More rust than usual has been showing up in alfalfa fields in Nebraska and neighboring states in recent weeks. Warm, humid weather is to blame, says Bruce Anderson, forage specialist with University of Nebraska Extension.

“Rust rarely infects our alfalfa before mid-July because it won’t overwinter here,” he says. “But if summer humidity is high, like we’ve experienced recently this year, rust blown up from the south can infect our fields.”

Rust usually causes little damage in fields harvested monthly. But more mature alfalfa or alfalfa grown for seed can be injured and defoliated. Heavy infections can also greatly reduce seed yield and quality. One way to minimize damage is to early harvest infected fields.

Late-summer seedings can be especially vulnerable. “Infected seedlings may be weakened and not develop as much winterhardiness as normal, making them more susceptible to winterkill,” says Anderson. “If your fields have this problem, plan to monitor them closely next spring to determine early if you need to change your cropping plans.”

Animals fed rust-infected hay can have allergic reactions, usually more of a problem with horses than ruminant livestock. Rust also lowers the digestibility of hay, and the lower energy value often isn’t detected well by standard laboratory tests. “So if you feed rust-infected hay, your animals may not get as much energy from it as expected.”

“There’s really nothing you can do economically to control rust. So monitor, harvest, and adjust plans if needed to minimize damage,” he says.

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