Degree-day accumulations above normal
Corn enters rapid growth stage
Soybeans enter agreement with bacterium
Corn rootworms set to hatch
Soybean Aphids found in N.E. Iowa
Check corn N status with LSNT

Introduction
A warm week is in store for western IA, which will benefit crop development.  Ample rains across the region over the last couple of weeks have left soil moisture supplies in good condition.  Soybeans are developing rapidly as new leaves are added and the root system is expanding.  Start looking for the formation of nodules which will create the soybean’s supply of nitrogen.  Corn rootworm hatch should be occurring this week along with the beginning of stalk borer migration.  The first soybean aphids (at very low levels) have been found in central and N.E. IA.  Soybean rust has not been spreading in the southern U.S., which is good news for IA.  If you plan to collect late spring nitrate samples, now is the time to do it.

Weather information
Growing Degree Day Last week northwest IA accumulated nearly 120 degree-days, greater than the average of 94 and greater that the same period in the previous two years.  If the weather forecast holds true, we should see another above average degree-day week.  A total of 159 degree-days are forecasted, compared to the average of 124.  By mid-week, degree-day accumulations will be at or above normal for all planting dates.  Accumulation and predicted plant phenology stages are shown in Table 1 and Figure 1 and is forecasted through June 12.  More detailed degree-day accumulations by planting date can be obtained at this URL:  http://www.extension.iastate.edu/nwcrops/degree-days-2005.htm

Table 1.  Degree-Day Weekly Accumulation
	2005	2004	2003	14-Yr Ave
May 30-05	119	93	84	94
Forcasted June 6-12	159	164	87	124

 

	Table 2.  2004 Degree-Day % of Average (by planting date)
	10-Apr	15-Apr	20-Apr	25-Apr	30-Apr	05-May	10-May	Ave
last week	107%	103%	95%	96%	102%	108%	101%	102%
this week	108%	105%	98%	100%	104%	109%	104%	104%

Crop Management
Corn development  Corn growth and development has really taken off in the last couple of days, and should progress quickly this week as well.  Corn across much of the region ranges from the V4 to V7 leaf stage.  Corn planted during the cool April 20 – 30^th time frame should be at or approaching the V6 stage and near V8 by early next week.  Corn planted in early April should be at the V7 leaf stage, approaching V8.   Corn at V6 is at the beginning of a period of greatly increased stalk elongation and nutrient uptake.  Total uptake of nutrients has been small through the 6th leaf stage of corn (N = 8lbs, P = 5 lbs, K = 8 lbs).  But rapid accumulation of nutrients and plant dry matter will occur from V6 through V18.  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.

Soybean Development  Soybeans range from VC to V2 (2 nodes) and like the corn, has been growing well in most areas. At V2, the lateral roots are proliferating rapidly into the top 6 inches of soil between the rows.  Cultivation during the early growth stages should be shallow to avoid damaging the developing, shallow root system. Nodule formation on the root system should become evident within the next couple of weeks.  Nodules are formed when a symbiotic relationship forms between the soybean plant and Bradyrhizobium bacteria.  The soybean plant provides nutrition (carbohydrates) to the bacteria while the bacteria fix nitrogen which is then used by the soybean plant. 

Pest Management
Corn Rootworm Hatch  Typically, corn rootworms begin to hatch around 500 degree-days (base 50) from May 1.  According to my calculations, west central IA will have accumulated 500 degree-days between June 6 and 10 in N.W. Iowa. Expect an accumulation of between 15 and 20 DD per day.  Plan your post-hatch rootworm treatments accordingly. 

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. Watch 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, northwest IA should reach 500 degree-days between June 5 and June 9.  For more information on stalk borers, refer to this ICM newsletter article:  http://www.ipm.iastate.edu/ipm/icm/node/35

Soybean Aphid:  The first soybean aphids have been found in ISU soybean research plots in central and N.E. Iowa.  Numbers found have been extremely low and are not of concern at this point.  But I think this is a good indicator that we should be looking over the next few weeks for soybean aphids in west-central IA soybean fields.

Weed Management:  For options with delayed post weed control in corn see the ISU ICM “Weed control options in late corn” (5/31/2004) article by Bob Hartzler.  http://www.ipm.iastate.edu/ipm/icm/2004/5-31-2004/weedsinlatecorn.html

Soybean Rust  There is no real news to report, which is good news.  There is a new addition to the USDA web page where you can get weekly state reports on the status of soybean development and soybean rust within each state.  View these comments at this web address:  http://www.sbrusa.net/ and click on the “state update map” on the right.

Fertility Management
If you plan to use the late spring nitrate test (LSNT) to determine the current N status of your corn fields, now is the time to take those soil samples.  Soil sampling should occur when corn is from 6 to 12 inches tall and samples should be collected within several test areas that are 1 to 10 acres in size.  Samples should be taken to a depth of 12 inches and should be comprised of 16 to 24 cores.  Refrigerate or air-dry the sample if it will not be at the lab within two days.  Often times the most difficult procedure with the LSNT is in interpreting the test results. Interpretations vary with cropping and manure history as well as with weather conditions prior to and after sampling.  Personal discretion should be used when results appear to be far from what would be expected. Increased use and field history will increase the reliability of the LSNT on individual fields.  Full information on the LSNT can be obtained from the ISU extension publication PM-1714 (http://www.extension.iastate.edu/Publications/PM1714.pdf)