Figure 10 illustrates the determination in chicks fed two commercial fish meals, one of standard quality and the other processed under low temperature conditions. In this study enzymically-hydrolysed casein supplemented with amino acids to meet all amino acid needs was used to measure basal endogenous loss at the zero test protein level. Instead of determining the basal endogenous loss in every trial, a mean value can be determined and used to correct apparent digestibility established at a single level of inclusion of test protein. Such values have been termed standardized ileal digestibility (Boisen, 1997; Jansman ., 1998; Rademacher ., 1999a,b).
Dietary protein is not used efficiently as a source of energy. Although the gross energy of protein is greater than that of carbohydrate (23.6 kJ/g v 17.4 kJ/g for starch), when protein is used as an energy source the N has to be excreted as ammonia (fish), urea (mammals) or uric acid (birds). The ME value of protein at zero N retention takes into account the loss of energy in the excreta, such that the ME of protein and carbohydrate are approximately similar. The ME value for mammals and birds, however, does not take into account the energy costs of synthesising urea or uric acid and the cost of excretion in the kidney. Net energy (NE) of the diet represents the useful energy used to replace the losses of maintenance and the net deposition of energy as new tissue in growth or milk secretion during lactation, after subtracting the heat losses of metabolism.
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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.
Dietary proteins, particularly the legume proteins, also have antigenic properties. These may have adverse effects on intestinal morphology, on intestinal myoelectric activity affecting the rate of passage of digesta and growth of calves fed milk replacers (Lallès, 1993). In the newly weaned pig they have transient effects, reducing growth in the first week post weaning, until tolerance is developed (Miller ., 1994; Rooke ., 1998; Figure 18). Similarly, soybean meal and an alcohol extract of soya meal (soybean molasses) caused inflammatory responses in the distal intestine of salmon (Krogdahl ., 2000). These reactions may predispose to infections. In both calves and pigs these adverse reactions may be accompanied by diarrhoea and death. The allergenic proteins, glycinin and b-conglycinin, in soya are resistant to normal proteolytic digestion, either in the abomasum or intestine. Both can be detected immunologically in the duodenum. b-conglycinin is most resistant to acid digestion in the stomach, whereas glycinin is more resistant in the intestine and can still be found in ileal digesta (Sissons and Thurston, 1984; Lallès ., 1999). Apparent N digestibility of commercial soya preparations in the calf varies greatly and is best predicted by the concentration of immunoreactive b-conglycinin (Lallès ., 1996). The antigenicity of these storage proteins is not removed by normal solvent extraction, heating or steam desolventisation. They can be denatured by hot aqueous ethanol or by partial acid or enzymic hydrolysis.