Crested wheatgrass (CWG) is commonly seeded in Nevada on sites in the 8 to 12 inch annual precipitation zone that was originally inhabited by Wyoming big sagebrush (Artemisia tridentata Wyomingensis) or basin big sagebrush (Artemisia tridentata tridentata). Many of these sites have burned in the past 20 years, and their lack of native perennial bunchgrasses facilitated the establishment of cheatgrass (Bromus tectorum), an undesired annual grass. When cheatgrass is abundant, CWG is often the best adapted perennial grass species to compete with this annual grass. A high density of cheatgrass, however, can result in poor establishment of CWG (Evans 1961). Numerous herbicides are known to control cheatgrass, but not all have a product label that permits application on rangelands (Prather 2009). An important management and regulatory question on recently seeded areas (burned and unburned) is how do these herbicides (labeled and non-labeled) affect CWG seedlings? If non-labeled herbicides are shown to have little or no adverse effects on seeded grasses, the product label of these herbicides can be updated to include use on rangelands. This study was conducted to determine the effect of six herbicides to CWG seedlings when applied at two different growth stages.

Methods

The study area was about 3.1 miles south of Winnemucca, at about 4,600 foot elevation and was initially inhabited with basin and Wyoming big sagebrush. The site burned in a wildfire in July 2007 and was seeded to crested wheatgrass in December 2007, with a calibrated rangeland drill. Six herbicides were used singularly or in combination to develop 11 treatments, plus a control (Table 1). Treatments were applied to 96 10 by 30 foot plots in a randomized complete block design. Half the plots were treated March 2008 (zero to one-leaf growth stage) and the other half April 2008 (three to five-leaf growth stage). Each treatment was applied to four plots, and each plot was treated once, except for the split Olympus treatment. Plots that received the split Olympus application received a 0.6 ounce application in both March and April. We mixed ammonium sulfate (AMS) with the glyphosate treatment and the non-ionic surfactant (NIS), Activator 90, with the other herbicide treatments.For the March 4 application, there had been very little emergence of crested wheatgrass seedlings, and those seedlings that had emerged were at the one-leaf growth stage. For the April 17 application, CWG seedlings were at the three to five-leaf growth stage. Each herbicide treatment was applied with a CO2 pressurized back-pack sprayer. Cheatgrass was nearly absent from all treatment plots; thus, it had no influence on the results.Table 2 describes the weather and soil conditions at the time of application, and the post-application precipitation pattern.

Data were collected in a 3 by 30-foot belt transect in the center of each plot, in mid-July 2008. Attributes measured were CWG seedling density and height (an indicator of plant vigor). For seedling density, we counted all crested wheatgrass seedlings in the belt transect. For seedling height, we selected the 10 seedlings closest to the center of each transect, or all seedlings if the transect contained fewer than 10. Values were averaged to determine mean seedling height in each treatment plot.

Both total density and mean height values for each treatment plot were transformed (square root) to equalize variances. A one-way analysis of variance was used to compare treatments within growth stages. The Least Significant Difference (LSD) mean separation test was used at the P≤0.10 level to assess whether differences in seedling density and height between treatment plots were due to the treatment or another factor. That is, was there a 10 percent or less probability that the differences found between treatment plots were because of a factor other than the herbicide treatment? The two sample t-test was used to compare treatments between the two growth stages (zero to one-leaf and three to five-leaf, respectively). P-values are reported on the graphs. The p-values show the probability that the difference between seedling density or height is due to some factor other than the growth stage at the time of treatment. For example, P≤0.12 means there is a 12 percent chance the difference in two values is due to some factor other than the treatment applied and an 88 percent chance the difference is because of the treatment applied. Statistics 9 (Analytical Software, 2009) describes all of the aforementioned statistical tests and procedures.

Results

Seedling Density

The application of Roundup (glyphosate) on March 4, 2008 did not eliminate most crested wheatgrass seedlings (Figure 1). This suggests that most seedlings had not emerged: probably due to low soil temperatures (Table 2).

  • Plateau, Oust, Sencor, and Olympus mixed with Sencor resulted in significantly fewer seedlings than the control plots (P<0.10). Few seedlings survived the applications of Plateau and Oust.
  • A split application of Olympus resulted in significantly fewer seedlings than either the control or the highest rate of Olympus.
  • The Outrider plots had many more CWG seedlings than the control, but the difference could not be attributed to the treatment.
  • The Olympus treatments did not result in statistically fewer CWG seedlings than the control, but there was a trend toward fewer seedlings at the lower application rates. This cannot be explained.

Treatments applied when seedlings were largely in the three to five-leaf stage resulted in a wide range of seedling densities, comparedto the control (Figure 2).

