MANITOBA FORAGE & GRASSLAND ASSOCIATION

Critical Harvest Period

By John McGregor, MFGA Extension Support

As we enter into the fall season many producers are looking at their alfalfa fields and wondering if they should harvest the standing alfalfa. Before making that decision there are a number of factors to consider that contribute to the potential for winterkill in the alfalfa.

Some of the factors that contribute to winterkill are:

  • Stand age- greater than 2 years
  • Variety- most varieties have a winter hardiness rating
  • Soil type- heavy or wet soils tend to be subject to heaving
  • Fertility- soils low in potassium hinder carbohydrate storage
  • Harvest management- alfalfa requires time to build root reserves for winter

Fall Rest Period(known as the Critical Harvest Period)-Harvesting alfalfa in the last 4-6 weeks of the growing period has been associated with reduced yields and stand longevity. Typically, alfalfa requires 6 weeks or 450 Growing Degree Days (GDD) toaccumulate root reserves, initiate crown buds and develop cold hardiness necessary to withstand normal winter temperatures.

To better understand the importance of the Critical Harvest Period you need to realize that after alfalfa is cut the new early growth draws carbohydrates from its root reserves. Once the alfalfa plant has enough new leaf material (6”-8”) the plant starts to produce enough surplus energy to continue growing and to replace root reserves. With a multi-cutting system (3-4 cuts per year) alfalfa doesn’t normally get enough time to fully replace root reserves and therefore a general recommendation is to allow the plants to go to full flower at least once during the growing season.

(click on map to enlarge)

For Manitoba, the critical harvest periods have been established based on averages indicating that after this date (see chart above) there is the likelihood that we will accumulate 450 Growing Degree Days (GDD)before a killing frost. A killing frost for alfalfa is -4 to -5 C but, as we get later in the season and the alfalfa is subject to cooler temperatures and shorter days, these factors increase alfalfa ability to withstand even lower temperatures. At this point, the alfalfa has stopped growing and has started to harden-off.

If less than 200 GDD are expected after the alfalfa has been cut, alfalfa will not grow to any significant degree. This can be useful if your alfalfa is in full bloom by the middle of September. Taking a final cut at this stage would normally be considered safe. For help determining GDD for your area click on Weather or go to the MAFRD Ag Weather site.

One final pointwhen taking a final cut: leave enough stubble to trap 15-20 cm of snow by leaving strips of standing alfalfa. Snow cover is necessary to insulate the crowns of alfalfa during the winter.

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