Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information

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CROP UPDATE 4/7/05

April 7, 2005

Soil Temperatures

The average soil temperatures in the upper 4-inch depth for the last 5 days (April 2-6) are:

April

2

3

4

5

6

Cedar Rapids

49

53

55

57

58

Crawfordsville

48

54

55

57

57

Muscatine

52

56

58

59

60


Soil Temperature and other weather data around the state can be found at the following ISU Dept. of Agronomy site: http://mesonet.agron.iastate.edu/agclimate/index.php.

 

 

Asian Soybean Rust Update

For the latest in Asian Soybean Rust Information, check the website: http://www.plantpath.iastate.edu/soybeanrust/.

 

Section 18 Label Changes
During the winter meetings we talked about limitations on the use of fungicides, i.e. don't use a strobi followed by a strobi, or don't use a triazole followed by a triazole, or don't use Section 18 products more than twice a season.   This is now being changed. At the discretion of the Minnesota Dept. of Ag., a maximum of 3 total applications using approved Section 18 products collectively may be made. This includes all products that have been approved or may be approved for use at a later date (except Domark 230 ME at this time since its label (tetraconazole) only allows one application). However, it should be noted that no more than 2 applications may be made with any given active ingredient, i.e. you cannot make 2 applications of Tilt (propiconazole) and 1 application of Stratego (propiconazole + trifloxystrobin), since both products have the same active ingredient (propiconazole).  Iowa is requesting the same amendment but EPA has not granted it yet. We anticipate that this be approved relatively soon.

New Products Labeled
Labels for all currently labeled Section 18 products for Iowa are found at:
http://www.agriculture.state.ia.us/labelsSec18quarantine.htm.

Domark was recently added to the list of Section 18 products. It appears that Headline STAR or Headline SBR will be approved for 2005.  Headline STAR is a premix, and Headline SBR is a co-pack of Headline (a strobi) and Folicur (a triazole).  The current list of Section 3 and Section 18 products can be found at: www.plantpath.iastate.edu/soybeanrust/files/Rust_products_application_notes.pdf.

 

Scouting Update
In case you have not been following the additions to the Soybean Rust Public Web Site at http://www.sbrusa.net/.
  So far so good, with rust still only found in 2 counties in west central Florida. To correctly identify Asian soybean rust, a good hand lens (20X or greater) is needed. A couple of places to order hand lenses are: http://www.akmining.com/mine/magnify.htm and http://gallery.bcentral.com/GID5072838P3166896-Magnifiers-Email-us-for-a-complete-catalog/LOUPE-BAUSCH-LOMB-POCKET-HASTINGS-20X.aspx.

 

 

ALFALFA

Stand Evaluations


It's time to start looking at alfalfa winter survival.  Although it seems likely that there will not be widespread problems with winterkill, every year there are some fields with enough stand loss to justify re-seeding. Stands can be estimated by either counting plants (crowns) or counting stems.

Plant Counts
The basic procedure to assess alfalfa stands has been through plant counts. Commonly recommended plant counts per square foot for a pure alfalfa stand are: > 20 plants in fall for the seeding year, > 12 plants in spring for the 1st production year, > 8 plants in spring for the 2nd production year, and > 5 plants in spring for the 3rd production year. Frankly, the plant count method seems to works fine for young stands (new seedings to 1 year old stands), but it does not correlate very well with older stands.

Stem Counts
A better method to evaluate alfalfa stands is the use the stem count method. However, this method requires alfalfa topgrowth to average at least 6 inches tall before the method can be used. Count stems per square foot in 4 to 6 representative areas in the field. Use the following table to estimate the yield potential of the stand. The yield potential is in context of realistic yields normally expected from that field. Actual yields can be less depending on problems with precipitation, insects, diseases, soil fertility, and harvest losses.

Table 1. Alfalfa stem counts and corresponding estimated yield potential.

Stem counts per square foot:

>54

50

45

40

35

30

25

Percent yield potential:

100

90

81

72

62

53

44


Table 2. Recommendations based on the stem count method.

Stems/sq.ft.

Recommendation

  >54

Not yield limiting.

  40-54

Usually keep. Some yield reduction.

  <40

Consider replacing. Significant yield reduction. May still keep it if significant grass forage is present.

 

Keeping the stand is an individual decision dependant on many factors, including: hay supply, available land, cash flow, etc. Table 2 provides general recommendations on whether or not to keep a stand. However, these recommendations are for pure alfalfa stands. Alfalfa-grass mixtures have a grass component that contributes to yield. If the desired stand was a 75:25 alfalfa:grass ratio, and assessment of alfalfa stem counts is about 41 stems per square foot (about 75% of a full stand), then the overall stand should provide 100% yield potential because the grass component makes up the other 25%.

Autotoxicity
Alfalfa produces compounds that inhibit the growth of other alfalfa plants. If an old alfalfa stand is rotated back into alfalfa, there is significant potential for the growth of the new seedlings to be inhibited by these compounds produced by the older plants. Recent research from the University of Wisconsin demonstrated that when seeding alfalfa into a recently killed current stand (plowed or herbicide application), the new alfalfa usually germinates, emerges, and survives, but yields tend to only be about 75% of normal. Basic recommendations are to rotate out of alfalfa for at least one year. If you must maintain a forage stand by seeding into a current stand, and you can't interseed some other forage (red clover, ryegrass), then it is probably best to plow down the old stand, wait at least 3 weeks, then seed the new stand. Even so, you can expect about a 10 to 30% yield reduction from the life of this stand compared to a rotated stand.  If you are interested, there is a summary of some of the research at:
http://www.uwex.edu/ces/forage/pubs/autotoxicity_files/frame.htm.

 

FOR YOUR CALENDAR

Saturday, April 16, 10 a.m. to 3:00 p.m., Transitioning to Organic Workshop, Kalona Family Cupboard Restaurant- - FREE

For producers interested in transitioning from conventional to organic agriculture.
Please register by April 14 by sending an e-mail to acmckern@iastate.edu. Call 515-294-5116 for more information.

Thursday, June 23, 1:00 p.m. SE Iowa Research & Demonstration Farm Field Day - Crawfordsville

 

If you have any questions, please feel free to contact the Iowa State University Extension Office.
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Last Update: April 12, 2005
Contact: Jim Fawcett fawcett@iastate.edu


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