PHOSPHORUS STUDIES IN PIGS .1. AVAILABLE PHOSPHORUS REQUIREMENTS OF GROWER FINISHER PIGS

Two experiments were conducted to determine the available P requiremen ts of grower and grower/finisher pigs and to define the conditions for conducting a growth assay for P availability. In the first experiment , diets with four levels of calculated available P (1-4 g/kg) and four Ca: available P ratios (1.7-2-9) were used to determine the available P requirements of grower pigs. The diets were formulated by substitut ing the required amounts of limestone and sodium tripolyphosphate for sugar in a soya-bean meal and sugar-based diet. In addition to measuri ng growth responses, a range of bones were examined to determine the m ost suitable criteria for assessing the response to available P. There was a small quadratic response of feed intake and growth rate of the pigs to level of available P, with maximum responses occurring to appr oximately 3 g available P/kg (P < 0-05). There were linear depressing effects of increasing Ca:available P ratios on carcass gain and feed c onversion ratio (P < 0.01) but most of these effects occurred when the ratio exceeded 2.5:1. All bone variables examined increased linearly (P < 0.05) or curvilinearly (P < 0.01) with increasing available P con centration. In general, these variables were not affected by the Ca:av ailable P ratio. The results of the growth responses and bone developm ent indicate that the grower pig requires approximately 3 g available P/kg. However, for availability assays, where linearity of response is needed, the dietary concentration of available P should be a maximum of approximately 2 g/kg. In the second experiment four levels of calcu lated available P (1-4 g/kg) with a Ca: available P ratio of 2.5: 1 we re used to determine the available P requirements of grower/finisher p igs from 20 to 90 kg live weight. At 50 kg live weight the dietary ava ilable P concentration for half the pigs fed at 2, 3 and 4 g available P/kg was reduced to 1, 2 and 3 g/kg respectively. The pigs were fed a d lib. and growth performance, bone characteristics, P retention and a sh concentration in the empty body were taken as response criteria to assess P adequacy. Among the variables tested, the ash concentration i n the radius/ulna bone and P and ash concentrations in the empty body appeared to be more responsive than other variables to the changes in dietary P levels. Based on these variables, the P requirements for gro wth and bone development of growing pigs from 20 to 50 kg live weight was 3 g/kg and reduced to 2 g/kg for finisher pigs from 50 to 90 kg li ve weight.