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ISU Extension Logo

Northwest Iowa Crop Update Newsletter
by Todd Vagts
ISU Extension Crops Specialist
Counties Served:  Carroll, Calhoun, Crawford, Ida, Monona, Pocahontas and Sac.

   
[Home][Special Topics][Weather Data][Subsoil H20][PDF Info] [ISU Extension][IA State University]
 

Volume 5, Number 08
Northwest IA Crop Update, May 17, 2005
Print or view this newsletter in  PDF or Microsoft Word format.

In this issue

bullet Cool and wet week delays crop development
bullet When will the corn emerge, week 2?
bullet Early planted corn approaching V4
bullet PEAQ measurements
bullet Black cutworm scouting
bullet Bean leaf beetle issues

Introduction
Another cool and wet week has further delayed crop emergence and development and has caused much anxiety with replant decisions.  Use the tools available in this newsletter as well as the ISU Extension Corn planting guide and Soybean replant guide to help make the often difficult decision on whether to leave or replant a corn/soybean field.  The extended forecast looks to be better, so hopefully area crops will progress rapidly from here on.  The earliest planted corn (if it didn’t freeze to death) should be approaching V4 by next week, and hopefully we’ll be seeing some soybeans at the VE/VC stage over the next few days.  Alfalfa is also progressing slowly as seen by the area PEAQ measurements.  Insects are still present even with the cool temperatures, black cutworm scouting should begin in corn field within a week to ten days and be observant for bean leaf beetle feeding in those first emerging soybean fields.

Weather information
Soil Temperature Area soil temperatures followed a downward trend during most of last week (4-inch daily average found in Figure 1) before leveling off over the weekend just above 50 degrees (F), and finally began moving upwards again on May 15 (Sunday, the 1st sunny day the area experienced for several days).  Interestingly, this same trend was observed the previous 3 years.  With the extended forecast looking to be relatively warm, soil temperatures should continue to climb.  Continue to monitor soil temperature at this web address:  http://mesonet.agron.iastate.edu/agclimate/display.php?src=/agclimate/daily_pics/4in-temp-out.png

graph

Growing Degree Day accumulation and predicted plant phenology stages are shown in Table 1 and Figure 2 and is forecasted through May 22.  Last week, N.W. Iowa picked up about half of the normal amount (29 vs. 59) of degree days and much less than predicted (66).  Fortunately, the week ahead looks to be slightly warmer than normal with 98 degree-days forecasted compared to a normal accumulation of 92.  Degree-day accumulation is near the 14-year average for the April 10 - 15 planting period, still much below normal for the April 20 - 25 planting period, and somewhat below normal for the May 1 – 10 planting period (Figure 2).  Corn planted on or before May 5 should be emerged with corn planted up to May 10 emerging by May 21. (Corn emergence typically occurs with an accumulation of 110 to 140 degree-days, base 50) 

Table 1.  Degree-Day Weekly Accumulation
  2005 2004 2003 14-Yr Ave
May 9-15 29 75 37 59
Forcasted May 16-22 98 87 58 92


graph 

Crop Management
Corn development   The earliest planted corn should be nearing V4 (345 GDD) by early next week and corn planted between April 20 – 30 should be nearing V2 (200 GDD) by this weekend.  Use the corn development chart (figure 2) to help predict timing of corn V stages.  If you want to figure you own crop development schedule, figure that corn requires about 75 degree-days (base 50) for each new leaf.   Pay close attention to the environment and your crop over the next couple of weeks as the V4 stage has major implications for potential grain yield.  The V4 stage begins the period when the ear is initiated in the growing point and the number of kernel rows is determined.  Any physiological (environmental or cultural induced) stress at this time may influence the potential size of the corn ear.  The V5 stage is also important, as this is the time when the growing point moves above the soil surface. 

Table 2.  Corn Yield at Various planting dates and stands, expressed as a percent of the optimum stand and date.

