Influence of Imazamox Rate and Tank-Mix Combinations on Winter Annual Broadleaf Weed Control and Yield in Imidazolinone-Resistant Wheat

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Kevin Bradley, Assistant Professor, Division of Plant Sciences, University of Missouri, Columbia 65211; and Shawn Conley, Assistant Professor, Department of Agronomy, Purdue University, 915 West State Street, West Lafayette, IN 47907

Abstract

Four field trials were conducted during the 2003-2004 and 2004-2005 wheat-growing seasons to evaluate the effect of imazamox rate and tank-mix combinations on winter annual broadleaf weed control and yield in imidazolinone-resistant (IR) wheat. Results from these trials indicate that timely fall applications of imazamox will provide good control of henbit and common chickweed in IR wheat, which are some of the most common winter annual broadleaf weeds encountered in many wheat production systems in the United States. No benefit in weed control was observed by increasing the imazamox rate from 35 to 44 or 53 g/ha or by the addition of a thifensulfuron plus tribenuron or 2, 4-D tank-mix combination. Results from these trials also indicate that the addition of a 20% urea ammonium nitrate (UAN) fertilizer solution to imazamox may cause some visual injury to IR wheat and that the addition of 2, 4-D or a 20% UAN fertilizer solution can reduce IR wheat yields compared to applications of imazamox alone.

Introduction

In the early 1990s Newhouse et al. (9) isolated a wheat cultivar that exhibited tolerance to the imidazolinone herbicides through seed mutagenesis techniques. This selection has led to the development of wheat cultivars that are now marketed under the name Clearfield. Imazamox is the primary imidazolinone herbicide that is registered for use on Clearfield, or imidazolinone-resistant (IR) wheat. Imazamox is registered for use on a number of crops including alfalfa, chicory, edible legumes, soybeans, and Clearfield canola, sunflower, and rice, and controls a number of grass and broadleaf weeds (1,2).

Imazamox offers growers new options for weed control in wheat. In many states there are few herbicides available for the control of troublesome grass weeds in wheat, or those that are available have rotational crop restrictions that often prevent their use. For this reason, most of the experiments conducted thus far in IR wheat have investigated the optimum imazamox rate and application timing for the control of grass weeds like downy brome (Bromus tectorum L.), Italian ryegrass (Lolium multiflorum (Lam.), jointed goatgrass (Aegilops cylindrica Host), and feral rye (Secale cereale L.). For example, Stougaard et al. (12) found that better downy brome control was achieved with fall compared to spring imazamox applications. Similarly, Geier et al. (7) observed greater than 95% control of jointed goatgrass with imazamox at 35, 44, or 53 g/ha while rates of 44 or 53 g/ha were required to provide consistent control of feral rye. Other researchers have found that imazamox treatments in IR wheat eliminated jointed goatgrass contamination of harvested grain and increased yields by as much as 41% (3). Experiments have also revealed that the use of imazamox in IR wheat provides growers with a viable alternative for the control of diclofop-resistant Italian ryegrass, which is a widespread problem in much of the southeastern United States (4).

In spite of all the experiments that have investigated the control of grass weeds with imazamox in IR wheat, few researchers have reported on the efficacy of imazamox on common winter annual broadleaf weeds like henbit (Lamium amplexicaule L.) and common chickweed [Stellaria media (L.) Vill.]. These species are some of the most predominant winter annual broadleaf weeds encountered in many wheat production systems in the United States (14) and may cause wheat yield losses when present at sufficient densities (5,6,10,13). Similarly, few researchers have investigated how imazamox might fit into the overall wheat management system. For example, some growers may wish to tank-mix imazamox with other herbicides to broaden the spectrum of weed control received, but little to no information exists on the potential interactions that may occur between imazamox and other winter wheat herbicides. Similarly, a common practice for many winter wheat producers is to apply a herbicide in combination with a higher concentration liquid nitrogen fertilizer solution, but little to no information exists on the effects of these kinds of co-applications on wheat injury, yield or weed control with imazamox. The objective of these experiments was to determine the influence of imazamox rate and tank-mix combinations on winter annual broadleaf weed control and yield in IR winter wheat.

