Researchers at the University of Missouri-Columbia, investigated the phosphorus availability of a genetically modified, low-phytate corn. The researchers were attempting to determine the effect diets formulated with low-phytate corn would have on grow-finish performance and nutrient excretion.

Approximately 70% of the phosphorus in a typical corn/soybean meal diet is unavailable to pigs, according to the National Research Council's 1998 Nutrient Requirements for Swine. This unavailable phosphorus ends up being excreted in manure. Reducing the amount of excreted nutrients in swine production systems is an environmental priority and important issue facing the swine industry. This means a great need exists to find methods to increase the bioavailability of phosphorus in feed ingredients used to formulate swine rations.

The first University of Missouri study used 50 pigs (20 lb.), individually penned, with 10 dietary treatments. This first experiment determined the phosphorus availability of low-phytate corn versus normal corn. There were five replications per treatment. A corn starch-soybean meal basal diet (.6% calcium, .2% phosphorus) was used. Treatments consisted of the basal diet plus .05, .10, .15% added phosphorus from low-phytate corn, normal corn and monosodium phosphate (MSP).

Pigs were killed after a 35-day feeding period, and the 4th metacarpal bone was collected for bone-breaking-strength tests. Bone-breaking strength was regressed on added phosphorus intake, and the bioavailability of phosphorus was determined by slope ratio. The bioavailability of phosphorus for low-phytate corn and normal corn (relative to the standard MSP) was determined to be 64% and 10%, respectively (Figure 1).

A second study was conducted with 20 barrows (45 lb.) individually housed in metabolism crates to allow for total collection of feces and urine. Four treatments were formulated utilizing two corn sources (low-phytate or normal corn) and two levels of added phosphorus from dicalcium phosphate (0% or .2% added phosphorus).

Diets with no added phosphorus were formulated to contain .95% lysine, .6% calcium, .34% phosphorus, and were adequate in all nutrients except phosphorus. Pigs were acclimated to crates and diets for a period of five days prior to a five-day total collection period. Feces and urine were analyzed for nitrogen, phosphorus and calcium.

No differences were seen between treatments for nitrogen digestibility, percent nitrogen retained, or amount of nitrogen excreted. However, there was a significant corn hybrid by phosphorus interaction for phosphorus between treatments (Table 1). Pigs fed diets formulated with low-phytate corn and no added phosphorus showed a 37% decrease in the total amount of phosphorus excreted compared to pigs fed a typical diet (consisting of normal corn and .2% added phosphorus from dicalcium phosphate). This resulted in a 55% increase in the nitrogen-to-phosphorus ratio of the total excrement. This makes the swine waste more economically and environmentally suitable as a source of fertilizer.

A third study utilized 210 high-lean-growth barrows in treatments with two corn sources (low-phytate and normal corn) and three phosphorus feeding regimens. Phosphorus was either supplemented throughout the grow-finish period (.2% grower and .15% finisher), only during the growing phase (.2% grower and 0% finisher), or no phosphorus addition at all from dicalcium phosphate throughout the entire grow-finish period (0% grower, 0% finisher).

All grower diets were formulated to contain .6% calcium and .9% lysine. Formulated levels of calcium and lysine in all the finisher diets were .5% and .65%, respectively. Upon reaching 250 lb., pigs were scanned via real-time ultrasound for determination of 10th rib backfat (BF) and loin eye area (LEA). All pigs were then slaughtered and the 4th metacarpal extracted for determination of bone-breaking strength.

Pigs fed low-phytate corn with no added phosphorus had a greater average daily gain (ADG), improved feed efficiency and increased breaking strength when compared to pigs fed normal corn diets with no added phosphorus. The pigs fed low-phytate corn with no added phosphorus had similar performance to those fed low-phytate corn and normal corn diets with added phosphorus (Table 2). When evaluating carcass characteristics, pigs fed diets formulated with low-phytate corn had an increased LEA compared to pigs fed normal corn diets. There was no effect upon final backfat.

These results show that you can increase the phosphorus availability of corn five to six times by genetically modifying it to contain lower concentrations of phytate, thereby making swine waste more economically and environmentally suitable as a source of fertilizer. The 37% decrease in phosphorus excretion in the second experiment translated into a 55% increase in the nitrogen-to-phosphorus ratio in the total excrement. Since swine waste is typically applied to soils on a nitrogen basis, the increased nitrogen-to-phosphorus ratio decreases the potential for phosphorus accumulation in soils where swine effluent is applied. This creates a decreased potential for phosphorus pollution of surface waters where erosion of soils might occur. These results further suggest that low-phytate corn can be fed to grow-finish swine resulting in a market reduction in phosphorus excretion. There would be no detrimental effects on pig performance, bone strength or carcass characteristics, the researchers conclude.

Researchers: Joel D. Spencer and Gary L. Allee, University of Missouri, Columbia, MO, and Tom E. Sauber, Optimum Quality Grains, L.L.C. (A Dupont/Pioneer Joint Venture). Contact Allee at (573)-882-7726.