Effects of dietary methionine and lysine levels on the performance of juvenile Nile tilapia

Global aquaculture production of tilapia was recorded to be 5.38 million MT in 2016 (FAO 2018) with an annual growth of about 11.7 % since 2010. Outstanding growth of tilapia production is a result of the market demand together with the success of intensive farming, where commercial feed play a significant role. Global tilapia feed production contributes to 17 % (8.16 million tons) of total aquaculture feed (48 million tonnes) produced in 2015 (FAO 2018). It is evident that in the intensive farming system, maintaining feed quality with regard to key nutrients (amino acids and energy) plays a vital role in the overall profitability. Commercial availability of supplemental amino acids provides the flexibility to balance the amino acid needs of fish while replacing the intact protein sources like fishmeal. Our previous study showed that feeding red hybrid tilapia with diets containing only 3 % fishmeal diet supplemented with MetAMINO® (DL-Met) yields significantly better growth performance than those fed with 42 % fishmeal diet (Facts & Figures No. 1610). Available data on the methionine (Met) and lysine (Lys) requirements of tilapia show high variability (Masagounder 2017). The objective of the present study was to evaluate the effects of feeding varying levels of dietary Lys and Met on the growth performances of juvenile Nile tilapia Oreochromis niloticus. This experiment was conducted in collaboration with Dr. Suphada Kiriratnikom at the Faculty of Science, Thaksin University, Thailand.

The study evaluated 12 experimental diets with 3 levels of Met (0.77 %, 0.95 % and 1.15 %) and 4 levels of Lys (1.2 %, 1.6 %, 2.0 % and 2.4 %) in a 3 x 4 factorial design. A total of 540 Nile tilapia fingerlings (0.90 g mean body weight) were allotted into 36 experimental tanks at 15 fish per tank. A glass tank containing 42.5 litres water was used as a replicate. Experimental tanks were operated in a flow through system. Fish were fed one of the 12 experimental diets with 3 replicate tanks per treatment. Diet 1 was formulated to contain 0.77 % Met and 1.20 % Lys (Table 1) and was supplemented with increasing levels of Met and Lys to obtain the desired levels of total Met and Lys in the experimental diets. All the diets were extruded to obtain floating pellets. Analyzed levels of dietary Met, Methionine + Cysteine (M+C) and Lys in the 12 experimental diets are summarized in Table 2. Fish were hand fed three times daily until apparent satiation over a 56-day experimental period. Feed fed per tank, survival, fish body weight were recorded for individual tanks in two-week intervals throughout the study. Mean feed intake of fish per tank was computed by dividing ‘the amount of feed fed to a tank’ with ‘the average count of initial and final fish survived in the tank’ during the two-week interval. Biweekly feed intake was then summed up for the total 8-week period to compute 

total feed intake per fish in each tank (g/fish). Feed intake and body weight data recorded for the whole study period were used to determine specific growth rate (SGR), feed conversion ratio (FCR) and protein retention efficiency (PRE): SGR (%/d) = ln (final body weight) – ln (initial body weight) x 100 / number of days); FCR (g/g) = feed intake per fish / body weight gain; PRE (% intake) = body protein gain x 100 / protein intake per fish, where protein gain and intake were computed by considering protein content in the whole-body of fish (initial and final) and that in the feed. Biomass gain per tank was computed by weighing total body weight of all fish at the final and initial study period in each tank. All the data were analyzed by SAS (version 9.4) using two-way ANOVA with a 5 % level of significance and followed by Tukey’s test when treatment means were different.