Evaluating Imazamox for Weed Control in IR Wheat

Separate field experiments were conducted at Lamar and Portageville, Missouri during the 2003-2004 wheat growing season and at Lamar and Columbia, Missouri during the 2004-2005 wheat growing season. The soil type at both Lamar locations was a Barden silt loam (fine, mixed, thermic Aquollic Hapludalfs) while that at Portageville was a Commerce silt loam (fine-silty, mixed, nonacid, thermic Aeric Fluvaquents), and Columbia was a Mexico silt loam (fine, smectitic, mesic Aeric Vertic Epiaqualfs). At all locations, AgriPro AP112CL winter wheat was planted conventionally in rows spaced 18-cm apart at a seeding rate of 3,700,000 seeds per ha. Fertilizer applications were made according to soil test recommendations provided by the University of Missouri Soil and Plant Testing Laboratory. Dates of major field operations for each location are presented in Table 1 while the average monthly precipitation and temperature at each experimental location is presented in Table 2.

Table 1. Dates of major field operations at the Portageville, Lamar, and Columbia research sites during the 2003-2004 and 2004-2005 wheat growing seasons.

Study	2003-2004		2004-2005
Study	Portageville	Lamar	Columbia	Lamar
Planting	Oct. 8	Oct. 15	Oct. 25	Oct. 19
Herbicide applications	Dec. 8	Dec. 5	Dec. 3	Dec. 10
Harvest	June 16	June 15	June 23	June 21

Table 2. Average monthly precipitation and temperature at each experimental location during the 2003-2004 and 2004-2005 wheat growing seasons.

Month	Portageville		Columbia		Lamar
	2003-2004		2004-2005		2003-2004		2004-2005
	Temp. (°C)	Precip. (mm)	Temp. (°C)	Precip. (mm)	Temp. (°C)	Precip. (mm)	Temp. (°C)	Precip. (mm)
October	16.2	47.5	14.4	97.3	14.9	40.1	16.0	143.5
November	11.7	120.9	8.0	133.5	8.9	67.8	9.2	174.0
December	5.0	53.8	1.8	29.0	3.5	112.8	3.4	47.5
January	2.6	82.8	-0.6	150.6	0.5	61.4	1.4	125.5
February	4.2	62.5	3.8	53.9	1.9	28.5	5.4	54.8
March	11.7	107.7	6.1	22.3	10.0	135.6	7.6	25.7
April	15.7	128.3	13.8	88.9	14.2	149.4	14.1	76.7
May	22.4	173.8	17.7	78.7	20.1	119.2	18.7	66.0
June	24.4	88.6	24.0	104.6	22.2	151.1	24.5	105.7

All herbicide treatments listed in Tables 4 through 6 were applied at a constant speed of 5 km/h with a hand-held CO₂-pressurized research backpack sprayer containing 8002 flat fan nozzle tips (Spraying Systems Co., Wheaton, IL) that delivered 140 liter/ha. The size and density of weeds at the time of each application are recorded in Table 3. Treatments were arranged in a randomized complete block design and were replicated four times. Individual plots were 1.5 by 7.5 m in size. Visual weed control and wheat injury ratings were taken at regular intervals throughout the growing season. Visual ratings were based on a scale of 0 to 100, with 0 equal to the vigor and weed ground cover observed in the untreated control plots or no wheat injury and 100 equal to complete weed control or complete wheat crop death. No weed control data was collected at the Lamar location in 2003-2004, however, as there were not sufficient densities of weeds present at this location. Wheat was harvested at all locations with a small plot combine and yields were adjusted to 13% moisture content. Grain test weight and moisture were quantified using a grain analysis computer (Dickey-John Corp., Auburn, IL).

Table 3. Average height and density of wheat and weeds encountered at each of the experimental locations.

