The amount of digestible calcium included in pig diets has a direct effect on phosphorus digestibility, but the optimum ratio between the two minerals has not yet been found. In a recent study from the University of Illinois, scientists established a first approximation of that ratio for 25-50 kg pigs.

“Because calcium is an inexpensive ingredient, the thinking was that we could add as much as we wanted. We discovered several years ago that may not be a good approach, because if you increase calcium in the diet, you reduce absorption of phosphorus,” explained Hans Stein, professor in the University of Illinois department of animal sciences and the division of nutritional sciences. “As phosphorus availability goes down, so does the pigs’ growth performance. Feed intake -- and, therefore, bodyweight gain and feed efficiency -- goes down.”

Stein and his collaborators formulated 20 corn/soybean meal-based diets varying in calcium and phosphorus concentration and fed them to 240 pigs over four weeks. Diets were formulated to contain 0.15%, 0.31%, 0.39% or 0.47% standardized total tract digestible (STTD) phosphorus and 0.13%, 0.27%, 0.42%, 0.57% or 0.72% STTD calcium. These values represented 48-152% of the STTD phosphorus requirement and 27-173% of the total calcium requirement.

By the end of the four-week trial, the researchers were able to determine pig growth performance in terms of average daily gain and gain-to-feed ratio, as well as incorporation of the minerals into bone.

In a separate trial, 120 pigs were fed the same 20 diets for two weeks, and their urine, fecal and blood samples were analyzed for calcium and phosphorus concentrations.

“The results confirmed what we’ve seen before: If you feed too much calcium, in particular with low or marginal phosphorus in the diet, pig growth performance goes down,” Stein said. “We still need to do more work to determine the optimum ratio between the two, but we have definitely confirmed that the ratio is very important.”

Stein said most pig diets are currently formulated with marginal phosphorus, partly due to cost of the ingredient and partly because producers want to avoid having to mitigate phosphorus excreted in manure. However, diets formulated with too much calcium or too little phosphorus could be reducing pig growth performance.

“If someone asked us today, we would say that to maximize average daily gain and gain:feed for 25 to 50 kg pigs, the ratio of STTD calcium to STTD phosphorus should be between 1.16:1 and 1.43:1. However, it is possible that we will have to change that ratio as we get more data. It is still very early,” Stein said.

The article, “Requirements for Digestible Calcium by 25 to 50 kg Pigs at Different Dietary Concentrations of Phosphorus as Indicated by Growth Performance, Bone Ash Concentration & Calcium & Phosphorus Balances,” was published in the Journal of Animal Science. Stein’s co-authors on the study include J.C. Gonzalez-Vega, C.L. Walk and M.R. Murphy.

Stein also will discuss calcium and phosphorus balances in the updated swine nutrition chapter in the upcoming Feedstuffs Nutrition Guidebook, which will be available the week of Oct. 2.