Rust usually causes little damage in fields harvested monthly, but more mature alfalfa or alfalfa grown for seed can be injured and defoliated by rust. One way to minimize damage is to harvest fields infected with rust early.
Rust can damage alfalfa in several ways. Heavy rust infections can cause leaf drop and defoliation if plants aren't cut on a timely basis. This type of injury also will greatly reduced seed yield and quality. Rust-infected hay also can cause allergic reactions in animals, more often with horses than with ruminant livestock. Rust also lowers the digestibility of hay, and this lower energy value often isn't detected well by standard laboratory tests. You may just notice that your animals aren't getting as much energy from it as expected.
One of our biggest concerns is late summer seedings infected with rust. Infected seedlings may be weakened and not develop as much winterhardiness as normal, making them more susceptible to winterkill. Plan to monitor affected fields closely next spring to determine early whether you need to change your cropping plans.
There's no good economic control for rust in alfalfa so if it develops, monitor the crop, harvest the field and adjust management plans to minimize future damage.
Bruce Anderson
Extension Forage Specialist
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