A Recent Evaluation of Phytase Enzyme For Grower-Finisher Pig Diets
Livestock Update, October 1996
Allen Harper and E. T. Kornegay
Phosphorus (P) is an essential nutrient for normal growth, feed conversion and bone development in pigs. However, over 60% of the P in corn and soybean meal is bound as phytate P and unavailable to the pig. Inorganic P sources such as monocalcium phosphate must be added to corn-soy feeds for normal pig growth and development. As a result, excessive amounts of P are excreted in pig manure.
Phosphorus is an essential crop nutrient and P from manure sources or commercial fertilizer can be used in crop production. Furthermore, P binds securely to soil particles so excessive application rates can result in P build-up in soils. Surface run- off and soil erosion of high P fields may cause P enrichment of streams, rivers and lakes. Under these conditions P becomes a pollutant rather than a crop nutrient.
Phytase is an enzyme that can liberate phytate bound P in the pig's digestive tract. Supplementing feed grade phytase to pig feeds may reduce the need for inorganic P supplementation and reduce P excretion in the manure. A feed grade phytase (BASF Natuphosr) was approved for use in the U.S. in 1996 but only limited data is available on phytase use in pigs during the growing-finishing stage. This is important because growing- finishing pigs consume more feed and excrete more manure than any other class of swine. In the spring of 1996, a set of Virginia Tech feeding trials were conducted with the objective of evaluating Natuphos phytase supplementation of growing-finishing pig feeds as a means to reduce P supplementation and P excretion.
Performance trials were conducted concurrently at the Tidewater AREC Swine Unit (140 crossbred pigs) in Suffolk and the Virginia Tech Swine Center (84 crossbred pigs) in Blacksburg.
Test treatments consisted of dietary alterations of inorganic phosphorus (P) supplementation rate from monocalcium phosphate and Natuphosr phytase inclusion rate in corn-soybean meal based diets. There were 6 dietary treatments:
Diet 1 - no added P, no phytase
Diet 2 - .04% added P, no phytase
Diet 3 - .08% added P, no phytase
Diet 4 - no added P, 167 U/kg of phytase
Diet 5 - no added P, 333 U/kg of phytase
Diet 6 - no added P, 500 U/kg of phytase
For the combined trials there were 16 replicate pens of 4 pigs each fed the negative control diet 1, and 8 replicate pens of 4 pigs each fed diets 2, 3, 4, 5 and 6. Pens were balanced across treatments based on body weight, litter of origin and sex (2 barrows and 2 gilts per pen).
Average initial pig weight was 65 lbs. The growth trials were terminated when average pig weight within a replication was at least 230 lbs. Grower phase diets were fed to 155 lbs. body weight and finisher phase diets were fed for the remainder of the trials.
Daily weight gain, feed consumption and feed conversion efficiency were the standard measures of performance.
Fresh grab fecal samples were collected from pigs within each pen for determination of P digestibility using standard methods.
After the growth trials, all barrow pigs were transported to a commercial slaughter facility for standard carcass quality evaluation. The tenth rib bone was removed and used for standard bone strength and ash content determination.
Supplementing Natuphosr phytase to pig diets with minimal inorganic P resulted in growth performance significantly above that observed for pigs fed similar diets with no added phytase. Furthermore, adding the phytase to low P diets resulted in pig performance essentially equivalent to performance of pigs fed diets containing "standard" levels of inorganic P. The growth rate for pigs fed no added P and no added phytase was 1.93 lbs. per day. For pigs fed .04% and .08% added dietary P, growth rates were 2.06 and 2.07 lbs. per day, respectively. For pigs fed the low P, phytase added diets, growth rates were 2.02, 2.05 and 2.06 lbs. per day for phytase addition rates of 167, 333 and 500 units per kg of diet, respectively. Feed conversion rate was also poorer with the low P diet with no added phytase at 3.24 lbs. of feed per lb. of pig gain. For pigs fed .04% and .08% added dietary P, feed per gain values were 3.11 and 3.18, respectively. For the low P, phytase added diets, feed to gain values were 3.19, 3.09 and 3.13 for phytase addition rates of 167, 333 and 500 units per kg of diet, respectively.
The P digestibility and bone assessment data clearly showed that this improved performance with low P diets is the result of enhanced dietary P digestibility with phytase supplementation. Phosphorus digestibility coefficients were 28.7, 32.6, 37.4, and 41.3% for diets containing 0, 167, 333 and 500 units of phytase per kg. For these same diets, ash content of the tenth rib bone was 32.7, 34.4, 36.7 and 38.7%, respectively.
The carcass evaluation data did not reveal any negative or positive carcass or pork quality effects due to phytase supplementation. External fat and muscling traits appeared to be more affected by genetics and slaughter weight of the pigs rather than phytase or P supplementation.
In conclusion, using supplemental phytase rather than traditional P supplementation in these trials resulted at least a 22% reduction in fecal P excretion while maintaining grower-finisher pig performance. Based on current prices for monocalcium phosphate and Natuphosr phytase, the cost for 500 U/kg phytase feed (diet 6) is $1.09 per ton or $.33 per market hog more than a traditional inorganic phosphate supplemented feed (diet 3). Under conditions in which P levels in manure increase land costs for manure disposal, this modest feed cost increase may be economically justified.
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