Cutting places stress on an alfalfa plant. Consider terminating healthy stands after 9-12 cuts, as they become very susceptible to disease and winterkill. Maintaining alfalfa past this point may not be as economical as terminating the stand, rotating the field into a different crop and establishing a new alfalfa stand elsewhere. Harvest is the most expensive part of growing a forage crop; planned termination keeps yield potential high enough to justify running the harvesting equipment through the field.
Other stressors, such as adverse weather, pests and diseases, may reduce the life of a stand before the target number of harvests is reached. Alfalfa stands should be assessed regularly to ensure they are healthy and productive enough to keep. These assessments include plant counts, root health checks and stem counts.
Plant Counts and Root Health Checks
When the crop has broken dormancy and is putting out new stems, it is time to do a plant count and root health assessment to determine how well the alfalfa overwintered. This first scouting acts as an early warning system, revealing any serious problems before field work starts.
The target number of healthy alfalfa plants per area varies depending on the age of the stand. Alfalfa stands thin naturally over time but can compensate because the crown of older plants is larger and sends up more shoots. Table 1 shows how many healthy plants should be present in the field. Where there are ranges, pure stands should be near the high end, while alfalfa/grass mixtures can be at the low end of the range.
Table 1. Target Alfalfa Stand Plant Count
Plants that have heaved are not healthy. Their crowns are exposed to drying winds, and their tap roots may be broken. These plants will not survive for very long.
Dig up some plants and cut open their roots. Healthy alfalfa taproots are white or cream-coloured, and firm like a potato. Unhealthy roots may be yellow or brown in the centre of the root, ropey or stringy, and may smell like rot. Plants with unhealthy roots are unlikely to survive.
If the number of healthy plants per area is lower than the targets in Table 1, producers have options and time to address their forage production needs. Fields could be patched with red clover (for protein) or grasses (for yield). Since patched fields should not be kept for another winter, this is a good opportunity to use Italian ryegrass for some very palatable haylage or baleage. Other grasses offer more summer growth. Another option is to terminate the stand and rotate it into something else. Alfalfa is autotoxic; it cannot follow itself in a rotation because seedlings will not germinate very well.
A plant count can also be done in the fall to assess the stand before winter. This can be used to make a termination decision after a tough growing year or can provide context for the spring plant count when assessing winter damage.
Stem Counts
While plant counts will alert producers to a serious problem early in the season, stem counts are a better way to assess the yield potential of alfalfa. Stem counts cannot be done until there is at least 15 cm (6 in.) of growth. However, because this is much later in the spring than a plant count, there may be fewer options available to address issues.
Each alfalfa plant sends up multiple stems. Figure 1 outlines how the number of stems per square foot relates to yield potential.
Figure 1. Alfalfa yield potential at various stem count densities.
Harvest is the most expensive part of growing a hay crop. In addition, there is very little difference in the per acre cost to harvest a low-yielding hay crop or a high-yielding hay crop. This means that a high-yielding hay crop is cheaper to grow per tonne of forage. If the yield potential is less than 75%, it is generally too expensive to keep harvesting that low-yielding crop. Consider replacing the stand if there are less than 108 stems/0.25m2 (40 stems/ft2) and the crown and root health are poor.
By the time there is enough growth to do a stem count, patching is not an option. The alfalfa will out-compete any new seeds for light and moisture. Most producers in this situation harvest first cut, terminate the stand and rotate it into something else.
Managing Alfalfa in Crop Rotations
Autumn is the best time to terminate an old stand of alfalfa so that a suitable seedbed exists in the spring and volunteer alfalfa is minimized. A mouldboard plow is the most effective tillage implement. Information on how to chemically terminate alfalfa can be found at the Ontario Crop Protection Hub at www.ontario.ca/cropprotection