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 11

Northwest IA Crop Update, June 3, 2002
(Word Document)

In this issue 
DD50 accumulation and Corn development
Soybean Stand Evaluation
Bean Leaf Beetles
Stalk Borer Migration

Introduction
Last week’s record warmth brought the local crop development back up to near normal.  Corn growth has really accelerated and soybeans have been emerging and growing well across northwest Iowa.  Most soybean fields have emerged extremely well considering the earlier cool conditions. The earliest planted corn is at or near the 5^th leaf stage, which means the growing point will be moving above ground.  Insect pest problems have been variable, Black cutworm and White grub have been found in some fields, but I’ve not heard of any major problems.  The Bean Leaf Beetle has been active in most soybean fields, but again, populations have been lower than expected and only a few fields have been treated.  We should be on the back end of the over-wintered population cycle.

Weather update
The record breaking warmth across west-central Iowa has come to an end this week; cooler temps should prevail for the next couple of days.  By the end of the week more moderate conditions should prevail, making for ideal plant growth and herbicide application conditions.  Rainfall across north-west Iowa varied from 0 to over 3 inches this last week.  Rain is forecasted through Tuesday and then again for this weekend. 

Growing Degree Day Accumulation
What a difference one week can make.  For the first time, the area accumulated more DD50’s than was predicted.  Last week the area accumulated 156 degree-days (forecasted 132), which was almost half of the total accumulated since April 20.  The area is forecasted to accumulate 118 DD50’s over the next 7-days.  Most of the corn should be at or near the 4^th true leaf and will be approaching the 6^th leaf by this time next week.  This means the growing point will be moving above ground (normally at the 5^th leaf stage of development) which makes the plant much more vulnerable to injury.  As mentioned last week, the V4 stage also has major implications for potential grain yield as this is the time when the ear is initiated in the growing point and the number of kernel rows is determined.

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.

Assessing soybean stand loss
For the most part, I think soybeans stands are good to excellent across most of the region, yet for those few fields or spots in fields where stands may need to be evaluated, the following information can be used to asses soybean stand.  John Holmes (NC IA Extension Crop Specialist) suggests the key things to remember when evaluating soybean stands is that research has shown (1) that roughly half a stand will produce yields equal to a full stand if weeds are controlled, and (2) that it’s critical to evaluate plants for bruising (hail) and/or seedling disease.  When soybeans are planted in the first week of June they normally produce 90 - 95% of typical yields -- depending on the fall weather.  Soybeans will develop from axillary buds located at each node in the plant if the top has been cut off.  Watch for regrowth from the buds when evaluating the stand.  Acceptable stands are 4 - 4.5 beans/foot of row in 30-inch rows, 3-4 beans/foot of row in 15-inch rows, and 80,000-100,000 plants per acre in drilled fields.  Losses from 18 inch gaps or less are insignificant.  When gaps range from 2-3 feet, yields are reduced by about 5%. 

If soybeans are going to be re-planted, use the same maturity as was planted originally.  The only exception would be for very full season varieties.  If a very full season maturity group was used originally, you may want to consider shifting to a slightly earlier maturity group.  Don’t make dramatic shifts to an earlier variety --- it will cost you yield especially in a late fall.

Additional considerations:
·         Planting or drilling directly into the existing stand without destroying the previous stand has given mixed results through the years -- more often bad than good.

·         Late planted soybeans will be shorter than when they are planted in May, drilling helps establish the canopy quicker thus shading weeds & utilizing sunlight more efficiently.

References:  http://www.extension.iastate.edu/Publications/PM1851.pdf

Pest Management
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  We should be scouting for cutworm activity now in area cornfields.  Corn is typically safe from BCW activity once it is past the V5 stage. 

White Grub:   I’ve heard of very few white grub problems so far this year.  Hopefully this will stay true for the remainder of the spring.  Yet I am looking for grub problem areas to do a White Grub control study.  If you know of an area or field that will be replanted due to grubs, please let me know.

Bean Leaf Beetle control threshold - Back by popular demand.  Bean leaf beetles are feeding in area soybean fields causing some defoliation, but in most cases, beetle numbers have not been enough to warrant treatment.  Table 1 can be used to help evaluate if treatment for over-wintering bean leaf beetles is warranted.  The threshold is based on economic feeding injury only; it does not account for the potential spread and injury from the bean pod mottle virus. 

Table 1. Early-season bean leaf beetle economic thresholds in soybeans (beetles per plant).^a

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

Stalk Borer Migration  Stalk borers will soon be leaving areas of tall grass (ditches, waterways, fence lines, etc.) for the edge rows of corn fields. Virgil Schmidt (ISU Extension Crop Specialist East-Central IA) recommends watching for heads of bromegrass and orchardgrass that turn brown early as these are the source of the insects that will move into the corn field. Scouting should begin when 1,300 - 1,400 Degree Days (Base 41) have accumulated. Based on current weather forecasts, we should reach 1,300 GDD south of Highway 20 by about June 9th and North of Hwy 20 by June 14th.

(Word Document)

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