V. Ravindran2
G. Ravindran2
W.L. Bryden3
1 Faculty of Veterinary Science, University of Sydney, Camden NSW 2570, Australia.
2 Institute of Food, Nutrition and Human Health, Massey University, New Zealand.
3 School of Animal Studies, University of Queensland, Gatton QLD, Australia.
Supplementation of lysine-deficient, but otherwise nutritionally adequate, diets with exogenous phytase or lysine monohydrochloride (Lysine HCl) increased weight gain and feed efficiency of broilers. However, significant treatment interactions were observed, as the effects of phytase on growth performance were more pronounced in lysine-deficient diets.
Phytase increased the apparent ileal digestibility (AID) of amino acids but additional lysine also increased the digestibility of certain amino acids and some significant treatment interactions were observed. It is suggested the increases in digestibility induced by lysine, and interactions with phytase, may be mediated via acid-base homeostasis and intestinal uptake of amino acids.
Introduction
The magnitude of the positive influence of phytase on the utilisation of protein/amino acids in poultry, and the corresponding negative effects of dietary phytate, remains a topic of debate (Augspurger and Baker, 2004). Moreover, the underlying mechanisms have not been completely identified. The present study investigated the effects of phytase supplementation in broiler diets containing two levels of lysine.
Materials and methods
The experimental design was a 2x2 factorial where treatments consisted of two levels of dietary lysine (10.0 and 11.8 g/kg), without and with phytase (Natuphos®, 500 FTU/kg). The phosphorus-adequate (4.5 g/kg nonphytate-P) diets were based on a wheat-sorghum blend with monocalcium phosphate (MCP) as the major P source.
The basal diet contained Celite as an inert marker and was formulated to contain 13.1 MJ/kg ME, 197.5 g/kg protein, 10.6 g/kg phytate and to meet the requirements of all amino acids except that of lysine.
Lysine HCl was included in the lysine-adequate diets and MCP was reduced (500 FTU ≡ 1 g P as MCP) in phytase-supplemented diets. The dietary treatments were offered to male broiler chicks (Cobb) from 7-28 days of age when growth performance was determined. From days 24-27, total excreta were collected to determine apparent metabolisable energy (AME) of the diets.
At 28 days the birds were euthanised and samples of ileal digesta and toes taken to determine the AID of amino acids and percentage toe ash by standard procedures (Selle et al., 2003). Experimental data was subjected to analyses of variance (SPSS Inc. Chicago, IL).
Results
Both phytase and lysine supplementation enhanced (P < 0.001) weight gain and feed efficiency without influencing feed intake (Table 1). Treatment interactions (P < 0.05) were observed for weight gain and feed efficiency because the effect of phytase was more pronounced in lysine–deficient diets with increases of 5.3% and 3.4% for weight gain and feed efficiency, respectively. Phytase increased (P < 0.001) AME of diets by 0.34 MJ/kg DM, which was independent of dietary lysine, and ileal digestibility of crude protein by 2.8%. Treatments had no influence on toe ash contents (data not shown), which is indicative of the P adequate status of the diets and that the effects of phytase were not related to enhanced P availability.
Phytase increased (P < 0.05-0.001) the apparent ileal digestibility of essential amino acids (Table 2). The percentage increase in AID of threonine (6.1%) was the most, and methionine (0.6%) the least, pronounced following phytase supplementation, which is a typical pattern.
Lysine supplementation increased (P < 0.01-0.001) the digestibility of some (isoleucine 3.9%, lysine 3.6%, methionine 0.9%, phenylalanine 3.8%, valine 4.0%) essential amino acids. Lysine also significantly increased the digestibility of certain non-essential (aspartic acid, glutamic acid, glycine, tyrosine) amino acids; whereas, phytase increased the digestibility of all non-essential amino acids (data not shown).
Significant treatment interactions (P < 0.05) were observed for the ileal digestibility of five essential (arginine, lysine, phenylalanine, threonine, tryptophan: and four non-essential (aspartic acid, glutamic acid, glycine, serine) amino acids, where responses to phytase were more pronounced in lysine–deficient diets.
Discussion
Phytase enhanced growth performance of broilers offered lysine-deficient diets, where the 4.5% increase in lysine AID was probably partly responsible. Increased digestibility of other amino acids and AME probably also contributed to this improved performance generated by phytase. It is interesting that AME responses to phytase were independent of dietary lysine levels. However, increases in AID of certain amino acids following the addition of lysine and, moreover, interactions between lysine and phytase in this respect, were unexpected and the underlying mechanisms are not clear.
In the present study, the dietary electrolyte balance (DEB = Na+ + K+ - Cl-) of the broiler diets was calculated to be 155 mEq/kg, which is less than the optimum range of 250-300 mEq/kg, recommended by Johnson and Karunajeewa (1985). Basic amino acids, including lysine, are thought to be important regulators of acid-base balances (Austic and Calvert, 1981). Although the effects of DEB on amino acid digestibility has received little attention, dietary electrolyte balance and amino acid metabolism are intimately related (Patience, 1990). Given the low DEB values in the present study, it seems possible that lysine influenced the AID of amino acids via its impact on acid-base balance and the uptake of amino acids from the gut. Na+-K+-ATPase activity transfers K+ into, and Na+ ions out of, enterocytes, which influences the uptake of nutrients from the gut (Gal-Garber et al., 2003) and a large proportion of amino acids are absorbed by co-transport with Na+ (Sklan and Noy, 2000). It is noteworthy, therefore, that Cowieson et al. (2004) reported that phytate significantly increased Na excretion in broilers to a substantial extent and phytase tended to reduce it. These workers suggested that the increased secretion of Na+ into the gut is to buffer the polyanionic phytate molecule, which would be countered by phytase. Possibly lysine altered the acid-base balance and absorption of amino acids from the gut and phytase also influenced, or was influenced by, the acid-base balance and this may be responsible for the significant interactions observed in digestibility of amino acids.
In conclusion, it is suggested that the influence of lysine on AID of certain amino acids, and its interaction with phytase in this respect, may stem from their effects on acid-base homeostasis in broiler chickens. The determination of the effects of phytase on AID of amino acids and growth performance of broilers offered diets with differing electrolyte balances appears to be justified.
References
Augspurger, N.R. and Baker, D.H. (2004). Journal of Animal Science, 82: 1100-1107.
Austic, R.E. and Calvert, C.C. (1981). Federation Proceedings, 40: 63-67.
Cowieson, A.J., Acamovic T. and Bedford M.R. (2004). British Poultry Science, 45: 101-108.
Gal-Garber, O., Mabjeesh S. J., Sklan, D. and Uni, Z. (2003). Poultry Science, 82: 1127-1133.
Johnson, R.J. and Karunajeewa, H. (1985). Journal of Nutrition, 115: 1680-1690.
Patience, J.F. (1990) Journal of Animal Science, 68: 398-408.
Selle, P.H., Ravindran, V., Ravindran, G., Pittolo, P.H. and Bryden W.L. (2003). Asian-Australasian Journal of Animal Sciences, 16: 394-402.
Sklan, D. and Noy, K. (2000). Poultry Science, 79: 1306-1310.
From Proceedings of the "17th Australian Poultry Science Symposium", New South Wales, Australia.
