Nutrition

Low-Phytate Diets with Phytase Drop Phosphorus Excretion

University of Kentucky researchers recently discovered that the combination of low-phytate feedstuffs and added phytase in pig diets resulted in a whopping 73% reduction in phosphorus excretion.

This discovery was made during two recent experiments to assess the value of feeding the low-phytate feedstuffs in combination with added phytase.

A number of experiments conducted the past several years have shown that reducing the phosphorus in the diet and adding phytase is an effective means of lowering phosphorus excretion. Additional experiments have shown that feeding of low-phytate corn or a combination low-phytate corn and low-phytate soybean meal with reduced amounts of inorganic phosphorus also reduces phosphorus excretion.

The University of Kentucky researchers conducted the first of the two research projects as a 40-day study with crossbred pigs initially averaging 33 lb. Six pigs were individually penned per treatment. Phosphorus excretion was measured using an indigestible marker in the feed. Metatarsal, metacarpal and femur bones were obtained from all pigs at the end of the study for analysis of bone strength and ash measurements.

The control diet consisted of conventional corn and soybean meal with 0.20% added phosphorus as dicalcium phosphate (Table 1). This diet contained 0.59% total phosphorus, which met the National Research Council (NRC) estimated requirement. The added phosphorus was reduced to 0.10% in Diet 2, and phytase (750 units/kg) was added to the reduced phosphorus diet (Diet 3). Diets 4, 5 and 6 consisted of low-phytate corn and low-phytate soybean meal with similar reductions in phosphorus and additions of phytase.

Diets 5 and 6 had no supplemental phosphorus and contained 0.22% less total phosphorus than the conventional corn-soy control diet.

The results showed growth rate was significantly (P 0.05) from the control.

Bone strength was reduced when phosphorus was lowered in both diets, but phytase addition to the lower-phosphorus diets resulted in bone strengths that were very similar to those of the control pigs. There was essentially no reduction in bone strength when the low-phytate corn and soybean meal diet without any added phosphorus was fed, as long as it was supplemented with phytase.

The most dramatic treatment effect was in respect to fecal phosphorus excretion, when the combination of low-phytate feedstuffs and phytase resulted in a 68% reduction in fecal phosphorus, as noted above. Adding phytase in conventional diets reduced phosphorus excretion by 18%. Feeding the low-phytate corn and soybean meal reduced phosphorus excretion by 38%.

Researchers also studied the premise that feeding phytase or low-phytate feedstuffs reduces total phosphorus excretion and may result in a dramatic increase in the solubility of phosphorus in the excreta. A second experiment was conducted to see if phytase or low-phytate feeds affected the type of phosphorus excreted by pigs.

Two, five-day balance experiments were conducted with crossbred pigs averaging 114 lb., with six pigs/treatment (Table 2). Four diets consisted of the two types of corn and soybean meal with or without phytase (750 units/kg.). All diets were calculated to have the same levels of non-phytate phosphorus (0.28%) and bioavailable phosphorus (0.27%), assuming that phytase would release one-third of the unavailable phosphorus. The normal corn-soy diet was slightly in excess of the NRC (1998) requirement of 0.47% total phosphorus and 0.20% bioavailable phosphorus for pigs of this weight.

Apparent phosphorus digestibility was increased by feeding the low-phytate vs. normal diets and by phytase addition (P < 0.05).

Daily phosphorus excretion in feces and total excreta were significantly affected (P < 0.05) by dietary treatment. Total phosphorus excretion decreased by 41% when phytase was added to the conventional diet, by 55% when the low-phytate diet was fed, and by 73% when phytase was included in the low-phytate diet.

Soluble phosphorus in the excreta, when expressed as a percent of the total phosphorus in excreta, increased significantly (P 0.50), researchers noted.

Results indicate that pigs fed low-phytate corn-soy diets combined with phytase require less supplemental inorganic phosphorus to optimize performance and bone density. Pigs also excrete 60-70% less phosphorus in their manure. Although the proportion of soluble phosphorus in the excreta increases with feeding of phytase or low-phytate feeds, the total amount of soluble phosphorus excreted by pigs is only marginally affected.

The amount of soluble phosphorus excreted is largely influenced by the amount of phosphorus in the diet that exceeds the phosphorus requirement of pigs. Excess dietary phosphorus is absorbed and excreted in the urine. Urinary phosphorus is highly soluble. In the experiment, the dietary phosphorus exceeded the pig's requirement by approximately 0.10%, which probably accounted for the slight increase in the proportion of excreted phosphorus that was in the soluble form.

Researchers: Gary L. Cromwell, Merlin D. Lindemann, Eduardo G. Xavier and L. Allen Pettey, University of Kentucky. Contact Cromwell at (859) 257-7534.

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