Abstract:
Proc. Aust. Soc. Anim. Prod. Vol. 18 RE-ESTIMATION OF THE PHOSPHORUS REQUI REMENTS OF CATTLE FED ROUGHAGE DIETS J.H. TERNOUTH* Controversy has raged for many years over the phosphorus (P) requirements of cattle and sheep (ARC 1980; Field 1983), due to the disagreement over the endogenous faecal P (EFP) losses and true absorption coefficients (TA). When plasma inorganic P (PP) is greater than 60 mg /l, surplus body P may be excreted in the urine and surplus dietary and salivary P excreted in the faeces (Challa et al. 1989) but when PP is less than 40 mg/l, cattle are in negative P Thus, when PP is 50 mg/l, the dietary P balance and losses are minimised. level is adequate, Kinetically, salivary secretion is related to dry matter intake (DMI) (Doyle et al. 1982) and ,total salivary P is related to PP (Challa et al. 1989). In addition, whenever moderate levels of P are fed, TA is greater than 0.75 and consequently EFP is a fixed proportion of salivary P. Ternouth (1989) has confirmed these relationships in cattle fed roughage diets containing low or moderate levels of P for extended periods as he found that EFP (mg /kg LW) is related to DM1 (g/kg LW) and PP (mg/l). The P requirements of cattle were calculated from this equation, with DM1 based on Minson and McDonald (1987) and a PP of 50 mg/L and then factorially using a TA of 0.75. A value of 8 g P/kg LW gain was used for growth (ARC 1980). Table 1 Estimates of the phosphorus requirements (g/d) of growing cattle The estimates of EFP (12.8 - 16.2 mg/kg LW) are in keeping with the field data obtained from cattle grazing buffel grass pastures (Ternouth et al. 1990). The P requirements are higher than those recommended by ARC (1980) at 200 and 300 kg LW but lower at 400 kg LW. They are also much lower than NRC (1984). However, when compared with data from practical feeding trials the calculated requirements are still higher than the 6.6 g P/d consumed by 160 kg cattle gaining 0.5 kg/d (Little 1980). The estimates appear to be in agreement with the 8-12 g P/d required for the maintenance of mature breeding cattle in longterm experiments (Call et al. 1986). The diets offered to the cattle in both experiments contained 1.2 g P/kg DM. (1980). 'The Nutrient Requirements of Ruminants'. (Agric. Res. Count.: London). CALL,J.W,, BUTCHER, J-E., SHUPE, J-C., BLAKE, J.T. and OLSON, A.E. (1986). Amer. J. Vet. Res. 47: 475. cm, J., BRAITBWAITE, G-D. and DHANOA, M-S. (1989). J. Agric. Sci. 112: 217. DOYLE, P.T., EGAN, J.K. and THALEN, A-J. (1982). Aust. J. Agric. Res. 33: 573. FIELD, AX, (1983). Livestock Prod. Sci. 10: 327. LITTLE, D.A. (1980). Res. Vet. Sci. 28: 258. MINSON, D.J. and MCDONALD, C.K. (1987). Trap. Grassl. 21: 116. NRC (1984). 'Nutrient Requirements of Beef Cattle', 6th edition. (Nat. Acad. Sci.: Washington, DC). TERNOUTH, J.H. (1989). In 'Recent Progress on Mineral Nutrition and Mineral Requirements of Ruminants', p. 19. (Kyoto University: Kyoto). TERNOUTH J-H., MCLEAN, R.W., MACDCNALD,R.N. and ADAMU, A.(1990). Proc. Aust. Soc. Anim. Prod. 18:396. ARC + Department of Agriculture, U'niversity of Queensland, St Lucia, Qld. 4067. 560