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Effective integrated weed management requires understanding the impacts of management strategies applied alone and in combination. The objectives of this study were to determine the effects of 2,4-D and repeated sheep (Ovis aries) grazing applied alone and in combination on spotted knapweed (Centaurea maculosa Lam.) and perennial grasses. We hypothesized that integrating a single spring 2,4-D application would remove the adult plants, repeated sheep grazing would control rosettes, and spotted knapweed density, cover and biomass would decrease, allowing residual grasses to reoccupy the sites. A single spring 2,4-D application at 2.1 kg a.i. ha1, repeated annual sheep grazing of 95% spotted knapweed or 60% grass utilization, and repeated sheep grazing and 2,4-D combined were applied to small pastures at 2 sites in western Montana beginning in 1997 and continued through 2001. Spotted knapweed rosette and flowering plant density, and spotted knapweed and perennial grass cover and biomass were sampled from 1998 through 2001. Spotted knap- weed rosette density was 61.7, 34.3, 44.3, and 0.3 m² in the control, sheep grazing, 2,4-D, and combined sheep grazing and 2,4-D treatments, respectively, at 1 site in 2001. Spotted knapweed flowering plant density increased from 3.7 in 1998 to 10.7 m² in 2002 in the 2,4-D treatment whereas there was no increase in the 2,4-D combined with sheep grazing treatment from 1998 to 2002. Perennial grass biomass was 6.9, 8.4, 25.7, and 19.7 in the control, sheep grazing, 2,4-D, and combined sheep grazing and 2,4-D treatments, respectively, averaged for both sites and 4 years of sampling. Herbicides released perennial grasses from weed competition and changed the weed population from mature, less palatable plants to juvenile plants that were preferred by sheep. [ABSTRACT FROM AUTHOR]

Prescribed fires are important for rangeland restoration and affect plant community composition and species interactions. Many rangeland plant communities have been, or are under the threat of noxious weed invasion, however there is little information on how fire effects weeds. Our objective was to determine the effects of prescribed rangeland fire on dalmatian toadfiax [Linaria dalmatica (L.) Miller] density, cover, biomass, and seed production. These plant characteristics, as well as density, cover, and biomass of perennial grasses and forbs were measured within burned and adjacent not-burned areas on 3 Artemisia tridentata/Agropyron spicatum habitat types in Montana. Areas were burned in the spring and measured in the fall 1999. Comparisons of plant characteristics between the burned and not-burned sites were made using t-tests and non-parametric Wilcoxon Rank Sum tests. After 1 growing season, fire did not affect density or cover of dalmatian toadflax. Burning increased dalmatian toadflax biomass per square meter at 2 sites, and per plant biomass at all 3 sites. Seed production of dalmatian toadflax was increased by fire at all 3 sites. Fire reduced forb cover at 1 site and increased grass biomass at 2 sites. The increases in dalmatian toadflax biomass and seed production suggest that fire used to restore healthy plant communities may increase dalmatian toadflax dominance. We recommend weed management procedures, such as herbicide control and seeding desirable species, be integrated with prescribed fire where dalmatian toadflax is present in the plant community. [ABSTRACT FROM AUTHOR]

Establishing competitive plants is essential for restoring spot ted knapweed infested grasslands. Revegetation attempts typically fail because of weed competition during the initial stages of establishment.We hypothesized that competitive interactions can be shifted from spotted knapweed to intermediate wheatgrass by increasing wheatgrass seedling density over 1,000 plants m-2. Spotted knapweed andintermediate wheatgrass were grown in addition series mixtures to assess their interference at low (0 to 1,000 plants m- 2) versus high (1,000 to 10,000 plants m-2) densities. In the spring of 1995, 7 densities (0, 100, 500, 1,000, 3,000, 6,000, and 10,000 plants m-2) of each species were seeded in a factorialarrangement (49 density combinations) in a randomized-complete-blockdesign and replicated 3 times at 2 sites in Montana. Plants were grown in pots (2,250 mm2 x 380 mm deep) for 60 days before harvesting. Regressions predicting shoot weight, root weight, total weight, leaf area, and root length were calculated using 1) low knapweed:low wheatgrass, 2) low knapweed:high wheatgrass, 3) high knapweed:low wheatgrass, and 4) high knapweed:high wheatgrass densities. Regression coefficients indicated intraspecific interference was most important in predicting intermediate wheatgrass weight at both sites. At the wet site (457 mm, annually), interspecific interference only occurred at high spotted knapweed densities. At the dry site (305 mm, annually), interspecific interference occurred at low densities. Increasing intermediate wheatgrass from low to high densities removed the effect of spotted knapweed on intermediate wheatgrass where interspecificinterference occurred. [ABSTRACT FROM AUTHOR]

