International Journal of Aquaculture 2015, Vol.5, No. 41, 1-20
14
levels compared to diets without phytase (Table 12). It
was also negatively correllated to FCR, PER and SGR
(Table 14). Nitrate levels were significantly affected
by soya bean and phytase inclusion in experimental
diets (ANOVA, P<0.05). Concentration of nitrate
increased significantly in tanks fed diets based on
25%, 50%, and 75% soya bean meal without phytase
(Duncan, P>0.05), but only declined with phytase
addition to diets based on 75% and 100% soya bean
meal compared to diets without phytase (Duncan,
P<0.05). Phytase addition to diet based on 75% soya
bean decreased nitrate level from 33.75±0.01mg/l in
tank of fish fed S3P0 to 1.78±0.01mg/l in tank fed
S3P4 (Duncan, P<0.05). Similarly, nitrate level
decreased significantly in fish tank fed 100% soya
bean from 1.84±0.01mg/l in fish tank fed S4P0 to
1.25±0.01mg/l in tank fed S4P4 (Duncan, P<0.05).
Irrespective of soybean levels, phytase addition of 750
FTU/g had a significantly lower nitrate level
compared to other diets (Tukey, P<0.05). Nitrate level
was lowest in tank of fish that received 100%
soyabean compared to other level of soya bean,
irrespective of phytase level (Tukey, P<0.05). There
was no significant difference between nitrate levels in
fish tanks fed S1P0, S3P4, S4P0, and S4P2 (Duncan,
(P>0.05).
Table 13 Correlation between water quality, treatment and phytase
Pearson
TRT
Phytase
DO
pH
Temperature
Ammonia
TRT
1
Phytase
.327*
1
0.035
DO
-0.047
0.262
1
0.767
0.093
pH
0.138
-0.183
-0.102
1
0.382
0.247
0.519
Temperature
.359*
.831**
0.282
-0.112
1
0.019
0
0.07
0.478
Ammonia
-0.249
-0.105
-.352*
-0.123
-0.254
1
0.112
0.507
0.022
0.438
0.105
Note: *. Correlation is significant at the 0.05 level (2-tailed), **. Correlation is significant at the 0.01 level (2-tailed)
4 Discussion
4.1 Growth and nutrient utilization
Phytase has been extensively used in animal nutrition
to enhance growth and feed and nutrient utilization
(Nwanna and Schwarz, 2007; Nwanna et al., 2008).
Phytase enhanced apparent digestibility of minerals
and their deposition in the fish (Nwanna, 2005). It
increased mineral utilization through the breakdown
of phytate-mineral complex (Singh, 2008), which
resulted in increased growth and feed efficiency of the
fish (Cao et al., 2007; Nwanna et al., 2008) through
improvement in phosphorus bioavailability (Kumar et
al., 2011). Improved growth performance and nutrient
utilization were observed in this research investigating
the possibility of substituting entirely or almost
entirely level of fish meal in fish diet (Cao et al.,
2007). Reduction in mean weight gain by increasing
soyabean levels in the diet (Figure 1) may be due to
high fibre (Table 1) and low phosphorus (Kumar et al.,
2011) and other antinutrient (Haghbayan and Mehrgan,
2015) in soya bean. Mean weight gain with phytase at
250 FTU/g in 75% soya bean (full fat) was better than
at lower (25% and 50%) and higher (100%) fish meal
substitution by soya bean (full fat) with phytase
addition at any level (Table 6). This may be explained
by fact that phytase is effective in hydrolyzing phytate
at low dietary phosphorus (Table 5) and low available
phosphorus level (Cao et al., 2007; Selle and
Ravindran, 2007; Singh, 2008). The total and
available phosphorus (Table 5) in 75% soya bean diet
(full fat) without phytase are much lower (0.75±0.01%
and 0.33±0.01%) than at 25% (1.15±0.01% and
0.63±0.01%) and 50% (1.01±0.01% and 0.44±0.01%).
Phytase addition to 25%, 50%, and 75% soya bean
diet (full fat) significantly improved feed intake
compared to control (100% fish meal). The
improvements in daily feed intake (Figure 3) and feed
conversion (Figure 5) may be attributed to phytate
