Values for the apparent total tract digestibility (ATTD) and the standardized total tract digestibility (STTD) of Phosphorus (P) and Calcium (Ca) have been measured in most feed ingredients. In almost all cases, these values were determined in growing pigs. Values for digestibility of Ca and P obtained in growing pigs are often applied to gestating and lactating sows, but research shows this may not be representative for what sows need.

Because of the binding of P to phytate in plant feed ingredients, the STTD of P in plant ingredients is low in both growing pigs and sows. Phytate can also chelate Ca ions from Ca carbonate or other feed ingredients, and reduce the digestibility of Ca. It is also possible that gestating sows and growing pigs respond differently to the phytate in diets, but data to demonstrate this have not been reported.

We conducted two experiments to test the hypothesis that values for digestibility of Ca and P obtained in growing pigs are also representative for gestating sows.

Experiment 1

In the first experiment, a basal diet containing primarily corn and soybean meal and two diets containing 60% of the basal diet and 40% of either full fat or defatted rice bran were formulated.

Diets were fed to gestating sows in mid-gestation and growing gilts (51.5 ± 3.1 kg) and both groups were provided feed at a level close to ad libitum intake while being housed in metabolism crates.

The ATTD of Ca and P was determined in all diets for both groups of pigs. Results indicated that the ATTD of both Ca and P was greater in the corn-soybean meal diet than in the two diets containing rice bran (Table 1), which is likely because rice bran has a very high concentration of phytate.

However, regardless of diet, gestating sows have much lower (P < 0.05) digestibility of Ca and P compared with growing pigs.

Experiment 2

In the second experiment, 48 multiparous sows in mid-gestation and 48 barrows (initial body weight = 19.8 kg) were housed in metabolism crates and fed 2 fed two diets that differed in concentration of phytate. The corn soybean meal diet contained 0.98% phytate, and the high-phytate diet (phytate = 2.94%) contained corn, soybean meal and 40% full-fat rice bran. A Ca-free diet and a P-free diet were used to determine basal endogenous losses of Ca and P.

Results show that basal endogenous losses of Ca and P from gestating sows (Ca = 1.58 g/kg dry matter intake; P = 0.78 g/kg dry matter intake) were greater (P < 0.05) than from growing pigs (Ca = 0.43 g/kg dry matter intake; P = 0.16 g/kg dry matter intake). Phytate level did not affect the STTD of Ca or Ca retention by gestating sows, but the STTD of Ca and Ca retention were greater if growing pigs were fed the normal-phytate diet than if they were fed the high-phytate diet (Table 2).

The STTD of P was greater for the normal-phytate diet than for the high-phytate diet, but the difference was greater for growing pigs than for gestating sows. Phosphorus retention by growing pigs fed the normal-phytate diet was greater than if they were fed the high-phytate diet, but P retention by gestating sows was not affected by phytate level. Regardless of phytate level, growing pigs had greater STTD (P < 0.001) and retention of Ca and P compared with gestating sows.

Keep the Sow’s Best Interest in Mind

Compared with growing pigs, gestating sows have lower digestibility of Ca and P, but increased basal endogenous losses of Ca and P. Gestating sows also respond differently to dietary phytate concentration than growing pigs. As a result, it may not always be accurate to formulate diets for gestating sows using ATTD or STTD values for Ca and P that were obtained in growing pigs.

Farm Journal’s Pork | Jennifer Shike | August 16,