Results showed both main effects of both Met and Lys as well as their interaction (Lys x Met) effects on the performance of tilapia (Table 3). Final body weight, SGR and biomass gain significantly improved for the increasing levels of the dietary Lys up to 2.0 % and Met up to 0.95 % (P < 0.05). Further increase in Lys from 2.0 % to 2.4 % showed no improvement in performance (P < 0.05), while increasing the Met level from 0.95 % to 1.15 % impaired the growth performance (P> 0.05). Looking at the interaction effects, it is clearly evident that the response of fish to Met was influenced by the dietary Lys levels (Table 3; Figure 1). At 0.77 % Met, performance of fish improved significantly when the total Lys in the diets was increased up to 2.0 %, while at 0.95 % total Met, a similar trend was noticed although fish fed 1.6 % Lys showed intermediate performance without any significant difference from other Lys levels. At 1.15 % Met, increasing the Lys from 1.2 % to 1.6 % showed no improvement in performance, while the diet containing 2.0 % or 2.4 % Lys resulted in significantly better growth performance compared with the diets containing 1.2 % or 1.6 % Lys.

Feed intake mirrored the response of body weight and growth in both main and interaction effects of dietary Met and Lys, except that at 0.95 % Met, differences in dietary Lys level did not affect feed intake. Increasing Met level from 0.77 % to 0.95 %, and Lys level up to 2.0 % significantly improved FCR (P < 0.05). Improvements in FCR albeit largely followed the trend of body weight gain and growth, only diet with least amount of Met (0.77 %) and Lys (1.2 %) showed significantly higher value (1.91) compared with other treatments (P < 0.05) (Figure 1). Increase in Lys from 1.2 or 1.6 % to 2.0 % significantly improved PRE (27-31 % vs. 37.5 %) (P < 0.05). However, dietary Met level did not significantly affect protein retention. Diet 1 with least amount of Met (0.77 %) and Lys (1.2 %) showed the lowest PRE which significantly differed from other treatment groups (P < 0.05). Within each Met level, diet containing 2.0 % Lys showed significantly better values than diets with 1.2 % Lys.

Increasing the dietary Lys level from 1.2 % to 2.0 % showed significant improvement in survival (65 % vs. 89 %), while further increase in Lys to 2.4 % showed a slight drop in survival and resulted in non-significant difference among other Lys levels. However, dietary Met level did not influence survival. Within each Met level, only at 0.77 %, diet with 2.0 % Lys showed significantly better survival relative to the diet with 1.2 % Lys (P < 0.05), whereas at higher Met levels, dietary Lys levels did not significantly affect the survival (P > 0.05).

Conversely, within each Lys level, increase in Met from 0.77 % to 0.95 % showed significant improvement in feed intake, survival, FCR, and growth performance (P < 0.05), when the dietary Lys level was very low (1.2 %). However, at higher Lys levels (2.0 or 2.4 %), increase in Met level did not influence the performance. This is interesting because when Lys was severely deficient (1.2 %), improvements in the performance of tilapia for the increasing dietary Met level (0.95 %) was unexpected. Nevertheless, better growth performance and feed utilization for the increased Met level (0.95 %) at 1.2 % dietary Lys suggest: (i) improvements in amino acid balance likely had increased feed intake of fish and thus other production parameters, (ii) the possible functional role of Met (e.g., production of glutathione and polyamines) in ameliorating the detrimental impacts of immune and/or metabolic stresses. Data thus indicate that Met requirement of tilapia increases when the dietary Lys is deficient.

In summary, results showed that dietary Lys level 1.2 % or 1.6 % significantly impairs tilapia growth performance, especially when the dietary Met level was also low (0.77 %). Increasing dietary Met level from 0.77 % to 0.95 % ameliorated the effects of Lys deficiency, however, performance of fish were not completely recovered. Increasing the dietary Lys level to 2.0 % showed significant improvements in the growth performance and feed utilization. At 2.0 % or 2.4 % dietary Lys level, Met level did not influence growth performance. Overall, results indicate that dietary Lys level of 1.6 % or less is deficient for juvenile tilapia. Juvenile tilapia performance is optimized when the dietary Lys is 2.0 % diet and Met level is at least 0.77 % diet (as-fed basis). Higher Met level (0.95 %) favours tilapia performance when the dietary Lys is deficient (1.2 %).