It is important to include calcium and phosphorus in the diets in the proper proportions because the excess or deficiency of one mineral may affect the utilization of the other. Calcium requirements in the 2012 NRC are based on a model, which used a 2.15 ratio of total calcium to standardized total tract digestible (STTD) phosphorus. An optimal ratio of STTD calcium to STTD phosphorus has not been reported because not enough data exist on the standardized total tract digestibility of calcium. However, recent studies conducted by the Stein Monogastric Nutrition Lab have determined values for STTD calcium for several calcium sources. With these data, it is possible to determine the requirement for STTD calcium. Therefore, an experiment was conducted to determine the requirement of STTD calcium to maximize growth performance and bone ash in 11 to 25 kg pigs.

Materials and methods

A total of 256 weanling pigs with an average initial body weight of 11.39 kg were used in the experiment. Pigs were fed eight diets based on corn, soybean meal, and lactose. Six diets contained 0.36% STTD phosphorus and either 0.32, 0.40, 0.48, 0.56, 0.64, or 0.72% STTD calcium. According to NRC (2012), the STTD phosphorus requirement for 11 to 25 kg pigs is 0.33%. Two additional diets containing 0.72% STTD calcium and 0.33% or 0.40% STTD phosphorus were also prepared. The former was used to determine if there were any negative effects of using 0.36% phosphorus in the experimental diets. The latter was used to confirm that calcium retention was not limited by the amount of digestible phosphorus in the diet.

Pigs were weighed at the beginning of the experiment, on day 10, and on the last day of the experiment (day 22). Average daily gain (ADG) was calculated for the first ten days and for the entire experiment. The amounts of feed offered and left in the feeder on day 10 and day 22 were recorded and used to calculate average daily feed intake (ADFI) and gain to feed ratio (G:F). The 3^rd and 4^th metacarpals and femurs were removed from the right front feet and right legs from one pig per pen and analyzed for bone ash.

Growth performance and bone ash results

The results from the experiment are shown in Table 1 and summarized below.

¹P < 0.05.

Body weight at all stages, ADFI at all stages, ADG from days 10 to 22, and G:F from days 0 to 10 were unaffected by the level of STTD calcium in the diet.

Pigs fed the diet containing 0.33% phosphorus and 0.72% calcium did not retain more calcium than pigs fed the diet containing 0.36% phosphorus and 0.72% calcium (Table 2), confirming that the phosphorus content of the experimental diets did not negatively affect calcium retention. Bone calcium in grams increased linearly (P < 0.05) as phosphorus increased, but bone calcium as a percentage of bone weight was not affected by phosphorus concentration.

A broken line analysis was used to determine the optimum calcium level for data with linear effects, and a quadratic analysis was used if there was a significant quadratic effect. If both were significant, the intersection of the broken line and the quadratic line was determined.

ADG started to decline at 0.54% STTD calcium and gain:feed started to decline at 0.43%. This may be due to the negative effect of increasing levels of calcium on phosphorus digestibility. The requirement for STTD Ca to maximize bone ash, bone calcium, and bone phosphorus in femurs, and bone ash in metacarpals were 0.48, 0.50, 0.56, and 0.54%, respectively.

• STTD calcium levels greater than 0.54% and 0.43% decrease ADG and G:F respectively, probably due to decreased phosphorus digestibility.
• Levels of STTD calcium greater than 0.48% are required to maximize bone ash, bone calcium, and bone phosphorus.
• In diets containing 0.36% STTD phosphorus, 0.48% STTD calcium yields a digestible calcium to digestible phosphorus ratio of 1.33 to 1.

Table 1. Growth performance and bone mineralization of pigs fed diets containing between 0.32 and 0.72% standardized total tract digestible (STTD) Ca and 0.36% STTD P

Table 2. Growth performance and bone mineralization of pigs fed diets containing 0.72% standardized total tract digestible (STTD) Ca and from 0.33 to 0.40% STTD P

This report is based on unpublished research by Caroline González-Vega, Yanhong Liu, Carrie L. Walk, and Hans H. Stein.