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Halloween hay and GDD
Forage minute

Halloween hay and GDD

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Winterkill in alfalfa field

Farmers don't like to see alfalfa fields in the spring like this one. Winter kill and winter injury is always on their minds in late winter.

Allowing for alfalfa to winterize before dormancy is a key factor preventing winter kill across a stand.

Traditionally, my recommendation has been to time the last cutting for roughly six weeks before the first frost. At a minimum, plants need three uninterrupted weeks to complete the transfer of carbohydrates to the crown and roots that is the winterization process. The additional three weeks gives us a cushion in case of an early frost.

While this general guideline has proven its worth over the years, many producers would love to have a bit more accurate method to time last cuttings. One way to narrow the no-harvest window down is by utilizing growing degree days (GDD).

Work from the University of Wisconsin calculated winterkill risk looking at GDD at a base 41 degrees accumulating until a killing frost of 25 degrees. The two GDD levels of importance for alfalfa stands were 500 and 200.

By providing at least 500 base 41 degress GDD after harvest, research trials showed that there was sufficient time for alfalfa to winterize. If harvest occurred with under 200 GDD left, alfalfa plants did now have sufficient time to regrow and deplete carbohydrate reserves to a level that would negatively impact winterization.

While other factors like ground cover and stress of the stand over the course of the year need to factor into the decision for a late cutting, this gives us a more accurate calendar point to shoot for if forage is needed.

As we’ve passed the 500 GDD threshold for most of the state, a tool like the High Plains RCC CLIMOD can be used to look at past years GDD and decide what the risk of getting more than 200 GDD going forward. If chances are low and extra hay is needed, it’s probably safe to take that final cutting.

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