Most of the phosphorus in corn and other plant-based feed ingredients fed to pigs is bound to phytate. Pigs do not secrete phytase in adequate quantities to liberate significant amounts of phytate-bound phosphorus, meaning that most of the phosphorus in these ingredients is not biologically available. Calcium digestibility is also negatively affected by the presence of phytate in feed ingredients. One way for producers to ensure that pigs' requirements are met is to add supplementary phosphorus to diets. However, this adds to the cost of diets. The excretion of phytate-bound phosphorus also may contribute to algae overgrowth in waterways.

Another approach is to add supplemental phytase to the diets, to allow pigs to better utilize the phosphorus present in the ingredients. Phytases can be obtained from plants, animals, or microorganisms (bacteria, yeasts, and fungi).

GraINzyme is a source of phytase produced by genetically modified corn plants with genes derived from E. coli. In this experiment, the effect on growth performance, apparent total tract digestibility (ATTD) of calcium and phosphorus, and bone mineralization of adding GraINzyme to weanling pig diets was tested.

Experimental design

A total of 60 weanling barrows and gilts with an average initial body weight of 10.78 kg were fed one of six diets. One was a positive control diet in which all nutrients were supplied according to current recommendations as set out in the 2012 NRC. These requirements included total calcium of 0.66% and standardized total tract digestible (STTD) phosphorus of 0.31%. The negative control diet was similar to the positive control except that inclusion of total calcium was reduced by 0.20 percentage units, and inclusion of digestible phosphorus was reduced by 0.18 percentage units. The remaining four diets were formulated by adding 500, 1,000, 2,000, or 4,000 units of phytase (FTU) to the negative control diet.

Use of GraINzyme increased growth performance in phosphorus-deficient diets

Final body weight increased (linear and quadratic, P < 0.05) as the inclusion of GraINzyme phytase in the diets increased (Table 1). Pigs fed diets containing 1000, 2000, or 4000 FTU of phytase had the same final body weight as pigs fed the positive control diet.

Linear and quadratic increases (P < 0.05) in average daily gain and gain:feed ratio, as well as a linear increase (P < 0.001) in average daily feed intake, were observed as the inclusion of phytase increased. For all growth performance parameters, pigs fed diets containing 1000 or more FTU of phytase had the same performance as pigs fed the positive control diet.

Increases in mineral digestibility and bone mineralization

The ATTD of calcium and phosphorus increased (linear and quadratic, P < 0.05) as GraINzyme inclusion increased (Table 2). Inclusion of 500 FTU of phytase was enough to increase ATTD of calcium and phosphorus to match that in pigs fed the positive control diet, and inclusion of 2000 units or more of phytase increased (P < 0.05) ATTD of phosphorus relative to the positive control diet.

Bone ash (in percentage and grams) in femurs increased linearly (P < 0.01) and quadratically (P < 0.05) as the concentration of GraINzyme phytase in the diets increased. Compared with pigs fed the positive control diet, pigs fed diets containing 2000 or more FTU of phytase had the same bone ash percentage, and pigs fed diets containing 4000 FTU had same bone ash concentration in grams.

The percentage of bone composed of phosphorus and calcium did not differ among pigs fed the different diets. The concentration of bone phosphorus and bone calcium in grams increased (linear and quadratic, P < 0.05) as inclusion of phytase increased. Bone phosphorus and calcium concentrations in pigs fed diets containing 2000 or more FTU of phytase were not different than in pigs the positive control diet.

Key points

• Addition of GraINzyme phytase to a negative control diet containing less calcium and phosphorus than is required by weanling pigs produced a linear increase in growth performance, ATTD of calcium and phosphorus, and concentrations of bone ash, bone calcium, and bone phosphorus.
• Adding 4,000 FTU of GraINzyme was sufficient to mitigate the effects of feeding a diet deficient in 0.20% total calcium and 0.18% STTD phosphorus.
• No upper limit for response was observed in this experiment. Therefore, it may be possible to replace more calcium and phosphorus in the diets by using GraINzyme phytase.

Table 1. Growth performance of pigs fed diets containing 0 to 4000 phytase units (FTU) of GraINzyme phytase for 28 days

Table 2. Apparent total tract digestibility (ATTD) of calcium and phosphorus, and bone mineralization of pigs fed diets containing 0 to 4000 phytase units (FTU) of GraINzyme phytase for 28 days

This report is based on unpublished data by Laia Blavi and Hans H. Stein.