A complex balancing act
When chicken litter is applied to cropland to meet nitrogen requirements, Malone says, the long-term consequence can be a buildup of phosphorus levels in soil. Chicken manure has a high phosphorus content, relative to its nitrogen content, he explains.
Chickens require phosphorus for muscle and bone development, Malone says. Yet, as much as 75 percent of the phosphorus in corn and other grains is poorly digested by chickens. That's because the phosphorus in these chicken feeds is locked within a molecule called phytic acid, or phytate. Low-phytate corn, therefore, results in higher levels of phosphorus available to the animal.
Chickens lack phytase, the enzyme required to digest the phytate that imprisons phosphorus, making it biochemically unavailable to the animals. Thus, phytate passes through chickens unused. To meet growth and health requirements, therefore, chickens' diets must be supplemented with a more readily available form of inorganic phosphate.
In an effort to produce corn with more digestible phosphorus, Raboy identified a recessive gene that results in corn with a low phytic-acid content. Just as a cake recipe calls for certain ingredients, genes may code for many different components of corn. This dominant gene in corn codes for almost 75 percent phytate, Raboy says. But, the recessive gene identified by Raboy codes for only 35 percent.
New diet shows promise
By crossing plants with the recessive, low phytic-acid gene, Raboy and his colleagues developed a corn containing low levels of phytic acid, resulting in a line of more high-availability phosphorus (HAP) corn, also known as low-phytate corn.
"The total phosphorus in grain remains the same," says Malone. "But, the amount of phosphorus available for digestion by the chicken increases."