Northwest IA Crop Update, May 14, 2001

Many are back in the fields this week, finishing up corn planting and moving to soybeans. The weather forecast looks to be fairly open, with warm (hot) days and only a chance of scattered showers across the region for the next several days. Fields in Calhoun, Pocahontas and Buena Vista counties are still drying out from rains over a week ago. It appears that corn survival in the wet and flooded areas may be better than expected. Low spots that sat under water for over five days appear to be at the highest risk. Continue to monitor these fields for emergence problems and damping off once the corn has emerged. Rotary hoeing has been necessary in many corn fields due to crusting and to reduce the potential for blowing soil and sand. A recent article in the Iowa State University Extension ICM newsletter provides more detailed information on the rotary hoe, this information can be obtained at the website http://www.ipm.iastate.edu/ipm/icm/2001/5-7-2001/hoecrust.html. Most corn in the eight county area is in the V1 to V2 stage and some of the earliest soybeans planted are just emerging. As post emerge herbicides are applied on corn ground, always be conscious of the wind speed. Most spraying is not recommended when winds are greater than 10 mph. Over-wintered Bean Leaf Beatles are emerging and can be found in last year’s soybean stubble and this year’s emerging soybean fields.

Scattered showers moved across the area last week, but most counties in NW Iowa received only minimal rainfall amounts. Most rainfall recently has been centered over southern and eastern IA counties, greatly slowing fieldwork in those areas. Dr. Elwynn Taylor predicts a fairly favorable week for field work across NW Iowa. He predicts the next major storm system to reach IA by Thursday but the moisture prospect from the system is still unknown. The last three-day’s and the forecasted 72-hour average soil temperature can be accessed online at: http://extension.agron.iastate.edu/NPKnowledge/

Crop Development

The graphics display of accumulated DD50’s has changed, but the message remains the same. This format will help visualize growth of the corn crop. Figure 1 should be used for Monona, Crawford, Carroll and southern Ida, Sac and Calhoun counties. Figure 2 should be used for Pocahontas, Buena Vista and northern Ida, Sac, and Calhoun counties. Corn planted up to April 26 should be at or approaching the V2 stage (two visible leaf collars). This is the time period in which many POST herbicides are recommended for application to corn. The growing point of corn is still below ground; it will not be above ground until about V5.

 

Table 1. Corn yield at various planting dates and stands, expressed as a percent of the optimum stand and date
	Planting Date
Stand	4/20 – 5/5	5/10	5/20	6/1	6/10
	-% Yield-
26 – 30,000	100	97	90	81	67
22,000	96	-	86	78	64
18,000	91	-	82	74	61
14,000	80	-	72	65	54
10,000	65	-	59	53	44

For the data set in table 1 the optimum stand level (established stand) is set at 100. From this base, relative yields for lower stands at different planting dates are suggested. Yields are based on stands that are normal in terms of uniformity of plant size and distribution. To use the table, determine the present corn population and find what the yield potential is compared to the desired stand of 30,000 at the original planting date. Compare this yield potential to the yield potential of the crop if it was replanted today (assuming a good stand was obtained with the replant). The yield potential of the replant may actually be more than the present stand, but it most likely will not be enough of an increase to cover the cost of the replant plus the risk of not obtaining a good stand with the second try. An excellent worksheet to use to make this comparison can be printed from the website at

http://www.agcom.purdue.edu/AgCom/Pubs/AY/AY-264.html

Alfalfa weevil: Alfalfa weevil scouting should continue across the region. No major feeding injury has yet been reported in NW Iowa. For more information on Alfalfa Weevil go to http://www.ipm.iastate.edu/ipm/icm/2001/4-30-2001/alfscout.html

Black Cutworm: We have accumulated enough GDD in NW Iowa to begin scouting for black cutworms. Scattered reports of Black Cutworm feeding has been reported in southern Iowa counties, but most of this feeding is from the Dingy Cutworm, which normally does not create economic problems in Iowa corn field. Corn is typically safe from BCW activity once it is past the V5 stage. For more information on cutworm scouting refer to http://www.ipm.iastate.edu/ipm/icm/2001/5-7-2001/earlyscout.html.

Bean Leaf Beetle: We are beginning to find bean leaf beetles (BLB) in fields. The majority of BLB over-winter in or near wooded areas where they can find good cover. Yet up to twenty percent may over-winter in the soybean stubble. As the beetles emerge, you may find these beetles on corn plants planted into last year’s soybean stubble. BLB typically will not feed on and cause injury to corn plants. The presence of BLB indicates that we did have a population survive the winter, which may cause problems this spring and summer. Late planted soybeans may be our best method of reducing the first generation population by not providing a food source when the over-wintering population emerges. Economic injury to soybeans typically does not occur until the second generation in August, yet with higher than normal populations the last couple of years, extensive feeding injury on soybeans has been observed from the over-wintered and first generation beetles. Additionally, the beetle transmits the Bean Pod Mottle Virus (BPMV), which alone can cause yield reducing injury to a soybean field. BLB samples were collected last fall and recent results indicate that most of the beetles were carrying the BPMV. This would indicate that the BPMV could be a problem again this year. The best way to control the BPMV is to control the BLB. If you suspect that you had BPMV in your soybeans last year, you may consider an early season insecticide application on the soybean field to prevent transmission of the virus by the feeding of the over-wintered beetles. For more information on early season control of the BLB, please contact me at the Carroll county extension office or visit the Website http://www.ipm.iastate.edu/ipm/icm/2001/5-14-2001/beetlevirus.html. and http://www.ipm.iastate.edu/ipm/icm/2000/4-24-2000/newsoyvir.html

For further information pertaining to this newsletter, please contact me or any of the county extension offices.  If you would like this letter to be emailed directly to you, please send an email with the desired email address to vagts@iastate.edu.

This page last updated on 02/03/04

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