Phosphorus in Swine Diets

by 5m Editor
20 September 2003, at 12:00am

By Dr Eric van Heugten, Assistant Professor, North Carolina State University - Several studies focusing on phosphorus in swine diets were presented at this year's national animal science meetings. Researchers from Iowa State University, T. Stahly and T. Lutz, evaluated the effects of dietary available/digestible phosphorus levels on phosphorus and nitrogen utilization in pigs.

Dr Eric van Heugten
Swine Nutrition Specialist
North Carolina State University

They used a total of 90 barrows and evaluated six dietary phosphorus levels during the growth period of 15.4 to 70.4 pounds of body weight. The basal diet consisted of a corn-soy-whey diet and contained 0.56 percent analyzed phosphorus. Dietary treatments were created by adding dicalcium phosphate at incremental levels to the basal diets, replacing starch and limestone; therefore, the diets were the same with the exception of the amounts of dicalcium phosphate, limestone, and starch.

Bioavailable phosphorus levels of the treatments (calculated from the actual analyzed phosphorus levels of the ingredients multiplied by the bioavailability of phosphorus, according the NRC (1998) for each of the ingredients) were 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7 percent. Dietary calcium levels were either fixed in each of six diets at 1.15 percent (dietary calcium to available phosphorus ratios were 5.8, 3.8, 2.9, 2.3, 1.9, and 1.6 percent, respectively) or they were adjusted in each of the diets to a ratio of calcium to available phosphorus of 2.5 to 1.

Phosphorus and nitrogen digestibility and accretion were determined in each pig for 4 days when they reached a body weight of 22 pounds and then 66 pounds. Digestible dietary phosphorus calculated from the phosphorus content of the diets and the measured digestibility of phosphorus increased (Table 1) as the bioavailable level of phosphorus in the diet increased, as expected. However, when comparing the digestible phosphorus level in the diet with the bioavailable phosphorus level, it is clear that digestible phosphorus levels are relatively greater than the bioavailable levels at low levels compared to high levels.

Table 1: Effects of dietary bioavailable phosphorus level on phosphorus
digestibility, phosphorus accretion and nitrogen accretiona

In the basal diet (0.2 percent bioavailable phosphorus; no added dicalcium phosphate), the digestible phosphorus levels were 1.7 times greater than the bioavailable levels. On the other hand, the authors calculated that the digestibility values for dicalcium phosphate were only 0.7 times the bioavailable reference value, which implies that the bioavailability of the dicalcium phosphate used in their experiment was overestimated. The accretion of both phosphorus and nitrogen increased with increasing levels of bioavailable phosphorus, but this increase became smaller at the higher levels of bioavailable phosphorus.

The magnitude of the response in phosphorus accretion to increased levels of bioavailable phosphorus in the diet was greater (P < 0.05) in the diets with a constant calcium to available phosphorus ratio and in the lighter pigs (weighing 22 pounds). The authors concluded that the dietary digestible phosphorus concentration needed to optimize the accretion of protein-rich tissues as well as the efficiency of phosphorus utilization in 22 pound to 66 pound pigs is 0.48 to 0.53 percent, regardless of dietary calcium to phosphorus ratio. They further conclude that the biological response to the intake of higher amounts of digestible phosphorus is dependent on the dietary calcium to digestible phosphorus ratio, which will influence whether the additional digestible phosphorus is retained in the body or excreted in the urine.

In a second presentation, Iowa State's Stahly and Lutz used the data from the same experiment as previously described to calculate the digestibility of phosphorus in dicalcium phosphate. They could do this by correcting all values obtained from pigs fed diets with dicalcium phosphate with those obtained from the basal diets (which did not contain dicalcium phosphate).

Once the contribution of dietary phosphorus and digestible phosphorus from the basal diet was eliminated, the phosphorus intake from dicalcium phosphate (which contained 17.94 percent analyzed phosphorus) was regressed against the amount of phosphorus absorbed from dicalcium phosphate. This ultimately allowed the authors to determine the average percentage of phosphorus from dicalcium phosphate that was absorbed.

The digestibility of phosphorus from dicalcium phosphate ranged from 67.5 percent to 73.3 percent, depending on whether the ratio of calcium to available phosphorus was kept constant, or depending on pig body weight (Table 2). The average digestibility of phosphorus from dicalcium phosphate was 68.1 ± 1.9 percent when considering all data points.

