutrients to the absorptive mucosa. Pea fibre has been shown to increase the flow of water, mucus and endogenous N to the ileum of pigs. The endogenous N loss was best described as a function of the water holding capacity of the diet (Leterme ., 1998). Antigenic proteins may also enhance the turnover of intestinal mucosal proteins. Desquamated epithelial cells and mucus in turn encourage the growth of bacteria in the intestine. Bacterial degradation of this protein may result in production of ammonia, which is absorbed and lost via urine. True endogenous faecal N loss is then underestimated and digestibility overestimated. In addition, and possibly of greater concern, are the additional energetic costs of enhanced intestinal protein turnover.
Appetite or voluntary feed intake is important in all species but especially so in aquaculture, where feeds must first attract fish or crustaceans and then be palatable to be accepted. Amino acids, betaine and inosine appear to act as attractants. Glycine, proline, taurine and valine appear to be preferred by carnivorous fish, while aspartic and glutamic acids are preferred by omvivorous fish (NRC, 1993). Trimethylamine and its oxidation products, as well as highly oxidised oil, are deterrents for salmonids. Thus freshness of fish used in the preparation of fish meal and stability of the oil, through use of antioxidants, are important factors for quality meals.
Fish meal and meat and bone meal are good sources of calcium and phosphorus in an ideal ratio of 2:1, and these are of high availability when included in diets for mammals or birds. Plant protein concentrates have much lower levels, especially of calcium, with a ratio more in the region of 1:2. Furthermore, the phosphorus is mainly present and bound as phytate, so the total phosphorus is about one third available for poultry and fish. The deficiency of calcium in both cereals and plant protein concentrates is readily and economically corrected with limestone, but supplementary phosphorus sources are expensive. The high level of phytate P also leads to high faecal P output and environmental pollution. Phosphorus is the main cause of eutrophication in aquaculture. In many countries legislation limits the amount of P that can be disposed of in manure on land. This has given added impetus to the development of phytase enzyme that can be added to the diet to hydrolyse phytic acid and improve the availability. Consequently, dietary P levels can be reduced and less P is excreted. In aquaculture, much of the calcium requirements are obtained by uptake from the water but P must be supplied from the diet. The digestibility in fish of P from fish meal is surprisingly low and variable and appears also to be inversely related to the ash content (NRC, 1993). Replacing a small part of the fish meal (51.8 percent reduced to 41.0 percent of diet) with 20 percent soybean, canola or peanut meals, increased diet true digestibility of P from 21.5,to 40.6 -and 43.4 percent respectively. Similarly, the replacement reduced total P from 1.74 to 1.5 - and 1.6 percent. If the same amount of P was absorbed, this reduction in intake would increase true digestibility to 24.6 percent. The much larger increase in digestibility reflects a 70.7 percent increase in the amount absorbed, despite replacing fish meal P with mainly phytate P of zero digestibility (Riche and Brown, 1999). The reduction in calcium supply by substitution of the part of the fish meal, is the most likely cause of the improved digestion of P.
Protein concentrates are also a source of many other nutrients that should be taken into account when formulating diets. These include the major minerals, Ca, P, Na, K, Cl, vitamins, including B12, choline and vitamin D and essential fatty acids. Consideration should be given to these nutrients because they may be either beneficial or in some cases, can be at such high concentrations as to be detrimental and limit the inclusion level.