Environmentally Friendly Phosphorus Feeding

New technologies on the horizon can cut the phosphorus (P) a pig leaves behind. That's good news for the environment.

University of Kentucky swine nutritionist Gary Cromwell lists some of the tactics to trim phosphorus excretion from pigs.

As a quick review, the reason swine manure is so high in phosphorus is because most of it is bound in an organic complex commonly called phytic acid or phytate.

1. Phytase

   First on his phosphorus-reduction list, this enzyme is widely distributed in yeasts, fungi and bacteria. From 55 to 80% of the P in cereal grains and oilseed meals is in the phytate form. To utilize P from phytate, the enzyme phytase is required. Pigs do not have sufficient amounts of phytase in their digestive tract to hydrolyze all the P from phytate, so most of the P from the diet is excreted.

   Adding phytase increases the bioavailability of P in a corn-soybean meal (SBM) diet three-fold — from about 15% to over 45%, explains Cromwell. As a result, the amount of supplemental inorganic P can be reduced such that total dietary P is lowered by 0.1%.

   Lowering calcium slightly also improves the response from phytase. Tests in the last five years indicate that less phytase may be required to release P from phytate if the dietary calcium level is also reduced by 0.05 to 0.1%. Studies at the University of Kentucky show that lowering dietary P and supplementing with phytase will lower P excretion by 30 to 40%.

2. Low-Phytate Corn

   A low-phytate gene reduces the phytate P in corn by half and triples the amount of inorganic P. As a result, bioavailability of P increases from about 20% in normal corn to over 75%.

   Experiments with both growing and finishing pigs indicated that feeding pigs low-phytate corn-SBM diets containing 0.10 to 0.12% less total P than normal showed performance and bone mineralization similar to pigs fed normal diets. This reduction in total dietary P, along with the greater bioavailability of P in low-phytate corn, is associated with a 43% reduction in excreted P.

3. Low-Phytate Soybean Meal

   Soybeans that are low in both phytic acid and oligosaccharides have recently been produced. Both compounds are reduced because oligosaccharides and phytic acid synthesis use similar metabolic pathways. Soybean meal produced from low-phytate soybeans has about half as much phytate P and more than twice as much inorganic P as soybean meal from conventional soybeans.

   University of Kentucky studies have shown that the P in low-phytate soybean meal is about 50% bioavailable, compared with 20% in normal soybean meal. Pigs fed both low-phytate corn and low-phytate soybean meal with no supplemental inorganic P grew as fast and efficiently, had similar bone traits and excreted 53% less P than pigs fed diets containing conventional corn and soybean meal supplemented with enough inorganic P to meet their requirement (Table 1).

   The composition of low-phytate corn and low-phytate soybean meal are essentially the same as their conventional counterparts. In fact, the total amount of P is also about the same. The only difference is in the type of P — less phytate P and more inorganic P in the low-phytate types.

   Low-phytate corn is on the horizon and low-phytate soybean meal is 4-5 years away, says Cromwell. Neither eliminates the need for phytase because the enzyme is effective when included in diets with either type of corn or soybean meal.

4. Phytase in other plants

   Back in the 1940s, researchers found that some crops naturally carry high levels of phytase in their seeds. Wheat, wheat byproducts, rye and to a lesser extent, barley, contain significant amounts of phytase. Cromwell has found a much higher bioavailability of P in wheat (50%), wheat middlings (41%), wheat bran (29%) and barley (30%) than in corn (14%).

   Biotechnology has been used to insert a phytase gene into alfalfa and canola, which increases their phytase content. Commercializing these crops, says Cromwell, could provide alternative ways of supplying phytase as a means to reduce P excretion.

5. The Enviropig

   Scientists at Canada's University of Guelph have produced several lines of transgenic pigs with high levels of phytase in their saliva. The P in soybean meal is highly digestible by the transgenic pig and excretion of P was reduced by as much as 75% in weanling pigs. It is estimated that these pigs can deliver as much as 200,000 units of phytase to the digestive tract during consumption of 1 kg. (2.2 lb.) of feed. This is much more than the normal 300 to 1,000 units of phytase supplemented per kg of feed.

   “Whether these pigs become practical to produce remains to be seen, but it opens a new biological approach to reducing phosphorus pollution,” explains Cromwell.

6. Diet Formulation

   Several technologies are available to nutritionists to formulate low-P, environmentally friendly swine diets. P excretion can be reduced by feeding diets that do not have excessive levels of P. For example, feeding 0.2% more P than is needed in a grow-finish diet — a common practice a few years ago — will result in a 70% greater P excretion, compared with feeding P levels that meet '98 NRC standards.

Balancing diets on a bioavailable P, or nonphytate P, basis also helps to avoid overages and is an effective tool to more precisely meet P requirements. An appropriate reduction in the dietary P level and the addition of a phytase supplement is presently one of the most effective strategies that can be used.