1 Professor, Graduate Research Assistant and Associate Research Scientist, University of Wyoming, Dept. of Plant Sciences.
Laramie, Wyoming, USA, 82071, Email sdmiller@uwyo.edu , akniss@uwyo.edu , cmalford@uwyo.edu
2 Professor, University of Nebraska, Panhandle Research and Extension Center, Scottsbluff, Nebraska, USA, 69361,
Email rwilson1@unl.edu
3 Professor, Colorado State University, Dept. of Bioagricultural Sciences and Pest Management Dept., Fort Collins, Colorado, USA, 80523, Email Philip.Westra@colostate.edu
The commercialization of numerous glyphosate tolerant crop varieties could potentially allow farmers in the Central High Plains to grow a three, four or five – crop rotation utilizing only glyphosate as the sole weed management tactic. Plots were established under irrigation at Torrington, Wyoming; Scottsbluff, Nebraska; and Ft. Collins, Colorado in 1998 to determine if glyphosate use pattern in glyphosate tolerant crops influences weed control by placing selection pressure on weed species, alters weed population dynamics, or leads to development of glyphosate resistant weeds. After six years there was no evidence that any species has developed resistance to glyphosate. However, at all three sites common lambsquarters (Chenopodium album) populations have increased in the treatments receiving 0.4 kg/ha of glyphosate. Rotating glyphosate with conventional herbicides was no more effective in slowing this population increase than the use of glyphosate at 0.8 kg/ha.
Reducing the glyphosate rate allows weeds with a slightly higher natural tolerance to survive and become the dominant species.
Key Words
Glyphosate rate, conventional herbicides, dominant species, weed shifts, herbicide resistance.
Glyphosate has been used to control unwanted vegetation for over thirty years. The commercialisation of glyphosate tolerant crops in 1996 has allowed growers to treat their fields with this herbicide without fear from damage to crops. Glyphosate tolerant varieties of maize (Zea mays), soybean (Glycine max), and canola (Brassica napas) have been commercially available to growers for seven to eight years. Glyphosate tolerant varieties of sugarbeet (Beta vulgaris), spring wheat (Triticum aestivum), and lucerne (Medicago sativa) have been developed and will likely be released in the near future. Farmers in the Central High Plains will potentially be able to grow a three, four or five-crop rotation utilising glyphosate as the main weed management tactic.
It is difficult to predict what effect a continuous glyphosate tolerant cropping system will have on the weed species composition of fields. Studies were initiated in the Central High Plains to determine if glyphosate use pattern in glyphosate tolerant crops influences weed control by placing selection pressure on weed species, alters weed population dynamics, or leads to the development of glyphosate resistant weeds.
Trials were established under irrigated conditions in 1998 at Research and Extension Centers near Torrington, WY; Scottsbluff, NE; and Ft. Collins, CO under conservation tillage techniques to evaluate the impact of herbicide treatment and crop rotation on weed population dynamics in glyphosate tolerant crops. The experimental design at all sites was a two factorial split plot set in a randomised complete block design with four replications. Main plots were crop rotation and sub-plots herbicide treatments. Sub-plot size was 9.1 by 30.5 m (12 crop rows wide). Treatments and crop rotations are described in Table 1.
Table 1. Description of herbicide treatments used in continuous corn and rotational plots from 1998 to 2003.
Designation |
Year |
Crop |
Herbicide |
Rate |
Applications |
LG-C |
1998 |
maize |
glyphosate |
0.4 |
twice |
1999 |
maize |
glyphosate |
0.4 |
twice | |
2000 |
maize |
glyphosate |
0.4 |
twice | |
2001 |
maize |
glyphosate |
0.4 |
twice | |
2002 |
maize |
glyphosate |
0.4 |
twice | |
2003 |
maize |
glyphosate |
0.4 |
twice | |
LG-R |
1998 |
maize |
glyphosate |
0.4 |
twice |
1999 |
sugarbeet |
glyphosate |
0.4 |
twice | |
2000 |
maize |
glyphosate |
0.4 |
twice | |
2001 |
sugarbeet |
glyphosate |
0.4 |
twice | |
2002 |
wheat |
glyphosate |
0.4 |
twice | |
2003 |
maize |
glyphosate |
0.4 |
twice | |
HG-C |
1998 |
maize |
glyphosate |
0.8 |
twice |
1999 |
maize |
glyphosate |
0.8 |
twice | |
2000 |
maize |
glyphosate |
0.8 |
twice | |
2001 |
maize |
glyphosate |
0.8 |
twice | |
2002 |
maize |
