Alfalfa growers whose stands are hit by frost — as growers saw in southeastern Manitoba this past Sunday and Monday mornings — will need to assess and get ready to mow if need be.
The growth of each alfalfa stem takes place at the tip — and a late spring frost may destroy the growing point of alfalfa, causing a stunting of the plant. A good “thumb rule” to follow is that if a third or more of the top growth is wilted by frost and drying up, immediate mowing will permit earlier development of a new crop.
If the damage is less than that, the plant should recover adequately to allow harvest at the normal time.
However, harvest of frosted stands that were clipped should be delayed to about the mid-bloom growth stage for the second harvest, to allow recovery of stored food reserves.
Alfalfa exposed to a light frost (-1 to -2 C) may look wilted for a few hours. Alfalfa plants experiencing this light frost often show frost damage on one to several sets of trifoliate leaves that were exposed at the top of the canopy. This may set the crop back a little but it should grow out of the damage. If standing yield is great enough, harvesting at this point is not a bad idea.
If temperatures get down to around -2.8 C or lower, you can expect damage to stands of alfalfa. If the temperatures drop to -3.8 to 4.4, you can expect serious damage, not only to the leaves but to the buds and growing points.
Gauging damage due to frost can be very difficult: if temperatures were around -4 C for an extended period of time, damage could be serious, but if the period of time for which the temperature was at -4 C was fairly short, damage could be minimal.
— John McGregor is the extension specialist and Green Gold program co-ordinator for the Manitoba Forage Council.