A study was conducted during the period May to September, 1999 to determine the effect of phosphorus (P) supplementation to beef weaners grazing kikuyu (Pennisetum clandestinum) foggage during winter followed by hay towards the end of winter at Dundee in northern KwaZulu-Natal. Foggaging is defined as the practice of allowing herbage to accumulate on a pasture during the growing season (Gertenbach, 1998). This accumulated herbage is then utilized by grazing animals during the dormant season. A total of 200 crossbred beef weaners (average age six months and average weight 175 kg) was used in the trial. Animals were randomly allocated into five groups, each receiving a different supplemental treatment. The treatments were different levels of P supplementation, administered through free-choice P lick supplements, viz. at levels of 0 g P, 2 g P, 4 g P, 6 g P and 8 g P per animal per day. The experiment was divided into two phases. During the first phase (08-06-99 to 20-07-99) the weaners grazed kikuyu foggage and during the second phase (05-08-99 to 16-09-99) they received Smutsfinger(Digitaria eriantha) hay ad libitum. During phase 1 the average weight gain of the animals in Group 1 (receiving 8 g P/animal/day) was significant higher (6 kg/animal during trial) than that of the control group, which received 0 g P/day/animal. Phase 2 which represented a transition from winter to summer showed that Group 1 had an average weight loss of 0.88 kg/animal but the control group lost an average of 5.13 kg/animal. Throughout the trial blood was collected from five animals randomly selected from each group. Plasma inorganic P (Pi) concentrations remained between 1.94 and 2.58 mmol/L. The average trend during phase 1 was that the Pi concentrations increased, while during phase 2, Pi concentrations dropped more (P ¡Ü 0.5) in the animals of Group 1 (8 g P/animal/day) than in the control. This resulted that during the entire experimental period plasma Pi in Group 5 increased while that in Group 1 decreased. Before the animals entered a grazing strip, herbage samples were collected at 14 to 20 day intervals from the five strip grazed kikuyu camps. Samples were divided into leaves and stems which were analysed separately. Mean foggage calcium (Ca) and P concentrations ranged from 25.0 to 29.0 g/kg DM and 20.0 to 27.0 g/kg DM, respectively, while the Ca:P ratios ranged from 1.20:1 to 1.60:1. Calcium concentrations were significantly lower (P = 0.0026) in the stems than in the leaves of the kikuyu foggage, while P concentrations decreased significantly over time, resulting in extremely low foggage P concentrations towards the end of the grazing season. This decrease in P concentrations resulted in Ca:P ratios of above 1:1 instead of the reported ratios of below 1:1 in kikuyu herbage due to low herbage Ca concentrations. The Ca concentrations in the kikuyu in this study were in the same range as herbage concentrations reported in the literature, but P concentrations were much lower than the reported herbage P concentrations. As with the summer pastures we found that magnesium (Mg) concentrations (ranging from 2.1 to 2.6 g/kg DM) were adequate in terms of the requirements of most classes of ruminants. However, potassium (K) concentrations (ranging from 11.4 to 20.3 g/kg DM) were well in excess of animal requirements, and Mg absorption could be severely inhibited by the oversupply of K The sodium (Na) concentration in kikuyu foggage was, as with herbage, inadequate in terms of animal requirements. Zinc (Zn) and manganese (Mn) concentrations were well above animal requirements but below toxic levels. Copper (Cu) concentrations was above animal requirements during May (onset of the study) but decreased significantly to levels below minimum animal requirements. Selenium (Se) concentrations were found to be largely inadequate in terms of animal requirements. The neutral detergent fibre (NDF) levels in kikuyu foggage tended to increase with maturity while acid detergent fibre (ADF) levels did not vary significantly. Crude protein (CP) concentrations decreased as winter progressed to reach levels of < 60 g/kg DM in the dry foggage, indicating that supplementation of CP would be required by animals grazing the kikuyu foggage in mid-winter. It is concluded that if little or no gain is expected from weaners in winter, the Ca and P concentrations in the kikuyu foggage should be adequate. However, if even slight weight gains are required, supplementation of Ca and P would be necessary when the foggage is dry. The P and CP composition of pastures in general were closely correlated and had a seasonal pattern with maximum levels during summer and minimum levels during winter. We can therefore expect that pastures and kikuyu foggage low on protein will also have a P deficiency. In the dry winter we should therefore firstly supplement for protein and energy and then P. The supplementation of P during winter has not only been very positive (in other studies) with positive results on weight gain during the winter period but also during the months thereafter. It is concluded that beef weaners grazing kikuyu foggage in northern KwaZulu-Natal would require supplemental P to maintain their body weights during winter as well as beef weaners on Smutsfinger hay. The recommended level of P supplementation is 8 g P/animal/day. / Dissertation (MSc (Animal Science))--University of Pretoria, 2007. / Animal and Wildlife Sciences / unrestricted