Location	Wheat		Henbit		Common chickweed
Location	Height (cm)	Tillers (#)	Height (cm)	Density (no/m²)	Height (cm)	Density (#/m²)
Portageville 2003-2004	15.0	4	2.5	300	2.0	138
Lamar 2003-2004	8.0	3	---	0	---	0
Columbia 2004-2005	6.5	3	1.5	213	1.5	37
Lamar 2004-2005	11.0	3	1.0	60	1.4	13

Table 4. Influence of imazamox and other herbicide treatments on henbit and common chickweed control in imidazolinone-resistant wheat 4 months after treatment (MAT) at Portageville, Missouri during the 2003-2004 growing season and at Lamar and Columbia, Missouri during the 2004-2005 growing season.

Treatments^x	Rate (g/ha)	Henbit (% visual control 4 MAT)			Common chickweed (% visual control 4 MAT)
Treatments^x	Rate (g/ha)	Portage -ville	Lamar	Columbia	Portage -ville	Lamar	Columbia
Untreated	--	0 c	0 b	0 c	0 c	0 c	0 c
Imazamox	35	91 a	98 a	98 a	72 ab	99 a	98 a
Imazamox	44	99 a	98 a	99 a	69 ab	99 a	99 a
Imazamox	53	89 a	99 a	99 a	80 a	99 a	99 a
Imazamox*	35	86 a	99 a	98 a	69 ab	99 a	99 a
Imazamox Thifensulfuron + Tribenuron	35 21 +11	99 a	99 a	99 a	80 a	99 a	99 a
Imazamox 2, 4-D Amine	35 280	91 a	99 a	92 ab	61 b	99 a	99 a
Thifensulfuron +Tribenuron	21 +11	93 a	99 a	99 a	79 a	99 a	99 a
2, 4-D Amine	280	58 b	98 a	89 b	13 c	89 b	86 b

^x All treatments except those with an asterisk were applied with urea ammonium nitrate (UAN) at 1% v/v and a non-ionic surfactant at 0.25% v/v. An asterisk indicates a tank-mix with UAN at 20% v/v and a non-ionic surfactant at 0.25% v/v.

^y Means followed by the same letter are not significantly different (LSD = 0.05).

Table 5. Influence of imazamox and other herbicide treatments on visual injury to imidazolinone-resistant wheat 4 months after treatment (MAT) at Portageville, Lamar, and Columbia, Missouri during the 2003-2004 and 2004-2005 growing seasons.

Treatments^x	Rate (g/ha)	Wheat injury (% visual injury 4 MAT^y)
Treatments^x	Rate (g/ha)	Portageville	Lamar 04	Lamar 05	Columbia
Untreated	-----	0 a	0 b	0 b	0 b
Imazamox	35	0 a	0 b	5 ab	5 ab
Imazamox	44	0 a	1 ab	6 ab	3 ab
Imazamox	53	0 a	1 ab	5 ab	10 ab
Imazamox*	35	0 a	2 ab	8 a	13 a
Imazamox Thifensulfuron +Tribenuron	35 21 +11	0 a	3 a	8 a	10 ab
Imazamox 2, 4-D Amine	35 280	0 a	3 a	5 ab	5 ab
Thifensulfuron +Tribenuron	21 +11	0 a	1 ab	5 ab	6 ab
2, 4-D Amine	280	0 a	2 ab	4 ab	3 ab

^y Means followed by the same letter are not significantly different (LSD = 0.05).

Table 6. Influence of imazamox and other herbicide treatments on imidazolinone-resistant wheat yield at Portageville, Lamar, and Columbia, Missouri during the 2003-2004 and 2004-2005 growing seasons.

Treatments^x	Rate (g/ha)	Wheat yield (kg/ha^y)
Treatments^x	Rate (g/ha)	Portageville	Lamar 04	Lamar 05	Columbia
Untreated	--	5095 a	5672 a	2563 a	3499 a
Imazamox	35	5595 a	5458 a	2668 a	3415 a
Imazamox	44	4710 a	5213 a	2685 a	3276 a
Imazamox	53	5539 a	5244 a	2383 abc	2364 bc
Imazamox*	35	4900 a	5007 a	2098 c	2050 c
Imazamox Thifensulfuron +Tribenuron	35 21 +11	4482 a	4765 a	2319 abc	3031 ab
Imazamox 2, 4-D Amine	35 280	4597 a	4452 a	2113 bc	2084 c
Thifensulfuron +Tribenuron	21 +11	5165 a	4620 a	2535 ab	3272 a
2, 4-D Amine	280	4830 a	4480 a	2123 bc	2325 bc

^y Means followed by the same letter are not significantly different (LSD = 0.05).

