An estimation of the minimal P requirements for growing-finishing pigs, as influenced by the Ca level of the diet

In a 3 ×; 3 factorial experiment (three dietary phosphorus (P) levels combined with three dietary calcium (Ca) levels), the minimal P requirements of 180 growing-finishing pigs within the live weight range from ±37 kg to ±100 kg were investigated. Analysis of the basal diet of wheat (37.7%), cassava (32.7%) and soybean oilmeal (24.7%), supplemented with 2000 IU vitamin D3, gave 0.36% total P (0.22% phytate P, 489 wheat phytase units kg−1) and 0.54% Ca. Phosphorus was supplemented as CaHPO4 to dietary levels of 0.40% or 0.44% and Ca, as CaCO3, to dietary levels of 0.74% or 0.94%. Daily gain and feed intake were significantly affected by the dietary P level, while feed efficiency was only improved during the first 5 week period. However, there were no main effects of the dietary Ca levels on any of the performance characteristics mentioned. Leg condition as well as carcass quality was not affected by Ca or P level. It can be concluded that 0.17% available (apparently digestible) P in the diet (0.44% total P) is close to the minimal requirements for growing-finishing pigs. At that dietary P level an increase in dietary Ca has no adverse effect on any of the parameters studied, while at the lowest dietary P level (0.11% available P, 0.36% total P) a decrease in daily gain and in feed efficiency, in serum inorganic phosphate and in bone mineralization is clearly related to an increasing dietary Ca level. An increase in dietary Ca had no influence on the availability of P. The normally recommended Ca/P ratio (1.3) or Ca/available P ratio (3.0) seems to be meaningless at the highest P level. The experimental figures support the hypothesis that the ratio of available Ca/available P is perhaps a better parameter to explain the apparent antagonism between Ca and P at low dietary P levels. The availability of Ca from CaCO3 is much higher than from CaHPO4, resulting in a rapid rise of the ratio of available Ca/available P when CaCO3 is added to a low P diet.