glyphosate |
0.8 |
twice | |
2003 |
maize |
glyphosate |
0.8 |
twice | |
HG-R |
1998 |
maize |
glyphosate |
0.8 |
twice |
1999 |
sugarbeet |
glyphosate |
0.8 |
twice | |
2000 |
maize |
glyphosate |
0.8 |
twice | |
2001 |
sugarbeet |
glyphosate |
0.8 |
twice | |
2002 |
wheat |
glyphosate |
0.8 |
twice | |
2003 |
maize |
glyphosate |
0.8 |
twice | |
RG-C |
1998 |
maize |
glyphosate |
0.8 |
twice |
1999 |
maize |
acetachlor + isoxaflutole |
1.7 + 0.05 |
once | |
2000 |
maize |
glyphosate |
0.8 |
twice | |
2001 |
maize |
acetachlor + isoxaflutole |
1.7 + 0.05 |
once | |
2002 |
maize |
glyphosate |
0.8 |
twice | |
2003 |
maize |
acetachlor + isoxaflutole |
1.7 + 0.05 |
once | |
RG-R |
1998 |
maize |
glyphosate |
0.8 |
twice |
1999 |
sugarbeet |
phenmidipham + desmedipham + |
0.18 + 0.18 + |
3 times | |
2000 |
maize |
glyphosate |
0.8 |
twice | |
2001 |
sugarbeet |
phenmidipham + desmedipham + |
0.18 + 0.18 + |
3 times | |
2002 |
wheat |
glyphosate |
0.8 |
twice | |
2003 |
maize |
acetachlor + isoxaflutole |
1.7 + 0.05 |
once | |
NG-C |
1998 |
maize |
rimsulfuron + thifensulfuron |
0.011 + 0.006 +0.14 |
once |
1999 |
maize |
acetachlor + isoxaflutole |
1.7 + 0.05 |
once | |
2000 |
maize |
acetachlor + isoxaflutole |
1.7 + 0.05 |
once | |
2001 |
maize |
acetachlor + isoxaflutole |
1.7 + 0.05 |
once | |
2002 |
maize |
acetachlor + isoxaflutole |
1.7 + 0.05 |
once | |
2003 |
maize |
acetachlor + isoxaflutole |
1.7 + 0.05 |
once | |
NG-R |
1998 |
maize |
rimsulfuron + thifensulfuron |
0.011 + 0.006 +0.14 |
once |
1999 |
sugarbeet |
phenmidipham + desmedipham + |
0.18 + 0.18 + |
3 times | |
2000 |
maize |
acetachlor + isoxaflutole |
1.7 + 0.05 |
once | |
2001 |
sugarbeet |
phenmidipham + desmedipham + |
0.18 + 0.18 + |
3 times | |
2002 |
wheat |
bromoxynil + MCPA + fluroxypyr |
0.42 + 0.42 + 0.21 |
once | |
2003 |
maize |
acetachlor + isoxaflutole |
1.7 + 0.05 |
once |
Nine soil cores were pulled from each plot in the spring prior to field preparation for weed seed bank analysis. The soil cores were bulked, weed seed extracted with a semi-automatic elutriator and weed seed counted by species. Weed density counts were made on three 3 m sections of row before herbicide treatment, two weeks after the last herbicide treatment, and again near crop harvest. Crop density and yield were determined from the same area where the soil cores were pulled and from which the weed density counts were made. Data were collected from the same site each year at all locations and the plot sampling arrangement is shown in Figure 1.
After six seasons (1998 to 2003) there was no evidence that any weed species had developed resistance to glyphosate. However, at all three irrigated locations common lambsquarters populations have increased in the treatments receiving the low rate of glyphosate when compared to the other treatments (Table 2). This effect was most pronounced at Scottsbluff and was similar in both, rotational (RC) or continuous maize (CC) plots. Rotation also had no effect at Ft. Collins, but at Torrington common lambsquarters populations were less in continuous maize compared to rotation plots.
Wild buckwheat (Polygonum convolvulus) which was not present in detectable quantities at Torrington in 1998 increased to over 4 plants/m2 in 2003 in the CC plots treated with the low rate of glyphosate (Table 3).
Figure 1. Plot sampling arrangement used at all experimental sites from 1998 to 2003.
Table 2. Influence of herbicide treatment and rotation on common lambsquarters populations in 2003.
Scottsbluff, NE |
Torrington, WY |
Ft. Collins | ||
Treatment |
Avg. Rotations |
CC |
RC |
Avg. Rotations |
plants/m2 | ||||
LG |
44 a |
3.5 b |
5.1 a |
6 a |
HG |
14 b |
0.5 c |
1 c |
2 b |
RG |
12 b |
0 c |
0 c |
1.5 b |
NG |
8 b |
0 c |
0 c |
0.5 b |
Table 3. Influence of herbicide treatment and rotation on wild buckwheat populations at Torrington, WY in 2003.
Torrington, WY | ||
Treatment |
CC |
RC |
plants/m2 | ||
LG |
4 a |
0.1 b |
HG |
0.2 b |
0 b |
RG |
0 b |
0 b |
NG |
0 b |
0 b |
The data indicate if the rate of glyphosate is reduced species that possess a slightly higher natural tolerance and survive the application are allowed to reproduce. Over time, these increase in numbers and become the dominant weed. For this study rotating glyphosate with conventional herbicides as in the alternating glyphosate treatment was no more effective on common lambsquarters than the high glyphosate rate at all three sites.