Fall Management of Alfalfa

Harvesting for quality and yield are always goals for the forage producer. Keeping the stand for more than two years is also a goal for most producers. Harvesting during critical times during the fall can have a major impact on both the survival of the alfalfa stand as well as the yield of the first cutting the following spring. I will review some important issues to think about in deciding a management program for the alfalfa crop in the fall.

Ideal Conditions

If there has been sufficient rest periods between cutting schedules during the season with at least one harvest with blossoms, a good level of carbohydrate in the alfalfa roots, a gradual hardening off during the fall months, at least four inches of snow present throughout the winter months, no exposure to alternate freezing and thawing during late winter or early spring then the risk of winter injury to alfalfa will be minimal.

Reality

In most cases, one or more of the above conditions will not be met and serious implications can result from cutting during the critical fall period which can cause alfalfa to be more susceptible to winter injury and reduce first cutting yields the following spring. Researchers have shown that 10-12 inches of top growth is necessary during the fall for alfalfa to maximize root reserve storage. It is estimated that about 50% of the available carbohydrates stored in the roots are used in respiration during the normal winter months. The remainder of the carbohydrates are used to initiate spring growth. Inadequate carbohydrate reserves in alfalfa roots usually results in either winter injury which can show up as dead plants or slower spring growth with depressed first harvest yields.

Fall Management

Much research in the Great Lakes States has shown that timing of cutting during September and October is critical in maintaining good stands of alfalfa. Recent research in Wisconsin showed when alfalfa was cut with short intervals between harvests (4 cuts), compared to longer intervals (3 cuts + 1) with the last cut after mid October, there was an associated reduction in the first cutting the following spring with the 4 cut versus the 3 cut + 1 system.

In another study recently conducted in the inter-mountain region in Utah, on late season management of alfalfa, researchers found that harvesting during the critical 2-6 week period before the mean killing-frost date (MKFD) overnight temperature <28 degrees f., resulted in consistent and significant reductions in root carbohydrates. Harvesting alfalfa during the critical period resulted in lower yields the following spring from 0 to 1.5 Tons/acre. The workers also found that harvesting during the critical period in newly established alfalfa resulted in no reduction in hay yields or stand losses, however, harvesting 4-year old stands during the critical period resulted in yield losses up to 1.25 Tons/acre.

What are appropriate fall harvesting recommendations

Allowing 4-6 weeks before the mean killing frost date (MKFD) in your area will allow alfalfa fields to store up carbohydrates adequately and help in preventing winter injury. Stands which were infested and yellowed by potato leafhoppers this summer may have been stressed and should especially be rested during the fall period. Older stands are most at risk from harvesting during the 4-6 weeks before the MKFD. If one is planning to plow up an alfalfa field next year, then that field may be a good candidate for harvesting if the forage is needed. Harvesting the same alfalfa field each year during the critical period before the MKFD will increase the risk of winter injury and reduced spring alfalfa yields. If forage is needed, then try and alternate harvesting of fields during the critical period rather than harvesting the same or all fields during this time period. Check your soil potassium levels with a soil test and if needed, apply potash this fall as proper potassium levels will aid in winter hardiness. Hay quality in the fall, because of cooler weather, is maintained for longer periods, thus, hay left standing during the critical fall period will not diminish in quality significantly.

Alfalfa is one of the best forage crops available in Michigan and with proper fall cutting management, one can reduce the risks of winter injury.

When is the first mean killing frost date expected?

Most growers have a pretty good idea when the average mean killing frost date (MKFD) will occur. Even though this will vary from year to year, by knowing the approximate date and avoiding alfalfa harvesting in the 4-6 weeks prior to (MKFD) will help to insure the alfalfa crop to survive the winter and be vigorous in the spring.

The following table compiled by Jeffrey Andreson, Michigan State University Extension Specialist, will be useful in determining MKFD for your particular area in the state.

The climatological normal date of first freezing temperature in the fall takes on added importance because of the impact on fall harvest management of alfalfa. We have chosen the 28 F mark for MKFD as alfalfa is susceptible to mean temperature at or below 28 F and because the temperature readings are taken at a 5-foot level, and temperatures at the ground or plant canopy surface are frequently colder. Remember that the date of first freeze can be significantly impacted by local factors, such as topography (especially low-lying areas), soil moisture content (the greater the soil moisture, the less likely the surface temperature to fall below freezing), and soil type (muck and coarse-textured soils are most susceptible to frost, especially when dry).

The average date of first Fall freeze for the period 1951-1990, is given in Table 1, courtesy of the Michigan Dept. of Ag. Climatology Program. Note the rapid change towards earlier dates north of a Muskegon to Bay City line. Earliest first freeze dates are in interior sections of western and central Upper Michigan and northern Lower Michigan. In table 1, the values are given in terms of percentiles. The 50th percentile is the value in which 50% of the observed data have fallen below that value, and 50% above. In general, the first Fall freeze date from year to year is very consistent, falling within 3 weeks either side of the normal value. While the values in the tables are not forecasts of future first freeze dates, the percentiles give one a feel for the expected range and magnitude of values one might encounter. Example: At Bad Axe, the normal first freeze date is October 13th. In 10% of the years between 1961 and 1990, first freeze has occurred as early as September 27th, and 90% of the seasons the date has been on or before October 28th. Another way of looking at this is that in about 1 out of 10 years the first freeze occurs before September 27th and 1 out of 10 years after October 28th.


Table 1
Statistics for first killing freeze temperature (28^oF or lower) of the fall season, 1951-1990.

Table 2
Effect of Cutting Management on Forage Quality at Arlington and LaCrosse, WI, in 1994-95.