Written in collaboration with Brigitte Lapierre, agronomist, specialist consultant in forages, silage preservatives and cereals at La Coop fédéreé.
Fall alfalfa cutting management represents a critical issue for alfalfa performance. A cutting strategy that respects the plants’ physiology, combined with an adequate fertilizer and liming plan, will let you get the most from your investment!
Although it has been extensively covered, the subject still raises many questions: is it detrimental to make a final cut after the first frost? What date should I stop harvesting my alfalfa to ensure a sufficient fall rest period for good winter survival? What is the ideal time to apply potassium in fall? Et cetera.
Fall cutting management
Traditionally, we have said that alfalfa needs a period of 45 to 50 days of regrowth before the killing frost (-3° C) in order to store sufficient nutrients in the roots to survive the winter. Following research performed by Agriculture and Agri-Food Canada, we can say more precisely that there must be a certain accumulation of degree-days above 5°C between the last two cuts. The conclusions of this study recommend a minimum of 500 degree-days.
So, whether you are in Gaspésie or around Montreal, the principle remains the same. What changes according to your area is the date of the last harvest before killing frost. No matter your in-season management practices, whether intensive management with Standfast alfalfas like Actis or Althea, with their 26-30 day cutting interval, or more conventional management with alfalfas like AAC Nikon and Akori, which require 35 to 45 days between cuts, we must observe the same fall rest period. This rest period corresponds with the part of the season that days are getting shorter and the temperature drops.
These conditions give perennial plants the signal that it is time to prepare for winter. The first weeks after cutting are devoted to regrowth, while the last three to four weeks before killing frost are reserved for filling up the plants’ energy reserves, in order to survive the winter. Over the course of this period, the plants physiologically mature, flower, and complete their life cycle for the year.
Fall Potassium application
It is well known and documented that supplying alfalfa with potassium in the fall improves survival and, by extension, their yield. Potassium facilitates the accumulation of carbohydrates in the root system and represents an important energy source. Potassium must be applied immediately following the last harvest, which marks the beginning of the critical fall rest period, so that the plant can make the most of this time.
The Co-op network has a new source of potassium available that contains 0.5% boron. This is our Aspire fertilizer (0-0-58 0.5 B). Each granule of potassium contains boron, which distributes this minor element more evenly over the soil surface.
Do we do it, or should we abstain? The decision is largely a question of need. It should be done only if there has been insufficient forage production during the season to meet the herd’s needs. Otherwise, it is best to leave it in the field.
Leaving the final regrowth of the season in the field offers excellent frost protection, as it causes a snow cover to accumulate, which acts as an insulating blanket against ice and cold. It has also been shown that regrowth the following spring and first-cut yields are higher when the last growth of the previous season is left in the field (see Les plantes fourragères, a guide published by CRAAQ).
In the case where the need for forage requires a final late fall harvest, it is important to leave stubble at least 10 cm high in the field. This will let a little snow cover build up. Even though this is very minimal, this snow can make a difference between the temperature at the soil surface and the ambient air temperature. Alfalfa resists cold down to -16 to -17 °C.
Mortality factors in alfalfa
There are 4 major causes of winterkill in alfalfa:
- Smothering: During periods of winter thawing or rain, water can accumulate at the soil surface and refreeze into ice when the temperature drops again, which asphyxiates the plant. Even if the plants are in winter dormancy, they are still metabolically active and require oxygen.
- Ice and frost heaving: the force of this natural phenomenon can destroy the crown and sever roots, as the soil freezes and thaws at the surface, while deeper soil remains frozen, breaking plants apart.
- Nutrient depletion: Plants that receive adequate fertilization and that benefit from sufficient time to refill the nutrient reserves in their roots have a better chance of surviving the winter.
- Cold: As previously mentioned, alfalfa can survive exposure to temperatures from -16 to -17°F. Field residue management that results in the maximum amount of snow cover can also increase chances of survival.
The first two factors are largely out of our control; it is Mother Nature that decides! But in the latter two cases, it is possible to reduce the mortality risk by good fertilization management, pH control, and drainage. And, of course, by following fall cutting practices that respect the plants’ life cycle.