Iowa State University Extension and Outreach

East-Central and Southeast Iowa Crop Information


CROP UPDATE 4/21/2006

April 21, 2006



Mid-April to late-April is the ideal seeding time for forages since they require soil temperatures similar to corn and soybean for germination. It is best to have forages seeded by late-April because as we get later into the spring, the soil surface tends to dry out more rapidly with the warmer temperatures, making successful establishment of forages more difficult. Seeding depth and seed-soil contact are critical for the establishment of alfalfa, smooth bromegrass, and other small-seeded forages. They should be seeded no deeper than 0.25-0.5 inches deep. Seed-soil contact can be improved by following the seeding with a cultipacker or harrow.


Stand Evaluations

Although it seems likely that there are not widespread problems with winterkill, every year there are some fields with enough stand loss to justify re-seeding. It's time to start looking at alfalfa winter survival.  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 full 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 top growth 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:








Percent yield potential:








Table 2. Recommendations based on the stem count method.




Not yield limiting.


Usually keep. Some yield reduction.


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%.



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:

Alfalfa Weevil

It is also time to start scouting hay fields for alfalfa.  Alfalfa weevils begin to hatch at 300 Growing Degree days (GDD) Base 48 and quit feeding at 900 GDD Base 48. Because south facing slopes are somewhat warmer than average, they should be scouted beginning at 200 (south of I-80) - 250 (north of I-80) GDD Base 48.

Following are GDD Base 48 accumulations as of the end of the respective day.



April 10           220              110.5         130.5           71

April 11           238              124            146.5           82.5

April 12           255.5           139            162              95

April 13           278.5           160            182.5          111.5

April 14           307.5           180.5         203             133

April 15           328              194            217.5          145.5

April 16           348              205.5         231.5          151

April 17           361.5           213.5         240.5          159

April 18           375.5           224.5         251.5          169

April 19           389              235.5         263             179


For details on managing this insect, see pages 22-23 of the April 19, 2004 Integrated Crop Management Newsletter Also, watch for updated information on the development of this and other insects during 2006.



Stalk Borers Begin to Hatch Soon


In areas of fields with high grassy weed or giant ragweed pressure in 2005, burning the residue or spraying an insecticide just prior to egg hatch are two strategies to consider if corn will be planted this year.  Stalk borers begin to hatch at 575 Growing degree days base 41 and hatch is complete at 750 GDD Base 41. Egg hatch is projected to reach I-80 within a week and highway 30 around May 2. 


Following are GDD Base 41 accumulations as of the end of the respective day.

April 10           447              247.5         301.5          181.5
April 11           472              268            324.5          200
April 12           496.5           290            347             219.5
April 13           526.5           318            374.5          243
April 14           562.5           345.5         402             271.5
April 15           590              364            422.5          289.5
April 16           617              382.5         443.5          302
April 17           637.5           397            458.5          315
April 18           658.5           411.5         473             328.5
April 19           679              428            491.5          345.5

For more details on managing this pest or for updated information on the development of this pest, see


Corn Flea Beetles in 2006

Corn flea beetles are a common pest in southern Iowa but seldom cause concern further north.  With the mild temperatures last winter, this insect may be prominent much farther north than usual in 2006.  In addition to the physical damage caused by corn flea beetles, the beetles also act as a vector for Stewart’s Disease.  Most commercial corn hybrids are resistant to this disease, but some are susceptible, and many corn inbreds and many sweetcorn varieties are susceptible.  As corn emerges, be sure to scout for this insect.  Information on scouting, thresholds, and management can be found on pages 63 – 64 of the May 7, 2001 Integrated Crop Management Newsletter or at  In addition to what is stated in the article, Poncho 250 and Cruiser 5FS (at the 0.125 milligrams per kernel rate) seed treatments also provide early season protection against corn flea beetles.

Bean Leaf Beetles in 2006

The mild winter may have also allowed very good survival of bean leaf beetles.  Information on managing the overwintering population can be found on pages 81 – 82 of the May 27, 2002 Integrated Crop Management Newsletter or  In addition to what is in the article, Cruiser 5FS provides excellent protection from the overwintering bean leaf beetles.  Also, watch as the season progresses.



Wednesday, June 21 – Thursday, June 22 (9:00 a.m. – 4:00 p.m. each day), Hay Expo 2006 - Strawberry Point, IA.  See for details.


Thursday, June 22, 1:00 p.m., SE Iowa Research & Demonstration Farm Field Day – Crawfordsville.  Watch as details emerge. Special Session for CCAs in the morning.


Wednesday, July 12, 9:00 a.m. to 4:00 p.m., SE Iowa Tile Installation Field Day – Tiling Field Demonstrations Will Include: 1) Plowing vs Trenching Equipment in Operation, 2) Shallow Vs. Deep Placement of Tile, 3) Controlled Drainage vs No Drainage, 4) Constructed Wetland to Reduce Nitrates in Tile Water. 3 hours of soil & water CCA credit for those who attend a special program from 11:00 a.m. to 3 p.m. ($40 fee for credit).


Monday, July 17 (tentative), Muscatine Island Research and Demonstration Farm Field Day (Horticulture) – Fruitland.  Watch as details emerge.


If you have any questions, please feel free to contact the Iowa State University Extension Office.

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Last Update: April 21, 2006
Contact: Jim Fawcett

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