All data were analyzed using the generalized linear model procedure in SAS (Version 8; SAS Institute, Inc., Cary, NC) and means were separated with Fisher’s protected LSD at the 5% level. There was a significant treatment by location interaction for both the weed control and wheat injury data; therefore the results are presented separately by location. All percent data were transformed using arcsine of the square root. Data transformation did not improve the model in either case; therefore nontransformed data are presented.

Imazamox Controls Henbit & Common Chickweed in IR Wheat

Weed control. At all locations, good control of henbit was observed with imazamox 4 months after treatment (MAT), regardless of rate, addition of 20% UAN fertilizer solution, or addition of 2, 4-D tank-mix partner (Table 4). The level of henbit control achieved with these imazamox treatments was similar to that achieved with thifensulfuron plus tribenuron at all locations. However, the level of henbit control achieved with thifensulfuron plus tribenuron and all imazamox-containing treatments was significantly greater than that obtained with applications of 2, 4-D at Columbia and Portageville, but not at Lamar. This may be a result of the lower density of henbit present at this location compared to that recorded at the Columbia and Portageville locations (Table 2).

Good control of common chickweed was also observed with all imazamox treatments at Lamar and Columbia, but results were more variable at the Portageville location (Table 4). For example, only the highest rate of imazamox and the imazamox plus thifensulfuron plus tribenuron combination provided 80% common chickweed control at Portageville, whereas essentially complete common chickweed control was achieved at Lamar and Columbia with all imazamox-containing treatments. This response may be a result of the greater height and density of common chickweed at the Portageville location compared to that at the Lamar and Columbia locations. At all locations, thifensulfuron plus tribenuron provided similar or better common chickweed control than the imazamox treatments, and all of these treatments provided better common chickweed control than applications of 2, 4-D alone.

Wheat injury. No treatment evaluated in these trials provided greater than 13% visual injury to wheat 4 MAT (Table 5). The highest wheat injury response was observed in Columbia during the 2004-2005 season, which may be a result of the relatively small size of wheat at the time of herbicide applications in this trial compared to the other trials (Table 3). Although the responses varied among locations, the imazamox plus 20% urea ammonium nitrate (UAN) solution resulted in some of the highest wheat injury levels at the Lamar 2004-2005 and Columbia locations. Similarly, the imazamox plus thifensulfuron plus tribenuron combination provided one of the highest levels of wheat injury at both Lamar locations and at the Columbia location.

Wheat yield response. In 2003-04, there were no differences in wheat yield between the herbicide-treated and untreated plots at either Lamar or Portageville (Table 6). Similarly, in 2004-05 there were no treatments that increased wheat yield compared to the untreated control at either location. The lack of yield response with weed removal may be explained by the relatively low winter annual weed densities at these locations (Table 3) compared to densities that might be required to cause wheat yield losses. Many authors have proposed that certain winter annual broadleaf weeds exhibit a general lack of competitiveness with wheat, especially when cultural weed control methods such as narrow row spacing and optimal plant populations are present (8). For example, Scott et al. (11) did not observe wheat yield losses even at henbit densities as high as 650 per m². Although we did not observe a yield response to the winter annual broadleaf weeds investigated in this research, it is important to note that other authors have reported densities of these winter annual broadleaf weeds that result in wheat yield reductions (5,6,10,13). Therefore, these results indicate that growers who choose to use IR wheat primarily for the control of troublesome grass weeds can also utilize this technology to eliminate potential yield reductions from winter annual broadleaf weeds as well.

