ndsucpr_L_sm_W.jpg (13414 bytes)
weeds_Logo_Lg.jpg (6562 bytes)


ISSUE 5   May 31, 2001

 

TANKMIXING PRODUCTS OF DIFFERENT FORMULATIONS

Reports have come in of solidifying when herbicides of different formulations are tankmixed. In particular, mixing DoublePlay + Atrazine DF resulted in caking. No trouble was reported when DoublePlay was mixed with Atrazine F. As a reminder, the recommended sequence for addition of various herbicide formulations to a tank partially filled with water follows the A.P.P.L.E.S. method: Agitate, Powders soluble, Powders dry, Liquid flowables and suspensions, Emulsifiable concentrates and Solutions. Add surfactants, petroleum oils, MSO (methylated seed oil) type and other adjuvants last.

Compatibility testing as described on labels can be used to determine if tank mixes of pesticides will form a uniform mixture in the spray tank. Herbicide combinations should be used with caution until experience or research has shown that the combination is effective and safe.

Many pesticide labels include information on approved tank-mixes. The tank-mix must be applied according to label directions. Non-registered tank-mixes may be applied if all pesticides in the mixture are registered by the EPA on the crop being treated. However, the user must assume liability for crop injury, inadequate weed control and illegal residues for non-labeled tank mixtures.

 

HARMONY GT BURNDOWN IN CORN OR SOYBEANS.

Harmony GT may be applied through supplemental labeling either preplant or preemergence burndown in field planted to corn or soybeans. Harmony Gt may be applied with other herbicides labeled for such use. Apply at 0.3 to 0.6 oz product/A. Add a nonionic surfactant at 1 to 2 qt/100 gallons of water. NOTE - the rate is equivalent to the rate used in small grains. DO NOT apply to emerged corn or soybean. This herbicide may provide control of weeds that glyphosate may not control such as wild buckwheat and common lambsquarters.

 

CRITICAL PERIODS OF WEED COMPETITION IN CORN

The critical period of competition defines how long weeds can compete with crops before affecting yield. Two critical periods are defined. The first involves weeds that emerge at the same time as the crop and compete until weeds are removed by chemical or mechanical means. Weeds emerging prior to crop emergence have the greatest potential to reduce crop yield. The second critical period involves weeds that emerge after crop emergence. As the interval between crop and weed emergence increases the less likely that weeds will impact yields.

Researchers in Canada investigated early-season competition from mixed weed infestations on corn yield in seven experiments (Table 1). Results were highly variable among locations, with early-season competition causing a 5% corn yield loss as soon as 2 weeks after planting or as long as 7 weeks after planting. Weed density was strongly correlated with the critical period, with sites with high weed populations generally having shorter critical periods than sites with low-to-moderate infestations. The shortest critical period occurred at the site having the highest weed population, 56 weeds per sq. ft.

Table 1. Days after planting required for native weed populations to cause a 5% yield loss in corn at several sites in Ontario, Canada.

 

Location

 

Year

DAP to cause 5% yield loss.

Corn lf stage at 5% yield loss.

Weeds/sq ft.

Kemptville

1988

50

12

3

Elora

1988

22

5

25

Woodstock

1988

24

8

4

Ridgetown

1989

40

10

9

Kemptville

1989

40

10

14

Elora

1989

52

12

14

Woodstock

1989

12

3

56

Modified from Hall et al. (1992). Weed Sci. 40:441-447.
DAP = Days after planting. Mixed weed populations include green foxtail, redroot pigweed, lambsquarters, and mustard.

Researchers in Michigan controlled weeds when they reached 2, 4, 6, or 8 inches in height. In 1992, yield losses were first observed when herbicide applications were delayed until weeds reached 6 inches, whereas, in 1993 losses did not occur until the 8-inch weed height application stage (Table 2). The difference in weed densities between the 2 years was relatively small and inversely related to critical period, thus it appeared that the difference in critical period was due to environmental factors rather than weed populations.

Table 2. Effect of weed removal time on corn yield loss. (Michigan State University).

Weed ht at application

Corn ht at application.

Corn lf stage (collars)

 

DAP

% Corn yield loss

1992

2 inches

3

2

12

0

4 inches

6

3

18

0

6 inches

12

5

25

10

8 inches

18

6

31

20

Full-season compeition

68

1993

2 inches

4

2

9

0

4 inches

6

3

15

0

6 inches

12

4

20

0

8 inches

18

4

23

8

Full-season competition

49

Source: Kells, J.J. (1999) 1999 Illinois Crop Protection Conference Proceedings, pp. 63-64.
Mixed weed populations include giant foxtail, lamsquarters, redroot pigweed, common ragweed, and velvetleaf. Total weed density approximately 55 to 80 plants/sq ft.

Monsanto sponsored a multi-state study investigating the critical period in Roundup Ready corn [Loux et al. (1998), Determining the critical period of competition in Roundup Ready corn, Proceedings of the North Central Weed Science Society 53:66-67]. Similar experiments were conducted at 22 sites in the NC region during 1998. Roundup was applied at different foxtail heights, ranging from 2 to 15 inches. Late flushes were controlled with a second application of Roundup. The critical period ranged from 4- to 12-inch giant foxtail, with a 6-inch height being the most common stage where yield losses were first observed.

The results of these studies illustrate the complexity of crop-weed interactions. The time at which weeds begin to affect yield is influenced by many factors, but weed density probably has the greatest influence, followed by soil moisture early in the growing season. Based on the information provided here, a conservative recommendation would be that initial postemergence applications should be made before weed reach 4 to 5 inches in height. In fields with low to moderate infestations, there should be little risk of yield loss following this recommendation. However, in fields with high weeds populations significant yield losses may occur if applications are delayed beyond when weeds are 4 to 5 inches tall.

Richard Zollinger
NDSU Extension Weed Specialist
rzolling@ndsuext.nodak.edu


cprhome.jpg (3929 bytes)topofpage.jpg (3455 bytes)tableofcontents.jpg (4563 bytes)previous.jpg (2814 bytes)next.jpg (1962 bytes)