VOLUME 22 NO. 13                                                                                                                                                                                 July 31, 2008

FIELD TOUR FOR RAGWEED

Glyphosate-resistant common ragweed may have been discovered in North Dakota. A field tour will be held August 5, 2008 near Mayville, North Dakota to demonstrate the possibility of glyphosate-resistant common ragweed and to gain a better understanding of glyphosate-resistance. Additional studies will show how to control the potential glyphosate-resistant common ragweed in sugarbeets, soybeans, and dry beans.  The tour of the research studies will begin at 9:30 a.m. and last approximately two hours. To get to the field, take exit 111 off Interstate 29. Go four miles west on North Dakota Highway 200. Turn north and go 2.5 miles on County Road 10 (158th Avenue). The research plot is in a wheat field on the east side of the road. Parking is available only along the side of the road.  In case of inclement weather, the tour will be canceled or postponed.  If inclement weather does occur, contact Jeff Stachler at (701) 231-8131, Mohamed Khan at (701) 231-8596 or Kendall Nichols at (701) 636-5665. 

START SCOUTING FOR THE RED SUNFLOWER SEED WEEVIL

Red sunflower seed weevil are emerging and can be found in R4 and early blooming sunflower fields in the major sunflower producing areas.  Peak emergence is usually in late July and early August.  The adult weevil is about 1/8 inch and a reddish-brown color.  Newly emerged adults feed on the bracts, sunflower buds, and pollen.  Female weevils must feed on pollen for fertile egg development.  Most weevils emerge from the soil by the first week of August. Field scouting for adults should begin when plants are showing yellow ray petals (R5.0) to 30% of the head shedding pollen (R5.3), and should continue until most of the plants have reached 70% pollen shed (R5.7). A plant that has reached R5.7 has few seeds still suitable for red seed weevil egg laying and should no longer be susceptible to further significant damage.  The economic loss due to larval feeding includes the loss of both seed weight and oil content. 

Economic thresholds for oilseed sunflower: 
Threshold (weevils per head) =
Cost of insecticide treatment
(market price x 21.5)(0.000022 x plant population +0.18)

The current high prices for sunflowers have lowered thresholds for the red seed weevil.  Typically 6-8 weevils per head is the economic threshold, however, this year thresholds are only 2-3 weevils per head.

Confection Sunflowers:  The economic threshold is only ONE WEEVIL PER HEAD.
Application timing targets the adult weevil to prevent egg laying.  Optimum timing is R5.0-R5.3, when more than half the plants are beginning to show yellow ray petals to 30% of the head shedding pollen and the rest of the plants are still in the late bud stage.  Insecticide applications at the late bud stage will kill weevils but are not economical.  First, seeds have not developed to a stage suitable for oviposition.  Second, eggs within the weevil are not mature, and third, adult weevil emergence is still occurring.  If weevil populations are high and an early application is made, re-infestation may occur which could mean a second insecticide application.  Banded sunflower moth and the red seed weevil may be controlled with one well timed spray.  Fields should be near 10% of the head shedding pollen (R5.1) when making this application.  This is the time when the banded sunflower moth eggs have hatched and larvae are present but before the head has seeds forming. 

SOYBEAN APHIDS POPULATIONS CONTINUE TO INCREASE

Soybean aphid numbers continue to increase in the Red River Valley Region.  Many fields are now in the critical reproductive stages (R1-R3) and will need to be treated when populations reach the economic threshold (250 aphids/plant).  Population will continue to increase if favorable weather conditions continue for aphid reproduction (low to mid 80�s).  Scouting at this time is crucial.

FOLICUR 3.6F RECEIVES SECTION 18 EXEMPTON IN ND FOR SUNFLOWERS

The Section 18 exemption label became effective July 24, 2008 and expires on August 8, 2008 for use on sunflowers to manage sunflower rust.  The label rate is 4 fluid ounces per acre, not to exceed a maximum of 8 fluid ounces per acre per year.  A copy of the Section 18 label must be in the applicators possession at the time of application.  Users must comply with all restrictions, precautions, and directions found on both the EPA-registered product label and the Section 18 exemption label.  Copies of the Section 18 exemption label may be found on the North Dakota Department of Agriculture website:  http://www.agdepartment.com/Programs/Plant/Section18Exemptions.html). 

DON�T FERTILIZE YOUR WEEDS

The following was written by Dr. Chris Boerboom, Weed Scientist, U of WI
Considering the cost of nitrogen, we can ill afford to allow weeds to use this expensive input. We conducted an experiment for the past 2 years that provided results that truly surprised us in regard to weeds and nitrogen.  Basically, we confirmed that weeds are highly effective consumers of nitrogen. As a consequence, the potential exists for weeds to capture some of the nitrogen that the corn needs for optimum yields. We just didn�t expect the effect to be so large. In terms of competition, the biggest risk would seem to be when weeds are allowed to grow too large before being controlled post emergence.  Therefore, our experiment was designed to determine how delayed weed control might affect optimum nitrogen rates in corn.  In the experiment, we applied increasing rates of nitrogen over the range of 0 to 200 lb/a. Within these increasing nitrogen rates, we controlled weeds either preemergence, at a recommended 4-inch height, at a 12-inch height, and a weedy check. Weeds were controlled post emergence with glyphosate in the Roundup Ready corn.  The 12-inch height was used to mimic a time delay that might occur because of rainy or windy weather or other delays. The time difference between the 4-inch height and the 12-inch height was 8 days in both years.  A few key findings:

� When weeds were sprayed at 4 inches, they had accumulated an average of 12 lb/a N. However, by 8 days later, they had accumulated an average of 25 lb/a N when they were 12 inches tall.

� The nitrogen uptake by the weeds reduced the nitrogen that the corn was able to accumulate. Corn had 15 lb/a less nitrogen (or 18% less) at tasseling when weeds were controlled at 12-inches tall as compared to when weeds were controlled preemergence.

� Surprisingly, corn yields were similar when 200 lb/a N was applied, regardless of whether the weeds were controlled preemergence or when they were 12- inches tall.  How could that be?  Apparently, we replaced the nitrogen that the weeds robbed from the corn early in the season to regain this yield. (When the weeds were controlled preemergence, the corn had excess nitrogen that it did not use efficiently.)  The economic optimum nitrogen rate was at least 100 lb/a lower when weeds were controlled preemergence or by 4-inches tall compared to when weeds were controlled at 12-inches (Table 1).

Table 1. Economic optimum nitrogen rates in corn with four weed management treatments at a 0.15 nitrogen:corn price ratio. 

	Economic optimum Nitrogen rate (lb N/a) Weed Management Treatment
	2006	2007
Weed Free (preemergence)	96	39
4-inch weed control timing	97	79
12 inch weed control timing	200	220
Weedy (nontreated)	200	193

- Corn yield loss averaged 15 bu/a or 7% in 2006 and 25 bu/a or 11% in 2007 when weeds were not controlled until they were 12-inches tall compared to the preemergence timing. This yield loss would not be acceptable at $2/bu corn and certainly would be outrageous at $5/bu corn. The bulk of this yield loss must have been caused by competition for nitrogen.

- At the end of the season, the best weed control (or fewest weeds) may not always provide the best corn yield.  In 2006, the plots sprayed at the 12-inch timing had the fewest weeds at the end of the season, but not the highest yield. Remember, the goal of weed management is to protect crop yield.