In both of the 2004-2005 experiments, the 2, 4-D alone, imazamox plus 2, 4-D, and imazamox plus 20% UAN treatments reduced yields compared to the untreated control. Similarly, although no statistical differences were observed in the 2003-2004 experiments, the 2, 4-D alone and imazamox plus 2, 4-D treatments also resulted in some of the lowest yields encountered in these trials. The reason for this response is unclear, as wheat was in similar stages of growth at the time of application in both years (Table 3) and all 2, 4-D applications were made according to labeled recommendations. In spite of this, the results from both years suggest that this tank-mix combination with imazamox may cause wheat yield reductions and should be avoided.

Conclusions

Collectively, the results from these experiments indicate that fall applications of imazamox will provide good control of henbit and common chickweed in IR wheat, which are some of the most common winter annual broadleaf weeds encountered in many wheat production systems in the United States. Additionally, the results from these trials indicate that there was no increase in henbit or common chickweed control as a result of increasing the imazamox rate or as a result of tank-mixing imazamox with thifensulfuron plus tribenuron or 2, 4-D. These results also indicate that the addition of a 20% UAN fertilizer solution to imazamox may cause some visual injury to IR wheat and that the addition of 2, 4-D or the 20% UAN fertilizer solution can reduce IR wheat yields compared to applications of imazamox alone. Based on these results, it seems that these tank mix options should be avoided. Additionally, for growers who choose to utilize IR wheat for the control of troublesome grass weeds like downy brome, Italian ryegrass, or jointed goatgrass, the results from these experiments indicate that some of our most common winter annual broadleaf weeds can be controlled in these systems as well.

Literature Cited

1. Anonymous. 2006. Beyond herbicide label. Online. Greenbook, Vance Publishing Corporation, New York, NY.

2. Anonymous. 2006. Raptor herbicide label. Online. Greenbook, Vance Publishing Corporation, New York, NY.

3. Ball, D. A., Young, F. L., and Ogg, A. G., Jr. 1999. Selective control of jointed goatgrass (Aegilops cylindrica) with imazamox in herbicide-resistant wheat. Weed Technol. 13:77-82.

4. Clemmer, K. C., York, A. C., and Brownie, C. 2004. Italian ryegrass (Lolium multiflorum) control in imidazolinone-resistant wheat. Weed Technol. 18:481-489.

5. Conley, S. P., and Bradley, K. W. 2005. Wheat (Triticum aestivum) yield response to henbit (Lamium amplexicaule) interference and simulated winterkill. Weed Technol. 19:902-906.

6. Farahbakhsh, A., Murphy, K. J., and Madden, A. D. 1987. The effects of weed interference on the growth and yield of wheat. Proc. British Crop Protect. Conf. 3:955-961.

7. Geier, P. W., Stahlman, P. W., White, A. D., Miller, S. D., Alford, C. M., and Lyon, D. J. 2004. Imazamox for winter annual grass control in imidazolinone-tolerant winter wheat. Weed Technol. 18:924-930.

8. Mertens, S. K., and Jansen, J. Weed seed production, crop planting pattern, and mechanical weeding in wheat. Weed Sci. 50:748-756.

9. Newhouse, K. E., Smith, W. A., Starrett, M. A., Schaefer, T. J., and Singh, B. K. 1992. Tolerance to imidazolinone herbicides in wheat. Plant Physiol. 100: 882-886.

10. Northam, F. E., Stahlman, P. W., and Abd El-Hamid, M. 1993. Broadleaf weed control in winter wheat. West. Soc. Weed Sci. Res Prog. Rep. 3:174-175.

11. Scott, R. C., Peeper, T. F., and Koscelny, J. A. 1995. Winter wheat (Triticum aestivum) yield response to winter annual broadleaf weed control. Weed Technol. 9:594-598.

12. Stougaard, R. N., Mallory-Smith, C. A., and Mickelson, J. A. 2004. Downy brome (Bromus tectorum) response to imazamox rate and application timing in herbicide-resistant winter wheat. Weed Technol. 18:1043-1048.

13. Vrabel, T. E. 1987. Effect of fall weed control on the yield of winter wheat. Proc. N. E. Weed Sci. Soc. 41:55-58.

14. Webster, T. M. 2004. Weed survey – southern states. Grass crops subsection