Environmental concerns are an increasingly important part of livestock management, especially with manure disposal. Not only are government regulations becoming more stringent, but industry concentration, with more animals per unit relative to the land available for manure application, adds to the pressure to minimize nutrient loading in the soil and reduce the risk of water pollution from runoff.
Phosphorus presents special challenges--as a potential pollutant and also a feed cost, since it's the most expensive of the minerals added to swine rations. The root problem is that as much as 80 percent of the phosphorus in cereal grains and oilseed meals is chemically bound in the form of phytate, which pigs digest poorly because they lack the enzyme phytase. All of this unavailable phosphorus is excreted in the feces. At the same time, however, producers must add inorganic phosphorus to the animals' rations to meet their needs--and part of this supplemental phosphorus is also excreted.
Scientists have known since the late 1960s that adding phytase to swine feed would enable the animals to digest much more of the phosphorus in feedstuffs and reduce the level of supplemental inorganic phosphorus. However, phytase was too expensive to produce for routine use until the 1990s, when biotechnology allowed large-scale production. Today, while practical issues still need to be resolved with commercial products, phytase offers significant promise as a means to reduce phosphorus levels in animal waste by 30 to 35 percent, while also reducing the cost of phosphorus supplementation.
Typical of the studies to date is one by Skaggs and Kornegay (1999) in which weanling pigs received a standard corn-soybean meal diet with low available phosphorus (0.39 percent). For the positive controls, the ration was supplemented to provide 0.6 percent phosphorus. The other treatments received from 0 to 1,000 units of phytase per kilogram of the base feed.
Animals showed linear improvements (P < 0.01) in phosphorus digestibility, average daily gain, feed intake and feed efficiency as the level of phytase in the diet increased. At 681,818 or 909,090 units per ton (750 or 1,000 units/kg) of complete feed, the pigs on the low-phosphorus diets had performance and also bone mineralization equal to or greater than the positive controls. Phosphorus concentrations in the feces decreased linearly (P < 0.001) as phytase levels increased. Based on both linear and nonlinear response equations, Skaggs and Kornegay (1999) reported that the average phosphorus equivalency of phytase was 500 units/kg for 1.06 grams of phosphorus fed as mono- or dicalcium phosphate.
In a related study (Harper et al., 1999), researchers considered the efficacy and safety of the same phytase product when fed to growing pigs with an average initial weight of 63 lbs (28.5 kg). The positive control pigs received a corn-soybean meal diet supplemented to provide 0.47 percent available phosphorus. The animals in the other treatments received the base diet (0.34 percent phosphorus) and phytase at the levels of 0, 250, 500, 1,000 or 10,000 units/kg.
Average daily gain (P < 0.01), feed efficiency (P < 0.01) and feed intake (P < 0.05) all improved as the level of phytase in the ration increased. Phosphorus digestibility increased and at the 1,000- and 10,000-unit levels was equal to or greater than the positive controls. There were no adverse effects on bone mineralization with phytase supplementation. The addition of phytase at the highest level of 10,000 units/kg continued to produce improvements in performance, bone mineralization and mineral digestibility, and there were no ill effects on the health of the pigs.
The amount that fecal phosphorus levels decline with phytase supplementation will depend on the percentage of phosphorus that is available in the base ration, according to Yi et al. (1996). With available phosphorus at 0.05 percent of the ration in this study, excretion of phosphorus was reduced by about 25 percent in pigs receiving phytase compared to the control animals. When the base diet in this study contained 0.16 percent available phosphorus, excretion was reduced by 50 percent in pigs receiving phytase but no inorganic phosphorus.
Because phytate can bind other minerals as well, researchers are also studying the utilization of other nutrients with phytase supplementation. Skaggs and Kornegay (1999) and Harper et al. (1999) reported linear increases in calcium digestibility, and decreased fecal concentrations of calcium, with phytase supplementation. It has been suggested that dietary zinc fortification may also be decreased with phytase supplementation.
Amino acid digestibility may also be improved with phytase supplementation. In a study with finishing pigs, Zhang and Kornegay (1999) reported that the digestibility of all amino acids except proline and glycine increased linearly (P < 0.10 to 0.01) as phytase supplementation increased. Based on those responses, the researchers calculated that 500 units of phytase per kilogram of feed had the equivalency of 0.76 percent crude protein. They estimated that the nitrogen content of the feces was reduced by 5.1 percent with this level of phytase added.
The savings from reduced phosphorus costs can virtually offset the cost of phytase itself, according to Dr. Trygve Veum of the University of Missouri. That would make the significant environmental benefits remarkably inexpensive to achieve.
Heat stability is a critical issue for commercial phytase products. Because phytase can be denatured and inactivated by the heat associated with feed processing, it's necessary to use a thermo-stable product form or post-processing spraying of a liquid phytase product so the animals receive the required levels. Apparently, phytase can be an important new nutritional tool to reduce environmental risk and feed cost while maintaining animal performance.
References:
- Harper, A.F., et al., 1999. Efficacy and safety of Novo SP938 microbial phytase supplementation of a corn-soybean meal diet fed to growing pigs. J. Anim. Sci. 77(1):174.
- Skaggs, J.H., and E.T. Kornegay. 1999. Dose response of Novo SP938 microbial phytase in weanling pigs fed a low-P corn-soybean diet. J. Anim. Sci. 77(1):174.
- Yi, Z., et al., 1996. Effectiveness of Phytase in improving the bioavailability of phosphorus and other nutrients in soybean meal-based semipurified diets for young pigs. J. Anim. Sci. 74:1601.
- Zhang, Z., and E.T. Kornegay. 1999. Phytase effects on ileal amino acid digestibility and nitrogen balance in finishing pigs fed a low-protein plant-based diet. J. Anim. Sci. 77(1):175.
