This year’s abnormally cool and dry spring has resulted in feed shortages across much of Western Canada. This will result in many producers trying to get a second cut of alfalfa off.
To ensure that your alfalfa goes into winter in the best condition, it is recommended that you observe a hiatus from cutting during what is referred to as ‘the critical harvest period.’
The critical fall harvest period for alfalfa is the 4-6 week rest period, 400 to 450 growing degree days, base 5 C, preceding the average date of the first killing frost, when alfalfa stops growing. Avoiding harvesting during this period allows alfalfa plants to re-grow and build up sufficient root reserves to survive the winter and grow more aggressively in the spring.
When cut early in the period the alfalfa will use the existing root reserves for regrowth, emptying the tank.
Later in the period, the alfalfa uses photosynthesis to produce carbohydrates and stores them as root reserves, refilling the tank. Cutting in the middle, of the critical period, the second or third week, when root reserves have been depleted and not yet replenished, is usually higher risk than cutting at either the beginning or end of the period.
The critical fall harvest period begins around August 10 in northern areas, August 15 for most of the prairies and as late as August 25 in southern Manitoba and southern Alberta.
However, it is difficult to predict when that killing frost will actually occur. The actual date seldom occurs on the average date, so the beginning of the Critical Fall Harvest Period is a guideline only.
Even when winterkill does not occur, the extra yield harvested during the critical period is typically offset by reduced vigour and lower first-cut yields the following spring. It can sometimes be difficult to observe, but still may be significant. Research shows that the yield sacrificed by not harvesting during the critical period is usually regained in first-cut yield the following year.
If fall harvest must be done, risk of winterkill can be reduced, but not eliminated, by cutting towards the end of alfalfa growth, close to a killing frost. A killing frost occurs when temperatures reach about -4 C for several hours. After a killing frost, alfalfa feed value will quickly decline, as leaf loss occurs and rain leaches nutrients quickly.
Low root reserves will be depleted by regrowth, but lack of stubble to hold snow to insulate the alfalfa crowns against damage during cold weather may be a problem. Increasing cutting height to six inches of stubble will help.
Try to limit late cuttings to fields that are otherwise lower risk, that are well drained, with good fertility, healthy crowns and roots, etc.
Another strategy that can be used to increase snow cover is to leave snow traps or snow catch strips. These uncut strips are left every 20 to 36 feet.
In dry years, these strips have been shown to increase soil moisture anywhere between five and 30 millimetres. These strips are recommended to be left perpendicular to the prevailing winter winds.
Thom Weir is a certified crop adviser and former professional agrologist working in the Yorkton, Sask., area. You can reach him at email@example.com.