 

Planting Dates

 

4/20 to 5/05

5/13 to 5/19

5/26 to 6/01

6/10 to 6/16

6/24 to 6/28

 

Relative Yield Potential (%)

28 – 32,000

100

99

90

68

52

24,000

94

93

85

64

49

20,000

81

80

73

55

42

16,000

74

73

67

50

38

12,000

68

67

61

46

35

For the data set in table 1 the optimum stand level (established stand) is set at 100. From this base, relative yields for lower stands at different planting dates are suggested. Yields are based on stands that are normal in terms of uniformity of plant size and distribution. To use the table, determine the present corn population and find what the yield potential is compared to the desired stand of 30,000 at the original planting date. Compare this yield potential to the yield potential of the crop if it was replanted today (assuming a good stand was obtained with the replant). The yield potential of the replant may actually be more than the present stand, but it most likely will not be enough of an increase to cover the cost of the replant plus the risk of not obtaining a good stand with the second try

Replant Decisions:  Research at Iowa State University comparing final plant stands at different planting dates (Table 2) should be used to help make the decision on whether to leave a corn stand or replant it.  Use the description at the bottom of the table to interpret the data.  Note that (according to this research) very a good yield potential can still be obtained with planting as late as May 19 if a good stand is obtained.  Replant decisions should not be based solely on plant stands, economics also should be considered.  An excellent worksheet to use with making economic comparisons of leaving the original stand vs. replanting can be printed from this website http://www.agry.purdue.edu/ext/pubs/AY-264-W.pdf

Soybean planting  Soybean planting and emergence progress has been greatly delayed due to the extended cool and wet conditions over the last 10 days.  Warmer temperatures this week should promote emergence and hopefully dryer soils will allow finishing of planting.    Be sure to monitor planted fields closely for problems with soil crusting and seedling disease (read more on potential disease problems here:  http://www.ipm.iastate.edu/ipm/icm/2004/5-10-2004/soyseeddis.html).  Also be aware that the soybean fields to first emerge in a region tend to have a higher risk for bean leaf beetle (BLB) feeding.  BLB numbers have been relatively low the last couple of years (read more here:  http://www.ipm.iastate.edu/ipm/icm/2005/4-25-2005/blbsurvival.html), still, fields need to be scouted, particularly if bean pod mottle virus (BPMV) has been a issue in the field in the past (read more about BLB and BPMV here:  http://www.ipm.iastate.edu/ipm/icm/2005/5-2-2005/integrated.html).  Refer to Table 3 for over wintering bean leaf beetle treatment thresholds.

Table 3. Early-season bean leaf beetle economic thresholds in soybeans (beetles per plant).a

PRIVATE

Cost of Treatment ($/Acre)

Market
Value ($/bu)

Growth Stage VC
Growth Stage V1
Growth Stage V2

$6

$7

$8

$9

$10

$11

$6

$7

$8

$9

$10

$11

$6

$7

$8

$9

$10

$11

$6.00

2.0

2.3

2.7

3.0

3.4

3.7

3.1

3.6

4.1

4.7

5.2

5.7

4.9

5.7

6.5

7.3

8.1

8.9

a For beetles per row-foot, multiply number by 7.6.

Table 3.  Relative Feed Value

County

Height (in)

Maturity

RFV*

Carroll

NA

 

 

Cherokee

17

Veg

230

Lyon

NA

 

 

Monona

17

Early bud

218

O’Brien

16

Lave Veg

237

Osceola

17

Lave Veg

230

Plymouth

18

Early Bud

212

Sioux

18

Early bud

212

*Relative Feed Value

Alfalfa Management  Many alfalfa fields are entering the bud stage. The alfalfa height ranges from 16 inches in to 18 inches in NW Iowa.  May 16 PEAQ results for NW Iowa showed Relative Feed Value (RFV) ranging from 230 to 212.  Fields are progressing slower than normal.  When fields enter in the 190 RFV range, then operators who want to harvest dairy quality forages need to watch weather and opportunities to harvest.

Pest Management
Black cutworm update. Significant numbers of BCW moths were captured in mid-April and, again, in late April. Dr. Marlin Rice will have a detailed article in the upcoming Integrated Crop Management Newsletter. Dr. Rice is recommending scouting fields in late May. Watch for leaf feeding from small black cutworms and dingy cutworms NOW.  And remember, just because scouting recommendations are issued, does not mean that a cutworm problems exists, it just means there is the potential for cutworm activity based off of black cutworm flights and egg laying across the region.  (Adapted from Crop Happenings, May 17, 2005; John Holmes, ISUE Crop Specialist)

 

 

Print or view this newsletter in  PDF or Microsoft Word format.

Todd Vagts
Iowa State University Extension
Field Crops Specialist
1240 D. Heires Avenue 
Carroll, IA 51401 
Office: 712-792-2364; Cell: 712-249-6025;  Fax: 712-792-2366
Email: vagts@iastate.edu  

For questions or comments please respond to vagts@iastate.edu

The information given in this publication is for educational purposes only.
Reference to commercial products is made with the understanding that no
discrimination is intended and no endorsement by Iowa State University with
any specific product(s) used in this is implied

This page last updated on 05/17/05

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