Dairy operations with excessive soil phosphorus (P) may be able to solve the problem with warm-season grasses.

Intensively managed warm-season grasses can be an effective tool for removing the nutrient, says Yoana Newman, a Texas A&M University forage specialist.

“It's a cost-effective way to remedy the situation, as opposed to using soil amendments that would bind the P,” says Newman.

The process is called “phytoremediation,” she adds. “It's a strategy to reduce nutrient loading of the soil through the harvesting of plant material.”

She says warm-season perennial grasses can extract more P than warm-season annuals like corn or sorghum because their potential dry matter yield is higher. They're better than cool-season grasses for the same reason.

“Cool-season grasses, either annuals or perennials, have a different photosynthesis and cannot produce the tonnage that warm-season grasses do,” says Newman.

For maximum yield and P uptake, the grasses must be cut every 28-30 days for hay, silage or green-chop, and nitrogen must be applied, if needed, she says. Intensive management can double the yield of some grasses.

Newman was a forage researcher at the University of Florida before recently moving to Texas A&M. In Florida, she was part of a team of researchers conducting a three-year study on the P extraction potential of three warm-season perennials commonly grown in that state: bahiagrass, limpograss and stargrass.

The soil was high in P from previous manure applications, but no additional manure was applied. The grasses were cut frequently and nitrogen was applied at 45, 60 or 90 lbs/acre/cutting.

“According to the first-year data, phosphorus removal capacity of the grasses increased with nitrogen fertilization,” reports Newman. “The annual phosphorus removal rate of 30 to 56 lbs translates into a reduction in the soil of 4-13 parts per million.”

She says the results also apply to other warm-season grasses because of their yield capacity.

These are important findings for dairies in Texas, Florida and anywhere large concentrations of animals are confined on land adjacent to or near watersheds.

“In Texas, the strategy of intensifying forage production systems can also be used to reduce phosphorus-based land area requirements in dairies with concentrated feeding operations,” she says. “Dairy producers are required to own or have access to sufficient acreage to apply excess manure phosphorus. By intensifying forage production, however, studies at Texas A&M show that acreage per cow can be reduced.”

In Texas A&M nutrient management studies, researchers are looking at Tifton 85 bermudagrass and other warm-season grasses, along with cool-season species.

“We're trying to create a year-round system,” says Newman.

She adds that, in some situations, manure applications can continue at low to moderate levels during phytoremediation. Intensively managed grasses may draw down soil P levels while also utilizing P from newly applied effluent.

“You can kill two birds with the same stone,” she says.