REDEFINING THE REQUIREMENT FOR TOTAL SULPHUR AMINO ACIDS IN THE DIET OF JUVENILE BARRAMUNDI (Lates calcarifer) INCLUDING ASSESSMENT OF THE CYSTEINE REPLACEMENT VALUE.
This study was designed to re-examine a previously published estimate of the methionine (Met) and total sulphur amino acid (TSAA) requirement of juvenile barramundi (Lates calcarifer) with a view for further study.
An initial experiment failed to find a plateau in response, despite dietary TSAA provision well in excess of the published requirement, suggesting this estimate may have been inadequate. A second experiment was then designed in which a greater range of methionine inclusion levels were assessed (8.6 - 21.4g kg-1 diet DM). Triplicate groups of fish (initial weight: 36.4g ± 8.3g) were fed the diets for a period of 49 days. A plateau and subsequent depression in growth, as well as significant (p<0.05) effects of dietary Met inclusion on %BW gain, FCR and PRE were observed at the conclusion of this experiment. The best fitting of nine nutrient response models, the five-parameter Saturation Kinetics Model (R2 = 99.97), predicted a requirement for TSAA of between 17.7 (95% of maximum response) and 19.8g kg-1 DM (99% of maximum response) in a diet with 592g kg-1 CP and 6.6g kg-1 Cys (1.9-2.2% CP Met + 1.1% CP Cys). This TSAA requirement is equivalent to 43-48% of the lysine content of the diets. The applicability of this mode of expression and it's relation to the ideal protein concept is discussed as is the application of different response models to the data. The impact of dietary Met:Cys ratio was also investigated with results suggesting at least 40% of dietary Met can be replaced with Cys without significantly affecting animal performance.
This study represents a more comprehensive assessment of the TSAA requirements of juvenile barramundi than that previously undertaken, providing a revised estimate while highlighting the importance of response model choice and mode of expression when reporting nutrient requirements of animals.