  • Split application Olympus, Sencor, Olympus with Sencor, Plateau, and Oust had 60 percent fewer seedlings than the control.
  • Plateau and Oust decreased seedling density more than any other treatment.
  • Olympus only treatments generally had slightly fewer seedlings than the control.
  • The high rate of Olympus with nitrogen had significantly (P≤0.10) fewer seedlings than the control or the 6 ounce-per-acre Olympus treatment. Nitrogen appears to enhance the activity of Olympus to CWG seedlings.
  • The presence of seedlings in the Roundup treatment (a nonselective herbicide treatment) suggests that additional seedling emergence occurred after this treatment.

There were substantial differences in seedling density between the pre-emergent or one-leaf growth stage and the three to five-leaf growth stage (Figure 3). For all treatments, the probability that the difference between the two growth stages was due to some factor other than the growth stage of the CWG seedlings ranged from 12 percent (Roundup treatment) to 99 percent (Olympus at 0.9 ounce active ingredient per acre).

  • Variable rate Olympus treatments had a distinct pattern. At the largely pre-emergent growth stage, increasingly higher application rates corresponded with increased density of CWG seedlings. At the three to five-leaf growth stage the response was the opposite: more seedlings at lower application rates.

Seedling Height

At the pre-emergent to one-leaf growth stage, shorter CWG plants were associated with all herbicide treatments, compared with the control (Figure 4). Differences for three of the treatments were not significant.

  • For the Roundup, Outrider and Sencor only treatments there was at least a 10 percent chance the difference was because of some factor other than the specific herbicide treatment.
  • Oust and Plateau had large reductions in seedling height, with a 99 percent chance the difference was due to the herbicide treatments.
  • All rates of Olympus, with or without other products in the application mixture, resulted in seedlings significantly shorter than the control plots. There was at least a 90 percent chance the differences were due solely to the treatments applied.

At the three to five-leaf growth stage, the Sencor treatment resulted in CWG seedlings slightly taller than in the control (Figure 5). All other treatments had CWG seedlings shorter than in the control.

  • There was at least a 10 percent chance that the differences in seedling height between the control treatment and the Roundup, Outrider, Sencor (4 ounces), and Olympus only treatments was because of some factor other than the herbicide treatment.
  • Oust and Plateau treatments had CW seedlings less than half as tall as the control, and the seedlings treated with Oust were significantly (P≤0.10) shorter than the seedlings treated with Plateau. Four treatments had an effect similar to Plateau: Olympus tankmixed with Sencor, the split Olympus treatment, Olympus with nitrogen, and the high rate Olympus treatment.

Most treatments applied at the three to five-leaf leaf growth stage (6 of 10) had taller CWG seedlings than treatments at the zero to one-leaf growth sage (Figure 6).

  • Late applications of Plateau and Oust resulted in substantially taller seedlings, but their heights were still shorter than any other treatment, and these treatments resulted in the fewest seedlings.
  • Seedlings treated with Sencor were among the tallest, especially those treated at the three to five-leaf growth stage, but their overall density was low, negating any gains in vigor for the surviving seedlings.

Conclusions

  • Outrider had little if any adverse effect on CWG seedling density and vigor.
  • Plateau and Oust had the most adverse effects to CWG seedlings.
  • Sencor had a relatively small effect on seedling height (vigor) with a pre-emergent application, and none at the three to five-leaf stage, but both treatments significantly reduced seedling density.
  • The Olympus only treatments generally had less adverse effects on CWG seedlings than Oust, Plateau or Sencor, but were more slightly more harmful than Outrider.
  • The response of CWG seedlings to the Olympus only treatments was complex and highly variable. Higher application rates had the least adverse effect on seedling density when applied at the pre-emergent to one leaf stage. This is counterintuitive, but the pattern held across all three application rates at the pre-emergent growth stage. The result is further complicated by CWG seedlings being shorter with an increasing application rate. One would expect the treatment with the lowest seedling density (0.6 ounces of active ingredient per acre) to also have the shortest plants, but the opposite occurred. Treatment at the three to five-leaf stage had a more consistent result, with seedling density and height increasing as the application rate declined. The high p-value for the seedling density response (Figure 3), despite a large difference in seedling density, suggests there was very high variability in seedling density among treatment plots in each herbicide treatment. It is possible that very high winds and associated dust storms in the spring of 2008 moved soil particles and attached herbicides among treatment plots and influenced the response observed in some treatment plots.

References

Analytical Software. 2008. Statistix 9. Analytical Software, Tallahassee. Fl. 454 p.

Evans, R. A. 1961. Effects of Different Densities of Downy Brome (Bromus tectorum) on Growth and Survival of Crested Wheatgrass (Agropyron desertorum) in the Greenhouse. Weeds 9:213-223

Prather, T. 2009. Weed Control in Pasture and Rangeland. In: Pacific Northwest Weed Management Handbook. Oregon State University. Corvallis, OR. Available at: PNW. Accessed on September 4, 2012.

Schultz, B. and Creech, E. 2012, The Response of Crested Wheatgrass (Agropyron cristatum) Seedlings to Six Herbicides, Extension | University of Nevada, Reno, FS-12-36

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