Field Studies to Determine the Suitability of Alternatives to Atrazine

David Pike, Bob Dunker, and Eric Adee

• Introduction
• Procedures
• Findings
• Summary Points
• Figure 1
• Table 1

Introduction:

Atrazine is used on more than 70% of field corn grown in Illinois. Its low cost and broad spectrum of weed control provides a high degree of value for producers. However, surface water contamination concerns has resulted in pressure to regulate atrazine more closely. Regulatory agencies must know the consequences of such restrictions if wise decisions are to be made. The purpose of this study was to determine the relative economic contribution of atrazine to field corn production. This was accomplished by comparing weed control and plot yields from a number of herbicides to atrazine standards.

Procedures:

Field corn studies were initiated at the Northwestern Illinois Research and Extension Center at Monmouth, at the South Farm Research and Demonstration center at Urbana, and at the Southwest Illinois Research and Demonstration center at Perry, Illinois to evaluate potential replacements for atrazine. Plots were 4 rows (10 ft) wide and 50 ft in length. Plots were replicated 3 times. Sieben 7685 hybrid was planted at Monmouth on 12 May 1997 at a population of 32,000. Pioneer 3335 was planted at Urbana on 30 Apr 1997 at a population of 27,500. Preemergence treatment were applied within one day of planting at both locations with air temperatures of 62 to 65 degrees Fahrenheit. Postemergence treatments were applied the 10^th of June at Monmouth and the 5^th of June at Urbana with air temperatures of 76 and 72 degrees F respectively. Weed densities were considered light at the Urbana and light to moderate at the Monmouth location prior to treatments. Weeds at the Monmouth location at the time of postemergence application were common lambsquarters (1-3"), tall morningglorry (1-2" scattered), giant foxtail (1-3"). Weeds at the Urbana location at postemergence application were common lambsquarters (2-3"), tall morningglory (2-3"), field bindweed (3-8"), giant foxtail (few), and velvetleaf (1-2"). Corn height at both locations was 6-8" at post applications.

All herbicides were applied at 30 psi at 25 gpa. At the Urbana location plots were sprayed with a hand held compressed CO₂ sprayer. At the Monmouth and Perry locations a tractor mounted compressed air sprayer was used. Plots were rated approximately two weeks after postemergence applications for weed control and visible crop injury.

Herbicides selected for comparison included those commonly used throughout the state of Illinois as well as newer products such as Hornet, Scorpion and Broadstrike. Identical herbicide treatments and rates were used at all locations with the lighter soils at the Monmouth and Perry locations best suited to the rates used. Although the higher organic matter soils at the Urbana location might typically require higher rates of soil applied herbicides, the rainfall and growing conditions present at Urbana during 1997 resulted in excellent weed control. A weed free control was established by using a standard rate of Dual herbicide over the weed-free plots followed by hand hoeing on the day post-emergence applications were made. The weed free plots will be listed as Dual+Cultivation. A cost of $24.00 per acre has been included to cover the expense of cultivation and hand hoeing in this treatment. Weedy check plots were also included for weed control and yield reference data. The experimental plots at Perry were destroyed in mid-season by a severe infestation of white-tailed deer. Data for this location is not reported.

Findings:

No discernable differences in weed control were observed except for the check plots which had no weed control. Weed control in all other treatments was above 97 percent at all locations. Some crop injury (stunting and minor leaf malformation) was noted at the Monmouth location from the Clarity and Marksman applications. Injury from dicamba compounds was not observed at either Urbana or Perry. Since these products rarely exhibit injury we speculate that the crop stunting and yield reduction at Monmouth is attributable to an interaction between weather conditions and hybrid sensitivity. Although the plots treated with Scorpion were among those with the smallest yield at the locations which went to yield, visible injury (stunting) was only observed at Urbana and not at the Perry or Monmouth locations.

In our study treatments containing dicamba (Clarity and Marksman) had yields significantly lower than comparable treatments. The treatment containing Scorpion had the lowest yields as a result of crop injury and resulted in a net loss. Considering the prevailing weather conditions and the size of the crop at application these treatments may have had much less crop injury if used in a directed spray rather than as a broadcast treatment. Although the weed pressure in these fields was estimated as light to moderate there is some evidence to suggest that the weed populations are equivalent to those found on many fields throughout the Cornbelt (Pike, unpublished data).

It is possible that the herbicide rates used in this study could have been cut back and would have given adequate weed control during the 1997 cropping season, thus increasing the net return by as much as 30 percent. However, the rates used were within the recommended use range and would be necessary under heavy weed pressure or under less favorable weather conditions.

The cost of each treatment was computed and the net return per dollar spent on herbicide was calculated. Net return was computed at $2.50 per bushel for every bushel of corn grain received above the weedy check yield. Costs of labor and application, other than for surfactants, were not included. The treatments showing the greatest net return (more than $2.50 for $1.00 invested. See figure 1) included the Harness Xtra ($3.30), Bicep ($3.15), and Dual f.b. (followed by) Buctril ($2.90). A number of treatments resulted in returns with just slightly less than $2.50 per dollar spent. These treatments included Balance+Harness+Atrazine, Dual f.b. Resource, Dual+Broadstrike, Balance+Harness, and Dual f.b. Buctril+Atrazine. Treatments with lower net return included Dual f.b. Exceed, Dual f.b. Marksman, Dual f.b. Hornet, and Dual f.b. Clarity. Within our study crop injury from the Dual f.b. Scorpion treatment was severe enough to result in a net loss of a few cents per acre.

Summary Points

Although this study was conducted during a single year only, the results suggest that the value of atrazine is significant to the corn growing farmer.

• The three treatments with the highest net returns all have atrazine as a component, suggesting that the low cost and broad spectrum weed control of atrazine was a significant contribution.

• Some atrazine alternatives had yields lower than atrazine even though weed control was excellent. These results indicate that these alternatives had a lower degree of crop safety, and that the crop injury early in the season did affect yields.

• Finally, the results from the study suggest that under conditions where minimal weed control is needed some alternatives could actually result in a net loss for money spent on weed control.

Figure 1. Net return to the use of different corn herbicides, compared to the weedy check.

Common name trade name cross reference

Dual metolachlor

Exceed rimsulfuron+nicosulfuron

Balance isoxaflutol

Marksman atrazine+dicamba

Buctril bromoxynil

Scorpion clopyralid+flumetsulam+2,4-D

Resource flumiclorac

Clarity dicamba

Broadstrike flumetsulam

Harness Xtra acetochlor+atrazine

Hornet flumetsulam+clopyralid

Bicep metolachlor+atrazine

Harness acetochlor

Table 1. Yields (Bu/A) from Atrazine alternative studies at Urbana and Monmouth Illinois for 1997.

 Within each column numbers followed with the same letter are not significantly different at the 0.05 level of probability.
f.b.=followed by