Weather
The ten-day forecast calls for mild temperatures but very little moisture.  Unfortunately, the current dry areas continue to get dryer.  We still have soil moisture reserves to work with, but plant stress will be inevitable in areas without rain since the first week in July.

Row-Crop and Forage Development
Figure 1 shows accumulated degree days (Y-axis) by planting date (X-axis) and in turn attempts to predict growth stage of corn based on planting date.  For a detailed discussion on how to interpret this figure, refer to the May 5 newsletter http://www.extension.iastate.edu/carroll/crops/vol_3_no_07.htm

Corn degree-days (Base 50) Mild temperatures and normal degree-day accumulation will continue on a near normal track from last week through the next seven days with 146 DD’s forecasted for this week.  The area has accumulated 89% and 86% of the degree-day totals compared to the 2002 and 2001 seasons.  Degree-days (average for NW Iowa) can be accessed at this web site:  http://www.extension.iastate.edu/carroll/crops/degree-days-2003.htm

Corn Development will approach early dough (R4) in many west-central IA corn fields.   R4 Stage (Dough) commonly occurs 24-28 days after silking.  Continued starch accumulation in the endosperm has now caused the milky inner fluid to thicken to a pasty consistency.  The reduced fluid and increased solids within the kernel at this time produce a doughy consistency. Just prior to R5, kernels along the length of the ear begin to dent or dry on top.

Corn Silage Harvest typically occurs between ½ to ¼ milk line, which under normal conditions will occur from 42 to 47 days post pollination.  According to projections based on the “Projected Pheonology Events” table, plan silage harvest to begin from August 26 to September 10 based on silking dates of July 15 – 30. 

Soybean Development has moved into the seed fill period in many fields.  This period is characterized by rapid seed filling and redistribution of dry weight and nutrients within the plant.  At initial R5 (the R5 stage is reached when one pod in the uppermost four nodes contains a seed 1/8 inch wide) reproductive development can range from flowers just open to pods containing seeds 8 mm long.  Root growth and expansion slows; therefore it's important that water be available where the developed roots are -- no longer can the tap and branch roots grow to where the water is.  At R5.5 the plant attains its maximum height, node number and leaf area.  The nitrogen fixation rate peaks and begins to drop.  The seeds at this point will begin a rapid period of dry weight and nutrient accumulation.  As development approaches R6, nutrients accumulated in the leaves and petioles begin to be redistributed to the seed.  

As mentioned earlier, this is a very critical period to maintain good moisture status in the soil.  If irrigation is available, make sure the soil water profile was full at R3 and again at R5.  The developing seeds need moisture throughout development.

Insect Pest Management

Soybean Aphid can now be found in almost all area soybean fields.  The aphids started to show up early last week and populations continued to climb through the weekend.  This is a pest that really needs to be monitored closely in soybean fields.  Typically at this period of soybean development, population thresholds would be near 1000 to 1500 aphids per plant.  But since 1) temperatures are mild (which enhance aphid development) and 2) the populations have established later in the season, thresholds have been lowered to an average of 250 aphids per plant.  Use the 250 aphids/plant as a treatment threshold to protect yield (it is believed that no yield will be lost at this level) or as a point where the field should be closely monitored for population increases.

Key points to consider when scouting for Soybean Aphid:

• Check the uppermost leaves for aphids first; this is where most aphids will be found.

• Later planted fields that are still growing new trifoliate leaves are at most risk.

• Aphid infestations can be very spotty. One field or spots in fields will have a heavy infestation; whereas the neighboring field will not.  Be sure to scout entire fields before making any decisions.

• Aphid populations can increase tenfold in seven days in optimum conditions.

• Aphid populations may persist into early September.

• It’s important to monitor the number of beneficial insects. Treating a field kills everything.

• Look for aphids with wings.  Aphid populations with a large portion that are winged will most likely decline with the next couple of days.

• Are there other pests in the field (grasshoppers, bean leaf beetles) that need to be controlled?

• If a treatment is made, leave an untreated check strip to make a yield check at the end of the season.  This is the only way that you will truly know if the insecticide application was warranted.

• Be sure to get adequate spray coverage, for aerial application the recommendation is a minimum of 5 GPA.

• Always double check your information, if in doubt, get a second, third or fourth opinion.

More information on soybean aphids can be found at:
http://www.extension.iastate.edu/carroll/crops/soybean-aphid.htm

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

This page last updated on 08/11/03