Northwest Iowa Crop Update Newsletter by Todd Vagts ISU Extension Crops Specialist Counties Served:  Carroll, Calhoun, Crawford, Ida, Monona, Pocahontas and Sac.
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Volume 2, Number 7

Northwest IA Crop Update, May 6, 2002
(Word Document)

In this issue 
• Soil Temperature Warming
• DD50 accumulation and Corn Emergence
• Soybean Inoculation
• Soybean Planting Rates
• Bean Leaf Beetle & Bean Pod Mottle Virus
• Iowa Bee Rule

Introduction
Spotty rain showers and thunderstorms kept many farmers out of the field again last week.  The 7-day forecast calls for warmer conditions but continued chances of rain.  Soil temperatures have warmed significantly the last couple of days so I would expect many corn fields to be emerging now.  Soybean planting is next, read up on soybean and inoculation and plant populations on page 2.  Bean leaf Beetles are in legume fields now and will be moving to the earliest emerging soybean fields.  If you’ve had problems with bean pod mottle virus, read the latest ISU ICM newsletter.  Before spraying insecticides be aware of Iowa’s Bee Rule and make the necessary phone calls.

Weather update
Once again, the west-central IA has been slowed by scattered showers and thunderstorms.  In general, west-central north-west IA has picked up from 0.5 to 2.0 inches of rain over the last 7-days.  Soil temperatures have warmed up significantly (figure 1) and I project the temperatures will remain warm this week.  Soybeans require temperatures at least 55 degrees for growth and development.  Average 4-inch soil temperatures have moved above 55 degrees (figure 1) in just the last couple of days.  I would not expect much germination and growth activity in soybean fields planted prior to May 4.

Growing Degree Day Accumulation
The area accumulated only 45 DD50’s over the last 7 days and is projected to accumulate a good 70 DD50’s over the next 7 days (Figure 2).  Emergence of corn should have occurred on fields planted from April 15th – 18^th and a large percentage of the corn planted over the last two weeks should be emerging this week as well.  According to DD50 accumulation, the earliest planted corn may be reaching the two full leaf stage (count the first leaf with the rounded tip). 

In Figure 2, the line with squares equates to Monona, Crawford, Carroll and southern Ida, Sac and Calhoun counties.  The line with Diamonds equates to Pocahontas, Buena Vista and northern Ida, Sac, and Calhoun counties.  The dotted line is predicted GDD and crop development in 7 days.

Soybean Planting
Roger Elmore (relmore1@unl.edu) Nebraska’s Extension Crops Specialist at the NE South Central REC wrote an excellent over-view of soybean inoculation and seeding rates in the 4/12/02 Nebraska CropWatch newsletter (http://cropwatch.unl.edu/).  His overview is as follows:

Inoculation guidelines
·         A well-nodulated soybean plant should have 5 to 7 nodules on the primary root two weeks after emergence or 12 nodules per inch of tap root at flowering.

·         Inoculation is not essential in soils that have had nodulated soybeans within the last five years.

·         Maintain soil pH in the 6 to 7 range for optimum nitrogen fixation.

·         Plant seed within four hours after inoculating.

·         Investigate fungicide-inoculant compatibilities with inoculant manufacturer. Read and follow product labels. If in doubt, use in-furrow inoculant applications.

·         Always inoculate after fields are flooded and on sandy soils 

Soybean Seeding Rate

The general recommendation of 120,000 to 150,000 viable seeds per acre is for wide rows (30”) with conventional tillage systems. Special circumstances such as tillage, planting date and row spacing will modify these recommendations:

·         Increase seeding rates 10% to 20% if a drill is used.

·         Increase seeding rate 10% for a poor seed bed.

·         Increase seeding rate up to 50% in no-till systems.

·         Increase seeding rate 20% when planting before or after the optimum date.

·         Decrease seeding rates with high-value seed, but do not, under most circumstances, seed fewer than 120,000 viable seed/acre.

The goal is to end with a harvestable stand of about 100,000 plants/acre. Since soybean seed size varies greatly, insure that seeding rates are based on seeds per acre rather than pounds per acre. For more information see the NU NebGuide, "Soybean Seeding Rates" (G99-1395).

Pest Management
Bean Leaf Beetle and Bean Pod Mottle Virus Control
Soybeans will be emerging across the region over the next couple of weeks and the question will arise as to whether the Bean Leaf Beetle (BLB) should be controlled or not.  In the past, early season damage by BLB feeding has been more of a cosmetic problem rather than a yield reduction problem.  But the last couple of years we have seen an increase in Bean Pod Mottle Virus (BPMV) which is vectored (transmitted) by the bean leaf beetle.  The result of ISU studies the last couple of years suggests that to prevent the virus, we need to control the beetle (as soon as they are found in soybean fields) before they can transmit the virus to soybean plants.  But just because BLB are present doesn’t mean we need to be spraying all soybean fields.  Early planted fields that have a history of BPMV (distorted leaves and green-stem) should be under consideration for control.  Fields that do not have a historical problem or that are planted later than surrounding fields should not have an early season control application.  For more information on early season control strategies with the BLB and BPMV, go the ISU’s ICM newsletter at  http://www.ipm.iastate.edu/ipm/icm/2002/5-6-2002/blbearlyman.html

The Iowa Bee Rule and Beehive Registration
http://www.ipm.iastate.edu/ipm/icm/2002/5-6-2002/beehivereg.html

Black Cutworm Trap Catch; Go to the following web page to see local BCW trap catch numbers http://extension.iastate.edu/carroll/crops/bcw_2002_trap_catch.htm

(Word Document)