Concerning the growth performance parameters, published data on the effect of encapsulated butyric acid (ButiPEARL) 0.3gm/kg feed mixed with L-threonine 0.48gm/kg feed on the growth performance and intestinal metabolism of Nile Tilapia fish are limited, however, in our study; this mix recorded the best result. As it had the best final body weight, length, length gain, weight gain, WGR, SGR, and FCR (58.03 gm, 15.55cm, 8.72cm, 43.28gm, 294.58, 3.26, and 1.02, respectively), at the same time the groups that supplemented with ButiPEARL alone or L-threonine alone showed a better growth performance parameters compared with the control one. Our result aligns with [43], who concluded that a butyrate-supplemented diet increased the weight of juvenile sea bream compared to the control one, which might be because the butyrate enhanced the availability of nucleotide derivatives and essential amino acids. At the same time, the energy provision for enteric cells is improved by a reduction in amino acid and glucose oxidation associated with using butyrate as fuel. Additionally, butyrate increased the activity of transmethylation. Also, [1] found that the dietary inclusion of a protected form of Butyric acid in aquafeeds improved growth performance and fish productivity. That might be due to increased butyrate efficiency when fed in an encapsulated form (ButiPEAR) [6]. Regarding the supplementation of threonine alone, our result aligns with [7], who concluded that dietary supplementation of threonine increased the growth rate of grass carp. Additionally, [46] found that supplementation of L-threonine (13.5 g/kg feed) for fish improved the growth performance parameters significantly (p < 0.05) in comparison with the control one. Our result disagreed with [29], who recorded a non-significant difference in dietary threonine supplementation.
Concerning the economic analysis. Although a relatively higher feed cost and total costs for treated groups compared with the control one, they showed a significant increase in the total return and net profit than the controldue to better FCR with higher weight gain. Our economic analysis showed the highest profitability was for the fish group that supplemented with ButiPEARL 0.3gm/kg feed mixed with L-Threonine 0.48gm/kg feed in its diet ($74.49). Our result agreed with [25] who concluded that cost efficiency reduces with the feed conversion ratio. Our study suggests that there are still opportunities to enhance feed efficiency and profitability in Egyptian fish farms.
In the present investigation, a fish-fed diet containing 0.48 g/kg L-threonine and 0.3 g/kg, especially in the mixed group (6), significantly enhanced biochemical, immunological, and antioxidative parameters and digestive enzymes. The activity of antioxidative enzymes, such as superoxide dismutase, catalase, and glutathione peroxidase, increased, indicating a reduction in reactive oxygen species and harmful free radicals [34].
The supplementation of L-threonine and sodium butyrate significantly increased the activity of CAT compared with the control, which is a marker of lipid oxidation [31]. Similar results were reported by [13] in juvenile grass carp. The supplementation of L-threonine and sodium butyrate also led to lower activity of aspartate aminotransferase and alanine aminotransferase, enzymes involved in amino acids metabolism for gluconeogenesis. The SOD enzyme transforms the superoxide anion to hydrogen peroxide, which is further used as a substrate by GPx and CAT enzymes [27]. This finding is consistent with previous studies by [51] for hybrid catfish and [20] for blunt snout bream.
In the present study, the least MDA was observed in the higher dose of L-threonine-(0.48 gm) and 0.3gm sodium butyrate within the diet, which might be because of a vital role of MDA as a margin for collapsed lipid molecules inside the fish tissues [17]. [51] showed the same results for MDA in juvenile hybrid catfish and carp. [7] also, found the same results but for grass carp.
AST and ALT activity lowered significantly for a fish supplemented with 0.48 gm L-threonine and 0.3 gm sodium butyrate in its diet. They have the principal role in the metabolism of L-amino acids for gluconeogenesis and have a role in the different physiological phenomena concerning protein and carbohydrate metabolism. Our findings align with [19], who found the same result for blunt snout bream and [23] for grass carp. Our study indicated that dietary L- threonine and sodium butyrate supplementation diets enhanced digestion and digestibility for Nile tilapia, which was reflected by a significant increase in lipase and amylase compared with the control group.
Total protein demonstrated a non-significant increase for the treated groups compared to the control in the 3rd week. But at the end of the experiment, it showed a significant increase, especially in the supplemented groups 4, 5, and 6 (5.07, 5.85, and 7.49, respectively). Also, the albumin level was significantly higher in treated groups at half and end of the experiment. The current findings agreed with those reported by [46]. Data from the current study shows that the nonspecific immune, especially the nitric oxide level, was significantly higher for the supplemented groups compared with the control one. That might be because of the ability of these dietary additives to improve the immune system activity. Supplemented groups increased the IgM significantly higher than the control group. The highest level was for the mixed group, as IgM is the primary defence molecule that mediates the immune response in freshwater fish [40].
In our study, there were significant improvements in intestinal villi height and their thickness. Meanwhile, an increase in surface area is capable of greater absorption of digested nutrients [5]. The data above suggested that diet-elevated threonine increases the absorptive capacity of Nile tilapia, as noticed by [23] in sub-adult grass carp and [13] in juvenile Jian carp; in terms of intestinal morphology, the supplementation of L-threonine and sodium butyrate resulted in improvements in intestinal villi height, thickness, and surface area. This suggests an increased absorptive capacity for digested nutrients [45]. Similarly, Threonine deficiency, on the other hand, reduced juvenile grass carp's intestinal function and morphology, as suggested by [16].