The majority of phosphorus in conventional soybean meal is bound in the phytate complex. Pigs do not secrete the phytase enzyme, so phytate-bound phosphorus is not digested and is excreted in the feces. Not only does this deprive pigs of an essential nutrient, but excessive phosphorus excretion can contribute to environmental problems as well.

Addition of microbial phytase is one strategy for increasing the digestibility of phosphorus in soybean meal. Another potentially effective strategy is to feed fermented soybean meal.  Fermentation may result in hydrolysis of phytate and release of phytate-bound phosphorus, thus making more phosphorus available to the pig.

An experiment was conducted to determine the digestibility of phosphorous in fermented soybean meal and to test the hypothesis that phosphorus digestibility  is greater in fermented soybean meal than in conventional soybean meal. Five diets were formulated. Two diets contained fermented soybean meal and either 0 or 800 units (FTU) of microbial phytase. Two additional diets contained conventional soybean meal and either 0 or 800 FTU of phytase. Finally, a phosphorus-free diet was used to measure basal endogenous losses of phosphorus. Values for endogenous losses were used to adjust values for the apparent total tract digestibility (ATTD) of phosphorus to determine values for the standardized total tract digestibility (STTD) of P.

Fermentation of soybean meal increases phosphorus digestibility

Feeding diets containing fermented soybean meal, with no added phytase, resulted in an increase (P < 0.01) in both ATTD and STTD of phosphorus (Table 1) compared with diets containing conventional soybean meal and no added phytase. A similar effect on phosphorus digestibility has been observed in fermented corn co-products, suggesting that fermentation is an effective way of improving the digestibility of phosphorus in feed ingredients containing phytate-bound phosphorus.

Adding phytase to the diets increased (P < 0.01) ATTD and STTD of phosphorus in both fermented and conventional soybean meal. The increase in digestibility was greater (P < 0.01) for conventional soybean meal. This is probably because fermentation decreases the amount of phytate-bound phosphorus in soybean meal, and therefore, the amount of phytate available to be hydrolyzed by phytase is greater in conventional soybean mealthan in fermented soybean meal. The fact that phytase addition resulted in STTD values around 71% for both fermented and conventional soybean meal suggests that this may be close to the maximum digestibility of phosphorus in soybean meal.

Feeding fermented soybean meal reduces phosphorus excretion

Pigs fed fermented soybean meal excreted less phosphorus in their feces, and in lower concentrations (P < 0.01), than did pigs fed conventional soybean meal. Use of phytase in the diet reduced both fecal phosphorus concentration and total phosphorus output (P < 0.01). The reduction in phosphorus excretion from adding phytase was greater (P < 0.01) in pigs fed conventional soybean meal than in pigs fed fermented soybean meal.

Key points

• The STTD of phosphorus is greater in fermented soybean meal than in conventional soybean meal. It is likely that fermentation caused hydrolysis of phytate bonds because less of the phosphorus in fermented soybean meal was phytate-bound compared with conventional soybean meal.
• No difference was observed in STTD of phosphorus between fermented soybean meal and conventional soybean meal if phytase was added to the diets. It is possible that the approximately 71% STTD of P obtained for both sources of soybean meal with added phytase may be the maximum achievable phosphate digestibility in soybean meal.
• Both the concentration and total amount of phosphorus excreted in feces were reduced in pigs fed fermented soybean meal compared with  pigs fed conventional soybean meal. The addition of phytase reduced phosphorus excretion in both diets.

Table 1. Effects of phytase on apparent total tract digestibility (ATTD) and standardized total tract digestibility (STTD) of phosphorus (P) in fermented soybean meal (FSBM) and conventional soybean meal (SBM-CV)

¹FTU = phytase units

This research report is based on unpublished research by O. J. Rojas and H. H. Stein.