Soybean aphid populations continue to rapidly expand in most area soybean fields.  Monitoring soybean developmental stage and aphid population growth is essential in making treatment decisions.  Corn development is moving forward rapidly as greater than normal degree-days are accumulated and soil moisture reserves are depleted.  Harvesting corn for silage at the correct moisture is important for maintaining quantity and quality, gage your harvest moisture by the storage structure.

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) The first extended heat wave of the season hit west-central IA, adding much above normal degree-day accumulations for the previous and forecasted next seven days.  Near 170 degree-days are forecasted for this week, the first such reading since the first week of July.  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 is nearing partial or full dent in many fields. At R5 (Dent, 35-42 days after silking) all or nearly all kernels are dented or denting. The kernels are drying down now beginning at the top where a small hard white layer of starch is forming. This starch layer appears shortly after denting.

Estimate corn grain yield

{(Ear #) x (# kernel rows) x (# kernels per row)}/90 = Estimated Yield bu/acre

Corn Silage:   Predicting when to begin harvest can be a challenging task.  Corn silage that is too wet will yield less, will have silo seepage and will produce sour tasting silage resulting in lower intake by livestock.  If corn silage is too dry then yield is often reduced, heat damage and mold more easily develops in the silo because fermentation is inadequate, and the silage has lower protein and digestibility.  Harvest moisture also depends on the storage structure.

Table 1 lists the recommended harvest moisture for different storage facilities.  Detailed information on harvesting corn for silage can be accessed at the following web site: http://www.uwex.edu/ces/crops/uwforage/Silage.htm

Soybean Development Criteria for determining soybean developmental stages:    

• R4 = Pod development at least 3/4 inch long at 1 of the 4 uppermost nodes on the main stem. About 7 days to R5.

• R5 = Seed is at least 1/8 inch long in the pod at 1 of the 4 uppermost nodes on the main stem. About 15 days to R6.

• R6 = Pod with green seed that fills the cavity at 1 of the 4 uppermost nodes on the main stem. About 18 days to R7.

Growth rate of the seeds and whole plant is still very rapid at early R6.  This rapid rate of dry weight and nutrient accumulation begins to slow in the whole plant shortly after R6, and in the seeds shortly after R6.5.

Insect Pest Management

Soybean Aphid populations continue to expand in most area soybean fields.  Scouting is essential to determine where the aphids are and if the populations are still increasing.  Most area soybean fields are in the R5 developmental stage and still have a lot of yield potential to protect.   The hot weather over the last several days does not appear to have slowed population growth like I had hoped, the cooler forecast may mean continued aphid population growth and population longevity (into September).  Be sure to include all aphids (including the small, pearly white aphids) in your counts.  It appears the small aphids may reproduce and feed just as well as larger aphids.

Key points to consider when scouting for Soybean Aphid:

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

• Count all aphids, including the small, pearly white aphids.

• Drought stressed fields may loose a greater percentage of final yield compared to a non-stressed field.

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

• Aphid infestations can be very spotty.  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.

• 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.  Be sure to get adequate spray coverage, for aerial application the recommendation is a minimum of 3 GPA.

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 08/20/03