Managing freeze-damaged alfalfa
The hard freezes last week could affect some alfalfa stands. If so, producers will have to decide how to manage their stands in the coming weeks.
Figure 1. Freeze-damaged alfalfa in Hodgeman County, taken April 17, 2013. Photo by Jim Shroyer, professor emeritus, K-State Research and Extension.
Figure 2. Closeup of freeze-damaged alfalfa. The apical meristems have been killed on most stems. These plants are not tall enough to cut or shred. Regrowth from the crown should begin as soon as it warms up. Photo by Jim Shroyer, professor emeritus, K-State Research and Extension.
Here are some key points:
* In established stands, the growing point is at the top of each stem, and is protected within a cluster of leaves. The leaves may have freeze damage, but the growing point might not be affected. If it is cold enough for a long period of time, such as occurred in most western Kansas last week, the growing point may also be killed by freezing temperatures. In the photo, many of the terminal buds are frozen and they have a bleached appearance.
No action will be needed on the alfalfa stands if new growth begins emerging from the tips of the stems, or if the plant begins branching out below the tips. In both cases, the new growth means the growing points were unaffected and the plants are recovering.
If new shoots are emerging from the crown buds, however, there will be very little regrowth from the damaged stems. In much of western Kansas there isn’t enough topgrowth to cut or graze, but that is an option, if there is enough growth for it to be worthwhile before the new growth gets tall enough to be damaged by the mowing. Do not cut or damage new regrowth from the crown buds. That could severely damage the stand.
If there is no regrowth occurring at all after 7-10 days of warm weather and the plants are severely wilted without recovery, mow or shred the plants to encourage new regrowth from the crown buds.
* If you plan to shred or cut the damaged stands, be sure to leave at least 2-3 inches of stubble. This will help encourage regrowth.
* Freeze-damaged alfalfa that is only 6-8 inches tall or less will be slower to regrow after mowing or shredding than taller alfalfa. That’s because alfalfa plants are depleting carbohydrate reserves from the roots during the first 6-8 inches of growth, and will not have as much carbohydrate reserves for regrowth as taller alfalfa. With slower regrowth, producers will have to watch especially closely for insect infestations and treat if necessary. Alfalfa taller than 8 inches will have manufactured a new supply of carbohydrate reserves for the root and crown, and will be able to regrow more quickly after mowing or shredding.
* If damaged stands are cut, producers should watch the regrowth carefully for further infestations of alfalfa weevil and possibly pea aphids, and treat immediately. Weevil larvae that survive in the leaf litter on the soil surface will start feeding on the new growth once the weather warms up. Alfalfa weevil has already been reported in Kansas this spring and the hard freezes probably didn’t kill the weevil larvae in all areas, so producers should not rely on that to have happened.
Some producers were able to get an insecticide application on fields between cold spells and wind gusts. They have reported some dead larvae. However, up until last night (the night of March 24/25),the cold temps in south central and north central Kansas had not been cold enough to harm the alfalfa weevil larvae. Whether it did last night or not won’t be known for a few more days. Thus, fields should continue to be monitored every three days (at least weekly) for a couple more weeks.
If the foliage is actually killed back to ground level, the weevils will continue to feed on the regrowth and thus hold that regrowth back significantly. The majority of the weevils are still very small, thus the majority of their feeding is yet to happen. The larvae will be in the foliage and if that foliage has been wilted due to the cold then it may form a protective canopy and the larvae will be well protected from a chemical application.
Insecticides and a hard freeze will both kill all the lady beetles, parasitic wasps, and any other beneficial insects that help keep the aphids under control, so aphids may rebound fairly quickly. Thus, scouting should continue for weevils and aphids even if an effective insecticide application has already been made. If an insecticide has already been applied, pay attention to the label for the pre-harvest interval (PHI) and number of applications allowed per cutting for the product used.
* If an insecticide had already been applied to the alfalfa for weevil control, producers will have to be aware of any residual insecticide in the alfalfa that may affect how it can be utilized.
Romulo Lollato, Wheat and Forages Specialist
Jeff Whitworth, Extension Entomologist
Holly Schwarting, Research Associate, Entomology