Sodium supplementation of steers grazing Stylosanthes native grass pastures in northern Australia.

Abstract

Proc. Aust. Soc. Anim. Prod. Vol. 17 SODIUM SUPPLEMENTATION OF STEERS GRAZING STYLOSANTHES-NATIVE GRASS PASTURES IN NORTHERN AUSTRALIA W.H. WINTER* and R.W. McLEAN** 485 Native grasses and introduced legumes growing in the tropics often have low sodium contents (Hunter et al 1979), but the degree to which sodium deficiency limits animal production in northern Australia is poorly documented. We present data showing a dramatic production response to sodium supplementation. A grazing study to evaluate low input management options in the semi-arid tropics was established at a site 50 km S.W. of Katherine (132oE, 14.30s) on an infertile loamy-red earth soil (Gn2.14). Native grass pastures dominated by Themeda triandra and Sehima nervosum were oversown with Stylosanthes spp. at two fertility levels (nil fertilizer (noF) and 100 kg/ha of superphosphate at establishment and 25 kg/ha annually (F)). There were three stocking rates at each fertility level (0.45, 0.6 and 0.75 steers/ha for noF treatments and 0.6, 0.8 and 1.0 steers/ha for F treatments). The pastures were stocked in December 1975 with 18 month old Brahman x Shorthorn steers (mean initial liveweight 240 kg). The noF 0.6 and F 0.8 treatments were given ad libitum access to salt as a compressed block from 2 March till 7 June 1978. The unprotected blocks were weighed weekly to assess intake. Samples of pasture and drinking water were analysed for sodium (Na) and bovine saliva samples for both Na and potassium (K) (Murphy and Plasto 1973). Table 1. The effects of fertilizer treatment and salt supplementation on the mean liveweight gain (kg/hd/day) of steers from 2 March to 7 June 1978. The mean liveweight gain of supplemented steers was 0.4 kg/day greater (P(O.01) than for unsupplemented steers. The response occurredin both the noF and F treatments,but gains were higher with fertilizer (Table 1). Saliva samples taken before supplementation had a mean Na:K ratio of 0.2:1. By June, the Na:K ratio of supplemented steers was 17:l compared to 1.3:1 for the control group. Salt intake, as measured by block disappearance, averaged 290 g/hd/day for the first week of supplementation then declined to 200, 185 and 140 g/hd/day during the next three four week periods. The pastures contained less than 0.01% Na. The liveweight response to sodium supplementation was nearly double that obtained by Murphy and Plasto (1973) with cows, and Hunter et al (1979) with steers and was consistent with the depleted sodium status of the cattle and the deficiency of sodium in the diet. Hunter, R.A., Siebert, B.D. and Webb, C.D. (1979). Aust. J. Exp. Agric. Anim. Husb. 21: 517. MurphnMrand Plasto, A.W. (1973). Aust. J. Exp. Agric. Anim. Husb. 13, 369. '\' CSIRO Darwin Laboratories, Private Bag No. 44, Winnellie, N.T. 5789. -'% CSIRO Cunningham Laboratory, ,306 Carmody Road, St Lucia. Q. 4067.