Timing critical in alfalfa harvest

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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 (http://ndawn.ndsu.nodak.edu/) 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.”

The result of harvesting at a less than ideal time can be different based upon stresses (for examples, spring freeze, competition from weeds, fertility, insect infestations, disease, too rigorous of a cutting schedule, cold winter with no snowfall, winter temperatures fluctuating wildly) the crop has experienced preceding and following the "less than ideal" harvest timing. Variety selection and leaving several unharvested strips in the field to help catch additional snow make a difference, too.

Taking a last scheduled cutting in early September, possibly followed by an “unplanned” cutting in late October, is preferable to a scheduled late September/early October “last” cutting.

“This is a good principle to follow, although the most conservative approach is to not harvest in the fall at all,” Schroeder says.

Weather conditions for producing hay after the alfalfa is dormant makes hay drying difficult, so harvesting the crop as silage or baleage is less risky. If a cool-season grass is growing with the alfalfa, an option might be to graze the residual growth if fencing is available and soil conditions permit grazing. Some strategies include leaving a tall stubble to reduce the chance of ice "hugging" alfalfa crowns, provided the alfalfa is tall enough to still yield forage that can be recovered economically.

Schroeder has this advice for producers trying to decide whether to harvest their alfalfa this fall:

  • If this is the last year for the alfalfa, growth is excellent now, its value as a forage exceeds the lost value of nitrogen for the crop that follows and weather is super, then harvest.
  • If this is year two or three for the alfalfa, crop vigor is good now and the forecast is for great weather for a week, but the potential loss from the ill-timed alfalfa harvest means that the producer’s financial status will be impaired, keep the mower-conditioner in the shed.
  • If you are going to cut the alfalfa, then do so immediately. The extended forecast is for warm weather next week, which would favor hay drying and regrowth.

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