Table 2: Digestibility of phosphorus in dicalcium phosphate

According to the authors, these data highlight the opportunities and incentives for technologies aimed at improving phosphorus digestibility in both phosphorus sources with a relatively high (such as inorganic phosphorus sources) as well as low (such as plant phosphorus sources) phosphorus bioavailabilities.

In a third study, researchers from the University of Kentucky—E. Xavier, G. Cromwell, and M. Lindemann—evaluated the effects of phytase addition to conventional or low phytate corn-soybean meal diets on phosphorus balance in growing pigs. Four dietary treatments were fed at 92 percent of ad libitum intake to 12 pigs (132 pounds) housed in metabolism crates. Diets consisted of:

  • Normal corn and soybean meal diet (0.75 percent lysine) containing 0.48 percent phosphorus and 0.55 percent calcium.
  • Normal corn and soybean meal diet containing 0.38 percent phosphorus, 0.50 percent calcium, and added phytase enzyme (750 units/kg).
  • Low phytate corn and low phytate soybean meal diet containing 0.35 percent phosphorus and 0.55 percent calcium.
  • Low phytate corn and low phytate soybean meal diet containing 0.35 percent phosphorus, 0.50 percent calcium with added phytase enzyme.

The control diet (diet 1) was formulated to meet the requirement for bioavailable and total phosphorus. The diets formulated using low phytate corn and low phytate soybean meal did not contain additional phosphorus from dicalcium phosphate. Phosphorus excretion in urine and feces, as well as amounts of phosphorus absorbed and retained each day was impacted by diet (P < 0.01; Table 3). Apparent phosphorus digestibility was increased when diets with low phytate ingredients were fed, and the addition of phytase to both of these diets improved the digestibility of phosphorus. The amount of soluble phosphorus in feces (expressed as a percentage of the total amount of phosphorus in feces) was not significantly affected by diet (P = 0.20); however, there was a numerical increase when low phytate diets and phytase were fed. The authors calculated that total phosphorus excretion was reduced by 31 percent when phytase was added to the normal corn and soybean meal diets. When the traditional ingredients were replaced with low phytate corn and soybean meal, phosphorus excretion was reduced by 48 percent. Addition of phytase to low phytate corn and soybean meal diets resulted in an additional reduction in phosphorus excretion, totaling 62 percent, compared to the control diets.

Table 3: Effects of phytase addition to conventional or low phytase
corn-soybean meal diets on phosphorus balance in growing pigs

The effects of pharmacological levels of zinc, commonly fed to early-weaned pigs, on phytase efficiency were evaluated by N. Augspurger and D. Baker at the University of Illinois and by D. Webel and J. Spencer at United Feeds Inc.

Ninety-nine pigs (15.8 pounds) were individually fed one of 11 diets. Diets consisted of a basal corn soybean meal diet (21 percent crude protein, 0.075 percent available phosphorus and 129 mg/kg of zinc), three graded levels of supplemental inorganic phosphorus (0, 0.075, and 0.150 percent), two levels of phytase (500 or 1,000 FTU/kg), and 1,500 mg/kg of zinc from either zinc oxide or tetra-basic zinc chloride, and all combinations of phytase and zinc.

Supplemental phosphorus improved pig performance and bone ash (measured in the fibula) in a linear manner (P < 0.01). The addition of phytase also improved daily gain, feed intake, and feed efficiency (P < 0.01). Bone ash was highest when 1,000 FTU/kg of phytase was added. The addition of zinc to the diet had no effect on pig performance, but it reduced bone ash (P < 0.05). When the effects of zinc on phytase efficiency were evaluated, the authors observed that 500 units of phytase released 0.130 percent phosphorus when no pharmacological levels of zinc were included; however, in the presence of 1,500 mg/kg of zinc, the amount of phosphorus released was reduced (P < 0.01) to 0.099 percent and 0.085 percent for the zinc oxide and the zinc chloride, respectively. At 1,000 units of phytase, the amount of phosphorus released by phytase was reduced from 0.195 percent in the absence of zinc to 0.124 percent and 0.140 percent for the zinc oxide and zinc chloride, respectively.

The authors suggest that growth-promoting levels of zinc may chelate (bind) the phytate complex, thereby reducing its availability for hydrolysis (causing the release of phosphorus from the complex and making it available to the animal) by phytase.

Reproduced Courtesy

Source: North Carolina State University Swine Extension - September 2003