Spotted knapweed (Centaurea maculosa Lam.) has reduced forage production, increased soil erosion, and lowered biodiversity on millions ofhectares of rangeland throughout the western United States. Objectives of this study were to quantify the interaction between picloram (4-amino-3,5,6-trichloropicolinic acid) and fertilizer on spotted knapweed density and grass yield. Four picloram rates (0.0, 0.14, 0.28, and 0.42 kg ha-1) and 4 fertilizer rates (N+P: 0.0+0.0, 10.5+12.2, 21.1+26.4, and 31.7+39.6 kg ha- 1) were applied to 3 spotted knapweed infested rangeland sites in a factorial combination arranged in a randomized-complete-block design during the spring of1994. Grass yield and spotted knapweed density were measured at peakstanding grass crop in 1994 and 1995. Data were analyzed as a split-plot in time using analysis of variance. Picloram and fertilizer did not interact to affect either spotted knapweed density or grass yield. All picloram treatments reduced spotted knapweed density to nearly zero. By 1995, all picloram treatments increased grass yield by an average of 1,500 kg ha-1. Fertilization did not affect spotted knapweed density, but the highest rates increased grass yield on those sites with a substantial residual grass understory. Combining fertilizer with picloram may enhance grass yield on sites with a residual of highly productive grasses. [ABSTRACT FROM AUTHOR]

Developing rangeland management strategies to minimize spotted knapweed (Centaurea maculosa Lam.) invasion will require understanding theeffects of intensity, frequency, and season of grazing on weed establishment. We studied the effects of hand-clipping 2-year-old Idaho fescue (Festuca idahoensis Elmer) plants 0, 30, 60, or 90%, 1, 2, or 3 times (14-day intervals) on spotted knapweed seedling emergence and growth in pots watered with 150 ml once weekly or 50 ml 3 times weekly. Pots were seeded with 5,000 spotted knapweed seeds m-2, replicated twice, and placed in a growth chamber in a completely randomized design. The experiment was repeated once. Plants were harvested after 50 days. Treatment effects on soil moisture, Idaho fescue andspotted knapweed shoot and root weight, and leaf area were compared using analysis of variance and regression analysis. At final harvest,Idaho fescue shoot weight and leaf area decreased with increasing defoliation level and frequency. Idaho fescue root weight was not affected by any treatment. A single Idaho fescue defoliation at 30% and 90% increased spotted knapweed weight and numbers per pot respectively,over those pots with undefoliated plants. The level of defoliation necessary to enhance spotted knapweed numbers was lower as defoliationfrequency increased. As defoliation level and frequency increased, soil water content increased resulting in a corresponding increase in spotted knapweed emergence and growth. [ABSTRACT FROM AUTHOR]

Community dynamics and dominance on cheatgrass (Bromus tectorum L.) and yellow starthistle (Centaurea solstitialis L.) infested rangelandappear to be influenced by resource acquisition rates and duration of growth. Objectives were to determine the effects of densities, proportions, and soil depth on the growth rate and duration of growth of these species. In 6 field experiments isolated individuals, monocultures (100, 1,000, 10,000 plants m{sup}-2{end}), and mixtures (same densities arranged factorially) were grown with unrestricted and restricted (0.2- and 0.5-m) soil depths. Shoot weights were determined on 12-day intervals beginning on day 24 and ending on day 72 for plants grown with restricted soil depth and day 96 (cheatgrass) and day 108 (yellow starthistle) for plants grown in unrestricted soil. Quadratic growth curves were fit for each replication for plants grown in isolation. Linear and quadratic models were developed for plants grown in monocultures and mixtures. Simple linear regression coefficients were used as growth rates and regressed over plant density. Time of injection (an indicator of the duration of growth) for plants growing in monocultures and mixtures was calculated from quadratic models. Growth rate of yellow starthistle was about 7 times faster and duration of growth 25 days longer, than those of cheatgrass when grown in isolation without soil depth restriction. As densities were increased and/or soil depth decreased, growth rate and duration of growth were loweredand the difference between species were masked. Growth rates and duration of growth of cheatgrass and yellow starthistle appear to dependon plant density, soil depth, and available soil moisture. [ABSTRACT FROM AUTHOR]

Diffuse knapweed (Centaurea diffuse Lam.), an invasive weed, has reduced forage production and biodiversity, and increased soil erosion on over a million hectares of rangeland in the western United States. This study evaluated the effects of a single grass defoliation on establishment of diffuse knapweed seeded at 2 rates into a bluebunch wheatgrass (Pseudoroegneria spicata [Pursh.] Scribn and Smith)/needle-and-thread (Stipa comata Trin. & Rupr.) community and a crested wheatgrass (Agropyron cristatum (L.) Gaertn.) community. Six defoliation levels (0, 20,40, 60, 80, 100 percent) and 2 seeding rates (3,000, 6,000 diffuse knaweed seeds) were applied to 1 m{sup}2{end} plots in a randomized-complete-block design (n=4). Diffuse knapweed was seeded in the fall of 1992, and grasses were defoliated on 28 April 1993. The number of flowering culms and weed seedlings were counted in September1993. Densities of diffuse knapweed seedlings, juveniles, and adults, as well as plant standing crop, were determined in May 1994. Seed rate had minimal effect on diffuse knapweed density. By May 1994, densities of diffuse knapweed were about 20 and 30 plants m{sup}2{end} onundefoliated bluebunch wheatgrass and crested wheatgrass plots, respectively, indicating that defoliation is not required for this noxious weed to become established. Higher levels of grass defoliation ( greater than 60 percent), especially of bluebunch wheatgrass, enhanced diffuse knapweed establishment, indicating that moderate ( less than 60 percent) defoliation would not necessarily accelerate invasion by diffuse knapweed. [ABSTRACT FROM AUTHOR]