April 1, 2016 - 10:40am -- Anonymous

The 2015 season was hard on some alfalfa stands.  We had an extended period of rainy weather from the later part of May until about July 19.  During this time many alfalfa fields were harvested under wet soil conditions resulting in soil compaction and damage to alfalfa crowns.  Plant roots that I dug in some fields during that time period exhibited root rot symptoms and diminished root systems. Then, to add insult to injury, it turned dry and alfalfa stands struggled to re-establish root systems and exhibited drought-stress symptoms.  Depending upon fall harvest management, alfalfa stands could have entered winter dormancy in a weakened state as a result of these compounding stressors.  The question is; how will alfalfa stands recover and respond in 2016?  To help answer this question alfalfa growers should be doing a stand evaluation when there is 3-4 inches of growth on the plants.

An alfalfa stand evaluation should include a stand count, a stem density count and a plant root health assessment. I suggest doing stand and stem density counts in 4-5 random locations within a field.  A stand count is simply a count of living alfalfa plants (crowns, not stems) within a one-foot square area.  The following table is taken from an Iowa State University Integrated Crop Management newsletter article from March of 2013:

 

 

Plants

Per square

foot

Stand Age

Good

Marginal

Consider reseeding

Year after seeding

+12

8-12

Less than 8

2 years

+8

5-6

Less than 5

3 years

+6

4-5

Less than 4

4 years and older

+4

3-4

Less than 3

 

The next part of the evaluation is to determine the amount or density of stems per crown.  Once again, this evaluation should be done in at least 4-5 random locations within the field.  Stem density counts provide an indication of the yield potential of the stand.  The following table is taken from University of Wisconsin Extension publication A 3620; “Alfalfa Stand Assessment: Is this stand good enough to keep?”

 

Stem number/square foot

Expected result or action

Over 55

Stem density not limiting yield

40-55

Some yield reduction expected

Less than 39

Consider stand replacement

 

If stand counts and stem density counts indicate that the alfalfa stand will be productive, then the final evaluation step is a plant root health assessment.  This is done by digging up some plant roots and splitting them open.  Consider digging up 5 to 6 plants in 4 to 5 random locations within the field.  This plant root health assessment can provide an indication of how the plant will hold up to stresses in the coming growing season.  Split the plant open.  A healthy root will have a creamy white color and no to very little discoloration at the crown.  These are also plants that have a lot of shoots and the shoots are evenly distributed across the crown of the plant. 

Plant roots that have health problems are discolored when split open.  They are a darker white, tending towards a tan color.  There may be obvious areas of root rot and crown rot that are a dark brown to black in color.  There may be streaks of brown running down the root.  These are plants that have fewer stems coming out of the crown and those stems may tend to be more numerous on one side of the crown as compared to the other.  These are plants that may appear to be productive but because of their compromised root system, they may not survive the entire production year, especially if we have a hot, dry year. 

In general if more than 30% of the roots that are dug and split have brown streaks running down the root and/or black areas of root/crown rot that cover greater than 30 to 50% of the roots diameter yield potential is significantly reduced.  The grower may want to consider alternative production options such as terminating the stand after first cutting and planting to corn for silage or possibly to a warm season annual forage crop such as sudangrass or a sorghum x sudangrass.  The previously mentioned University of Wisconsin publication has a root health rating system along with color photo illustrations that can be used to make a root health assessment.