PHOSPHORUS STUDIES IN PIGS .2. ASSESSING PHOSPHORUS AVAILABILITY FOR PIGS AND RATS

Experiments were conducted with pigs and rats to determine the availab ility of P in feeds. Initially, the availabitity of P in a soya-bean m eal and field peas (Pisum sativum cultivar Early Dun) was assessed usi ng a slope-ratio assay for grower pigs. Three different levels of eith er monosodium phosphate (MSP), soya-bean meal or field peas were added to a basal sucrose-soya-bean meal diet (2.5 g/kg P) to give three lev els of P (3.0, 3.5 and 4 g/kg) for each source. The diets were offered for 35 d at three times maintenance energy requirements to female pig s initially weighing 20 kg live weight. Several bone variables and the ash and P concentration and retention levels in the empty body were u sed as criteria of availability. The responses to MSP were linear for all variables. However, responses to P in the test proteins, particula rly soya-bean meal, were mostly non-linear, except for ash and P conce ntrations and retentions in the empty body. The estimates of P availab ility in the soya-bean meal and in the field peas were dependent on th e criteria used to assess availability. Using bone variables as the cr iterion, the availabilities of P in soya-bean meal and field peas were approximately 0.17 and 0-38 respectively. Using ash or P concentratio ns or retentions in the empty body as the response criterion, the avai labilities of P in soya-bean meal and field peas were 0.61 and 0.38 re spectively. The mean retention values for P from MSP, soya-bean meal a nd field peas were 0-74, 0-33 and 0.21 respectively. Experiments were then conducted to define the conditions necessary for a slope-ratio as say for P availability with growing rats to determine if they could be used as a model to assess P availability for pigs. These experiments examined level of P in the diet, the effect of Ca: P ratio and criteri a for assessing response (growth rate, feed intake, feed conversion ra tio, bone ash and bone bending moment). The results indicated that a s uitable range of dietary P for a slope-ratio assay in rats was 1.8-3.5 g/kg, with Ca: P ratio of between 1.3 and 6.2, and that ash content o r bone bending moment of the femur bone were suitable criteria of resp onse. However, using bone bending moment as the criterion of response, the availability of P in soya-bean meal was 0.81, which was considera bly higher than the estimate with pigs. Overall the results indicated that the estimates of the availability of P in feeds for pigs were dep endent on the criteria used to assess availability and that the value for rats led to an overestimate of the value for pigs.