Difficult weather conditions have thrown alfalfa harvesting off schedule this year.
“This raises the question of best management for the alfalfa harvest as the end of the growing season approaches,” North Dakota State University Extension Service dairy specialist J.W. Schroeder says.
For good winter survival, alfalfa must be cut early enough in the fall to regrow and replenish root carbohydrates and proteins or so late that the alfalfa does not regrow or use any root carbohydrates.
This has resulted in the recommendation of a “no-cut” window from Sept. 1 to a killing frost. However, recent research in Quebec has helped define this window by indicating that alfalfa needs 500 growing degree days (GDDs) after the last cutting to regrow sufficiently for good winter survival and yield the next year. This means alfalfa can be cut in the fall as late as 500 GDDs continue to accumulate without hurting the winter survival, according to Schroeder.
GDDs are a measure of the amount of heat needed for plants, insects and microorganisms to grow and develop. If a plant or insect is too cold, it cannot grow. However, at some minimum temperature, growth begins. The warmer the plant or insect is, the faster it grows, up to a maximum temperature, when growth stops.
The minimum temperature for alfalfa growth is 42 F and the maximum temperature is 110 F.
GGDs are calculated by determining the average of the daily (24-hour) maximum and minimum temperatures, compared with a base temperature. If the mean daily temperature is lower than the base temperature, the GDD is zero.
The Quebec research also showed that cutting later in the fall was acceptable as long as less than 200 GDDs accumulated after cutting. When less than 200 GDDs accumulate after a late-fall cutting, little regrowth occurs to use up valuable stored carbohydrates and proteins in the alfalfa crowns and roots. This would result in good winter survival of the alfalfa plants. The North Dakota Agricultural Weather Network () or a nearby NDSU Research Extension Center are sources of GDD information.
Fall harvesting increases the risk of stand loss, so deciding whether to cut in the late summer or fall requires weighing the risk of winter injury against the need for the forage.
“We are always concerned about winterkill in North Dakota, but that may be a bit of a misconception because fall practices seem to be of far less concern than what happens in April and May,” Schroeder says. “The worst scenario is a warm April, with spring growth followed by a hard freeze in May.”