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The study was designed to evaluate the effects of phosphorus and non protein nitrogen supplementation on weaner cattle grazing natural pastures on the Fitzroy Pastoral Research Station, north Western Australia. The study was repeated over the period July 1971 to April 1976, with four drafts of weaners which grazed Mitchell grass pastures during the dry season and curly spinifex-ribbon grass pastures during the wet season. In each year, one of four groups received either no supplement (control), urea plus sulphuric acid, phosphoric acid, or urea plus ammonium sulphate plus phosphoric acid. The supplements were measured into and mixed with the drinking water. Weaners given phosphoric acid gained between 10.5 and 21.7 kg a head more than those not given phosphorus. The response was significant in two of the four dry seasons, three of the four wet seasons and for every ye= when both seasons were considered together. Urea supplements increased liveweight gain over the dry season and depressed liveweight gain over the wet season. There was no advantage to urea when the two seasons were combined. Cattle given supplements through the drinking water drank less water and more of those given nitrogenous compounds died than those not receiving supplements.