EFFECTS OF NA INTAKE ON NA CONSERVATION, DIGESTION AND MINERAL METABOLISM IN GROWING RUMINANTS FED STYLOSANTHES-HAMATA CV VERANO

Effects of adequate (2 g NaHCO3/1 drinking water) or low Na intake on Na conservation, liveweight gain, roughage digestion and mineral metabolism were studied in nine young goats and six cattle for 3-6 months fed a diet of Stylosanthes hamata cv. Verano containing < 0.5 g Na/kg. Growth rate, OM intake and digestibility were not affected by level of Na intake (1.42 g/d vs. 0.33 g/d) in goats even though Na secretion in saliva and loss in urine was actively restricted by elevated plasma aldosterone concentration in the low Na group. OM and Na intake (2.58 g/d) were relatively higher in steers and consequently there were no signs of active Na conservation. In goats, unavoidable Na loss in faeces and urine were calculated from regression equations to be 0. 14 g and 0.001 g, respectively. Calculated daily Na maintenance requirements for a 20 kg goat and 200 kg steer are less than 0.33 g/d and 1.88 g/d, respectively. A 0.8 g reduction in Na intake to 0.33 g/d in goats evoked a 3-fold increase in plasma aldosterone which persisted for 2 months until a new Na equilibrium was established. Once equilibrated, a further small decrease (0.1 g) in Na intake prompted a similar aldosterone response. Aldosterone induced conservation of Na maintained goats in positive balance, but salivary Na:K ratio did not fall below 4:1 which would be indicative of a deficiency. Data are supportive of the 'set point' theory of Na conservation in which aldosterone is secreted to defend a critical mass of body Na which is close to maintenance requirements. Level of Na intake did not significantly affect macromineral and N metabolism although goats appeared to have a higher apparent absorption rate (> 7 %; P < 0.05) of K, Ca, Mg, P and N than steers which compensated for the relatively lower intake of these nutrients in goats. In conclusion, Na conservation mechanisms re-established Na equilibrium and thus prevented a deficiency in goats fed a low-Na verano diet. Na requirements for growing goats and cattle in this study appear similar when related to metabolic body size but goats were in better N balance than cattle.