• Sep 08, 2021

Late Summer Cutting Management of Alfalfa

Woman standing in front of farm equipment
Difficult alfalfa harvesting conditions sometimes result in farmers being off schedule for late summer harvesting alfalfa. This raises the question of best cutting management of alfalfa harvest as the end of summer approaches. 

If we want good winter survival and rapid green-up for optimal yields next year, alfalfa must either:
  1. be cut early enough to regrow and replenish root carbohydrates and proteins before a killing frost (24oF) or
  2. be cut so late that the alfalfa does not use significant root carbohydrates for fall regrowth.
This has resulted in the recommendation of a ‘no-cut’ window from Sept 1 to killing frost for states along the Canadian border with the ‘no-cut’ window extending from Sept 10 to frost in states immediately to the south. In areas that experience a killing frost in early October, it is recommended to have an even earlier ‘no-cut’ window that may extend as early as mid-August.

However, these dates are based on weather averages and the ‘safe’ cutting date may be either earlier or later in some areas. Research has helped define this window by indicating that alfalfa needs 500 growing degree days (GDD, [{(maximum temperature + minimum temperature)/2} minus 41oF], accumulated until a killing frost of 24oF) after the last summer cutting to regrow sufficiently for good winter survival and yield the next year. Thus, the date is not important but rather temperature following cutting and alfalfa regrowth; provided we allow for at least 500 GDD to accumulate, we can cut as late in the season as possible.

On the other extreme, we can also cut sufficiently late that little regrowth occurs. Cutting when 200 GDD or less will occur indicates that there will be insufficient regrowth to use significant amounts of root carbohydrates. These plants would also have good winter survival, especially with moderate snowfall. It is important to remember that we do not need to wait for a killing frost to take the last cutting. Cutting ahead of when a killing frost is predicted is important as haymaking conditions after a killing frost are seldom good.

In summary, we want either to take the last alfalfa harvest either early enough that regrowth will replenish reserves or so late that little to no growth occurs.

Calculating both probabilities tells us the risk of winter injury or kill when harvesting at different dates during September and October. This data in the figures below was calculated for Wisconsin sites where we had 42 years of weather history. In each graph, the blue is the probability of accumulating 500 GDD after each week. The maroon area is the probability of accumulating less than 200 GDD. The top line is the probability of accumulating either 500 GDD or less than 200 GDD after the indicated date and shows the probability of no injury or kill to alfalfa stands harvested on that date. We should assume that the graphs are for very winterhardy varieties (winter survival score of 2 or less) and that less winterhardy varieties would be at more risk.  

The effect of timing late summer cuttings on winter survival and next year yield depends on the weather following cutting and the graphs give the risk associated with cutting times over the last 42 years. We can see that, at Eau Claire, WI, (east of Minneapolis, MN) 500 GDD or more always accumulated by September 1 but at Beloit, WI (on the Illinois border) 500 GDD always accumulated until Sept 10. The middle of September through the middle of October was the riskiest time to cut alfalfa over the last 42 years.

Thus, not harvesting between Sept 1 at Eau Claire or Sept 10 at the WI/IL border and a killing frost is the safe alternative but often times harvesting a week late was not detrimental. The last half of September was the riskiest with low probability of either more than 500 GDD or less than 200 GDD accumulation. Waiting till mid Oct or later was often safe whether or not a frost has occurred. Alfalfa forage quality changes little during mid-September to October, so harvest can be delayed till close to a killing frost if the stand is to be kept.

Many will take a late fall cutting this year due to hay shortages and high hay prices, but we should remember that there is some risk of stand injury if harvesting at or near frost. Leaving alfalfa residue improves overwintering of alfalfa since the reside provides some insulation of the alfalfa crown from cold air temperatures and helps hold snow which further insulates the crown. Data has shown that the risk of taking the late fall harvest is diminished if the harvest intervals have been longer than 32 days during the growing season. Optimum soil test levels of soil pH (6.5 or higher) and potassium can also enhance winter survival of late harvested alfalfa. Cutting high to leave 6 inches of stubble is recommended in the Plains to catch and hold snow.

Fall Regrowth 42 Yearrs Eau Claire WI chart
 
Fall Regrowth 42 Yearrs Beloit WI chart 
Data collected at University of WI sites where there was 42 years of weather history

 



Thank You for Signing Up.