Background

Increasingly, producers are induced to purchase agricultural chemical program packages by marketing and price incentives. This trial was designed to look at the performance of insecticide/herbicide/fungicide compounds marketed by several companies. In addition to overall yield, information on the interactions between corn rootworm insecticides and herbicides and the relationships between yield and corn rootworm damage were to be examined.

Site and application information

The trial was located at the University of Minnesota-Southwest Research and Outreach Center, Lamberton, MN. The trial was planted into an area that was corn the previous year. Adult northern corn rootworm beetles were abundant (>5/plant) in the area of the trial during August of 2001.

Cooperators: University of Minnesota Southwest Research and Outreach Center

County: Redwood, MN

Nearest town: Lamberton, MN

Soil type: Nicollet/Clarion

PH: 6.0

P: 34 ppm

K: 192 ppm

Cultivar: DeKalb DK5332B.

Planted population: 33,000

Row spacing: 30”

Planting date: May 4, 2002

Design: Randomized complete block with 4 replications

Plot size: 10’X30’

180 lbs of N was applied as urea in the spring of 2002. Rootworm insecticides were applied at planting in a T band. Pre emerge herbicide treatments were applied 5/04/02 and Post emerge herbicide treatments were applied 6/05/02 to 1-3” tall weeds using a tractor-mounted sprayer delivering 20 gpa at a pressure of 40 PSI. The sprayer was equipped with 8002 flat-fan nozzles on a 15-inch spacing. Fungicide treatments (Quadris) were applied 6/29/02 to V8-V9 stage corn with a CO2 pressurized backpack sprayer, 8002XR flat fan nozzles on 18-inch spacing, 20 gallons/acre and 35 PSI. Temperature and Precipitation data are available at: http://swroc.cfans.umn.edu/Weather/Y2002/year2002_previous.htm

Results and discussion

Stand – No differences in stand between treatments were observed with emerged stand averaging 27,209 plants per acre. Minor feeding by dingy cutworm was observed but was less than 2 % of the plants in all treatments.

Weed control- Weed pressure was very light in the trial area with Green foxtail, Common lambsquarters and Eastern black nightshade at densities of less than 0.3/ square foot. All herbicide combinations provide 99 percent or greater control of all weed species. The lack of weed pressure is illustrated by the yield data presented in Table 1. The weedy check (Trt. 1) yielded the same as herbicide treatments without insecticide (Trt. 2, Trt 9)

Effect of Fungicide on yield - The bulk of the growing season was hot and dry. As a result, no significant foliar diseases were observed in the trial in spite of non-rotated corn. No yield difference occurred by adding a post emerge fungicide (Quadris) treatment (Trt. 3 and Trt. 4).

Crop injury – Although 4.5 percent of the plants showed emergence difficulty, differences between pre emerge treatments were not observed. Post emerge herbicide crop injury symptoms began to appear by 2 days after post herbicide treatment (DAT). Crop injury was rated on 6/12/02 or 7 DAT (Table 1). When applied after Counter CR, Callisto injury symptoms (chlorosis Figure 1.) were observed on 38% of plants. Steadfast injury symptoms (discoloration and shortened internodes Figure 1.) occurred on over 90% of the plants when this herbicide was applied after Counter CR. Callisto symptoms and most Steadfast injury symptoms had disappeared by 15 DAT and injury could no longer be detected by 21DAT. By 15 DAT, only 2.5% and 5% plants showed injury symptoms in the Steadfast + Clarity after Counter CR and Steadfast + Callisto after Counter CR treatments respectively. When Callisto + Steadfast were applied after Counter CR, the Steadfast injury symptoms were dominant. Yield reductions from herbicide x insecticide interaction were not observed and plots that showed herbicide injury were high yielding.

Rootworm damage - High winds and heavy rain occurred the evening of July 28,2002. Plots without insecticide treatment were severely lodged and impassible for obtaining root samples for rating. Several roots were dug from untreated border rows and rated 5-6 on the Iowa 1-6 root rating scale indicating severe corn rootworm larval pressure in the trial. To estimate rootworm injury by treatment a 1-5 rating lodging scale was used before harvest.

1- No lodging

2- Minor goose necking within row

3- Lodging into inter-row

4- Lodging across rows

5- Lodging across rows and stalks on ground

All treatments without rootworm insecticide averaged a rating of 5. The Lorsban and Force treatments averaged 4.1 to 4.3 respectively. Counter CR treated plots had slightly less lodging ranging from 3.1 to 4.1.

Additionally, a 1-9 scale for harvestability was used as plots were combined (Table1, Figure 2). Fortunately, these two independent scoring systems provided similar results. Unfortunately, both of these assessments of rootworm damage are subjective and discontinuous. Technically, the latter precludes an analysis of variance (ANOVA) but the results are presented in Table 1.

Rootworm control did increase yield in this trial. When similar herbicide programs with and without insecticide (Trts. 3 and 2, Trts. 9 and 7) are compared the insecticide treatments have greater yields. Several of the plots without insecticide showed nitrogen and potassium deficiency symptoms which presumably were related to greatly compromised root systems. Both Force 3G and Counter have been consistent performers in University of Minnesota. The somewhat greater level of lodging associated with Force 3G in this trial may be due to its lower solubility and availability in the root zone under the dry conditions and T-band placement during June and July. Additionally, none of the insecticides tested completely prevented lodging under the intense Northern corn rootworm pressure in this trial.

Lodging was not well correlated with yield and treatments containing any of the rootworm insecticides yielded better than those without insecticide but significant yield differences were not observed between rootworm insecticides. Although higher lodging scores tended to have lower yield, this is not true for every plot (Figure 3). The greater variability in yield among highly lodged plots is typical of rootworm damage and is partially related to harvesting difficulties in small plots. The regression line shown, although appropriate for this trial, is not likely to hold true under other situations.

Table 1. Corn stand, injury, rootworm induced lodging and yield

University of Minnesota - Southwest Research and Outreach Center 2002
Maxim XL seed treatment at 0.167 oz/cwt seed was applied to all treatments

Figure 1. Herbicide x insecticide interaction injury.
University of Minnesota - SWROC 2002
Callisto (left) and Steadfast (right)

Figure 2. Relationship of lodging and yield across herbicide and insecticide treatments.
University of Minnesota-SWROC 2002

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This page was created 11/15/02 by B. Potter with assistance from M. Werner.

A Comparison of Corn Herbicide/Insecticide Programs