Alfalfa Stand Assessment
Table of Contents
Assessing alfalfa stands has been an issue due to the winterkill that much of Ontario has experience in the past 3 to 4 years. A decision must be made to either keep and manage a reduced stand, or replace the stand with another crop.
Alfalfa winterkill can be caused by oxygen starvation to the plant due to flooding and sometimes ice sheeting. Ice sheets can cause less oxygen to reach the dormant alfalfa crowns, but wide scale stand losses are uncommon. Usually enough stems will stick out of the ice sheet to allow some oxygen to reach the crowns. Stand losses where there is severe icing generally shows up in patches. These patches occur where the plants were already under stress, or where the ice was deep enough to completely seal the area. Generally those areas are depressions where water collects and the ice is deeper and stays for over one to three weeks.
A bigger concern is the constant freeze-thaw cycles that much of the alfalfa has been subjected to over winter. In areas that have good soil moisture the constant freezing and thawing can cause heaving. Heaved plants have their crown moved above the soil level. Severe heaving can break the taproot, subjects the crown to desiccation and the chances of survival is small. Slightly heaved plants can survive but their longevity and productivity are lessened.
The decision to keep a reduced stand or replace it has to be one
of the toughest decisions alfalfa growers face every year. To properly
assess forage stands you must inspect the field in several locations.
This involves digging up plants and looking for leaf and bud vigour,
resistance to bark peeling and a good internal root colour (white
to cream colour). Research has found that root resistance to squeezing
and the visual appearance of fungal growth on the root surface do
not give as accurate an estimation of the alfalfa plant survivability
and future yield potential. Roots with broken lateral roots have poor
chances of survival, particularly in a dry spring as in 1998 and 1999.
Estimating Yield Potential of Alfalfa
Alfalfa has the ability to produce its maximum yield potential over a range of plant stand densities. Therefore plant density is a poor estimator of yield because individual plants range in the number of stems that they will produce. Stem density is the best indicator of yield potential from a stand.
The chart gives an estimate of potential yield of an alfalfa stand relative to the number of stems assuming no additional yield contribution from other plants such as grasses.
The following table gives the yield potential based on the number of stem counts per square foot:
Table 1. Stem Counts (# per square foot)
|Stems per square foot||% Maximum Yield|
|55 or more||100%|
|40 to 50||75% to 92%|
|Less than 40||Stands too weak to keep|
The following table gives the minimum number of healthy plants per square foot for a desirable alfalfa stand:
Table 2. Plant Count ( # per square foot)
20 + plants/sq.ft.
12 to 20 plants/sq.ft.
8 to 12 plants/sq.ft.
|Year 3 or older||
Assessing whether to keep an alfalfa stand or not, usually begins in the spring time before the plants have had a chance to produce stems. At this time, you will need to count the number of plants to estimate the stand density. The best time to do plant counts is in the spring after the plants have broken dormancy to assess the health of the plants in the stand.
Other factors to consider when deciding whether to keep a stand depend on:
- the other forage species in the stand
- your forage needs over the course of the summer, fall and winter
- alternative forage options
- crop rotation
- and the availability of equipment and/or custom operators in the area.
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