Paul Peterson and Dan Undersander
A balmy weather forecast for Minnesota and Wisconsin for the week of 28 March 2010 (>70°F forecast across much of the region!) indicates that alfalfa stands could fully break their winter 'dormancy' earlier than we might like. If April comes and goes without any hard freezes, this should be of little concern; but the odds of hard freezes in April are too high to not be alert to the condition of alfalfa-dominant stands as spring unfolds and we plan for 2010 forage supplies.
Additional alertness may be warranted this spring due to fall 2009 weather, when an exceptionally warm November following a cold, wet October may not have enabled full dormancy acquisition. Indeed, some late-2009-emerging alfalfa and grass stands 'emerged' from the early March 2010 thaw with short green 'preserved' fall 2009 regrowth. However, the extended insulating snow cover of this past winter should have limited potentially damaging cold-exposure through mid-March (though these same conditions can actually favor brown root rot infection). April weather could be the major determinant of how these stands 'decide' to proceed into the 2010 growing season.
On 23 and 24 March 2010 on the UMN-St. Paul Campus, temperatures at 2" below a bare soil surface reached 56 and 58°F for 3 hours each day, respectively. Low temperatures March 23 through March 26 reached 34ºF 4 days in a row, and 29ºF at 0.4" below the bare surface two consecutive days March 25-26. Under sod on the St. Paul Campus on March 23 and 24, high temperatures at a 2" depth reached only 51°F (~6ºF cooler than under bare soil). Low temperatures at 2" beneath the sod surface were 35ºF on March 23 and March 26, and 33ºF at a 0.4" depth beneath sod on March 26 (i.e., 4ºF warmer than under a bare surface).
The bare soil data may approximate fields with pure alfalfa stands cut late in 2009. Sod temperature data might approximate mixed-species stands, particularly those with some fall growth/residue. Thus, forage stands with dense insulating ground cover and/or residue are less susceptible to the big spring temperature swings that can cause early dormancy breakage and crown (bud) sensitivity/exposure to freezing. Heaving damage could also be possible with April freezes; especially in wet, clayey soils with pure alfalfa stands. Alfalfa in mixture with sod-forming grasses is less likely to heave.
There are many good resources detailing alfalfa 'winter'-survival assessment and injury management options, accessible via the UMN and UW Extension Forage websites at www.extension.umn.edu/forage and www.uwex.edu/ces/crops/uwforage/uwforage.htm.
In a nutshell, consider the following:
• Closely monitor all alfalfa-dominant fields multiple times as spring progresses. This requires walking fields multiple times, as drive-by assessments can be deceiving.
• Dig up some alfalfa plants to assess crown/root health. Assess root integrity and split roots to assess rot. Do this as soon as soil conditions allow and again in early May.
• Count stems in representative areas after average stem height is >6". Stands need ≥55 stems per square foot to be at maximum yield potential.
• If stands look questionable, consider the N-credit value of turning them under early or after 1st cutting and planting a grass (corn) crop in 2010.
• Mixed forage stands approximate 'sod', so are less likely to break dormancy early and less susceptible to crown-damaging low temperatures than are alfalfa monocultures.
• Alfalfa stands stressed by age, suboptimal fertility or drainage, disease, insects, and/or prior management are more apt to succumb to an April freeze than are less-stressed stands if they break dormancy early.
• Alfalfa varieties with greater fall dormancy are less susceptible to spring freezing.
• Stands on sandy soils and/or southern slopes will break dormancy sooner and thus be more susceptible to spring freeze damage. Stands in clayey soil are more susceptible to heaving.