Protein efficiency ratio was higher and nitrogen load was lower in groups fed with Ž
. SM than in groups fed with FM Table 7 . Protein retention averaged at 41.7 of intake
with no significant differences among the treatments. Supplemental phytase did not affect protein utilization by the fish. P load was significantly lower in soy-fed groups
Ž .
than in FM-fed groups means 8.48 vs. 4.57 . Supplemental phytase had no effect on P load values. Algal availability of P was significantly higher in the diets than in the fecal
Ž .
Ž matter 35 vs. 23 , and lower in fecal matter of SM than FM groups 9 vs. 27;
. Fig. 2 .
4. Discussion
Decreasing the share of FM protein from 91 to 25 significantly improved the growth of large rainbow trout. Previous papers have reported unaltered or inferior
growth rate by replacement of FM for soy products, while the present experiment is the first to show that growth of trout can be increased by partially replacing FM for
Ž .
commercially available soy products. Olli and Krogdahl 1994 , Pfeffer and Henrich- Ž
. Ž
. freise 1994 and Kaushik et al. 1995 have reported no growth reduction by FM
Ž .
Ž .
replacement for SPC. Pfeffer and Henrichfreise 1994 and Kaushik et al. 1995 totally replaced FM for SPC and supplied the SPC diet with methionine and inorganic
Ž .
phosphate in 60–80 g trout, whereas Olli and Krogdahl 1994 replaced up to 56 of the dietary FM protein in 70 g trout. In the latter paper, methionine was supplied but
composition of mineral premix was not presented. On the contrary, despite phosphate, lysine, methionine and threonine supplements, 75 replacement level decreased the
Ž .
Ž .
growth rate of 12 g trout in Stickney et al. 1996 . In Rumsey et al. 1994 , 100 replacement of FM drastically decreased growth of 3 g trout fed diets with phosphate
Ž .
but without amino acid supplements, whereas in Medale et al. 1998 , increasing the
´
protein ratio between SPC and FM from 3:1 to 4:0 decreased the weight gain of 97 g trout fed closed formula diets. The soy diets in the present trial were supplied to meet
Ž the requirement for lysine and methionine in large trout Rodehutscord et al., 1995,
. 1997 . In addition to differences in dietary amino acid and mineral concentrations, levels
Ž .
of ANF in soy products Rumsey et al., 1993 and quality of FM may affect how SPC and SBM compare to FM.
Supplemental phytase at 1200 U kg
y1
did not improve growth of fish. Several recent papers show enhanced amino acid availability by supplemental phytase in land animals
Ž .
e.g., Sebastian et al., 1997; Martin et al., 1998 . The negative effect of phytates on Ž
protein utilization by fish has been shown Singh and Krikorian, 1982; Spinelli et al., .
1983 , but in feeding experiments with practical-type diets, plant protein utilization has Ž
. been reported to increase Storebakken et al., 1998; Vielma et al., 1998 , remain
Ž .
Ž .
unchanged Lanari et al., 1998 or even decrease Teskeredzic et al., 1995 by phytase Ž
. supplementation of diets or feedstuffs. In Storebakken et al. 1998 , soy protein and in
Ž .
Teskeredzic et al. 1995 , rapeseed protein concentrate were pre-treated with phytase, Ž
. Ž
. whereas in Lanari et al. 1998 and Vielma et al. 1998 , phytase were sprayed onto dry
pellets at 1000 U kg
y1
. According to the only dose–response assay we are aware of, 500 U kg
y1
produced maximum growth effect although fecal P decreased even at 2000
y1
Ž . Ž
. U kg
in channel catfish Ictalurus punctatus Jackson et al., 1996 . Differences in
protein quality are better assessed by feeding a range of dietary protein levels in a Ž
. slope–ratio design March et al., 1985 , and the absence of positive response in the
present study may indicate that the use of supplemental phytase is not economically feasible at soy and protein levels used.
Ž .
Bone ash is a sensitive criteria for available P supply Vielma and Lall, 1998 , which was also shown in the present trial. The slight decrease in operculum ash content in fish
fed high soy levels without supplemental phytase shows that dietary P content of 7 g kg
y1
was marginally deficient for maximal bone mineralization under the present experimental conditions. Requirement of large trout for dietary P has not been deter-
mined, but trout weighing 50–200 g require 0.25 g available P per megajoule available Ž
. Ž
. energy Rodehutscord, 1996 . Based on the data by Vielma and Lall 1998 , the decrease
of 1.7 U in bone ash, as found in the present study, corresponds to about 0.6 g deficiency in available P supply per kilogram diet. Whole body ash or P was not
significantly lower in fish fed soy-based diets, which also suggests that P deficiency was
Ž not severe. P deficiency may increase fat deposition in tissues Eya and Lovell, 1997;
. Skonberg et al., 1997 , but no such effect was evident, thus further indicating only a
marginal P deficiency of fish. Decreased bone ash without changes in growth response is consistent with several P requirement studies which show that to some extent, fish can
Ž adapt to sub-optimum P supply by homeostatic regulation of phosphate balance Vielma,
. 1998 .
Supplemental phytase tended to increase bone ash of soy-fed fish, thus indirectly indicating successful gastrointestinal hydrolysis of phytate in the soy diet. Enhanced
phytate P availability by supplemental phytase has been reported in rainbow trout by Ž
. Ž
. Ž
. Cain and Garling
1995 , Rodehutscord and Pfeffer 1995 , Lanari et al.
1998 , Ž
. Ž
. Storebakken et al. 1998 and Vielma et al. 1998 .
Due to the similar feed efficiencies and whole body P contents, P load decreased by the decrease in dietary P content in fish fed soy-based diets. To further decrease dietary
P content of low-pollution diets, P availability of feed ingredients should be increased to avoid P deficiency syndromes. As demonstrated by the factorial model of Shearer
Ž .
1995 , better feed efficiency would increase the requirement for dietary P. Therefore, changes in factors influencing feed efficiency, e.g., dietary available energy content or
fish size, should also be accompanied by changes in dietary P concentration or availability.
Algal availability of P was lower for fecal matter than in the diets. This would agree with the fact that hydrolyzed phosphates are gradually absorbed from the intestine
Ž .
Nakamura, 1985 , and therefore, the percentage of poorly available P complexes is expected to rise in the food chyme during the food passage. Accordingly, Petterson
Ž .
1986 reported that the share of organic P is lower and HCl-soluble P is higher in the Ž
. fecal matter than in the diets, and Vielma and Lall 1997 found that the percentage of
reactive phosphate in the food chyme is higher in the proximal than in the distal intestine. Algal availabilities of P both in soy diets and in fecal matter of soy-fed fish
Ž .
were lower than those of FM group, thus suggesting that similarly to fish Lall, 1991 , P present in phytic acid is poorly available to algae. Low algal availability of P present in
fecals of soy-fed fish may also indicate that under hypophosphatemic threat, fish utilized
available P effectively, thus increasing the share of poorly algal available P in the fecal matter. It is of future interest to investigate the possibilities to reduce algal availability of
fecal P by a feed formulation.
5. Conclusion
