No abstract

Reduction of weed competition using herbicides is critical to the successful revegetation of weed-infested rangeland. However, little is known about the influence of persistent broadleaf herbicides on the establishment of many desired grasses. The objective of this study was to determine the influence of various rates and times of application of picloram and clopyralid before seeding on two native and two nonnative grasses important to western United States rangeland. The study was conducted in 1997 and 1998 in Wyoming, Montana, and Washington. Five herbicide treatments (none, picloram at 0.14, 0.28, 0.56 kg ai/ha, clopyralid at 0.14, 0.56 kg ai/ha), three timings of herbicide application (44, 24, and 0 d prior to seeding[DPS]), and four grass species (Idaho fescue, bluebunch wheatgrass, crested wheatgrass, and pubescent wheatgrass) were factorially arranged in a randomized split-plot design with three blocks. The whole-plots were the seeded grass species, and the subplots were the herbicide treatments and timings of application. Treatments were performed in the spring of 1997. Treatment effects on grass vigor (1997) and biomass (1998) depended on grass species, herbicide and rate, and timing. Idaho fescue established at low densities, and it was not possible to determine the treatment effects. In general, crested and pubescent wheatgrass had higher vigor estimates and biomass than did bluebunch wheatgrass. Herbicide applied 24 or 44 DPS had less effect on grass vigor and biomass than did herbicide applied at seeding. Picloram at 0.28 and 0.56 kg/ha tended to reduce vigor and biomass more when compared with the 0.14 kg/ha rate and with both rates of clopyralid. On the basis of these experiments, applying picloram or clopyralid at rates as high as 0.56 kg/ha 24 d or more before seeding bluebunch wheatgrass, pubescent wheatgrass, or crested wheatgrass may allow effective weed control and long-term management through grass competition.

Herbicides with residual activity more effectively control infestations of yellow starthistle, a facultative winter annual, because seed banks quickly furnish replacement plants after nonresidual herbicide treatments. Picloram has been applied to rosettes in fall or spring, but new infestations of yellow starthistle are often discovered when plants are more noticeable in bud or flower stages. Eradication, containment, and revegetation are facilitated if weed seed rain can be stopped. This study evaluated whether registered rates (0.14, 0.28, and 0.42 kg ae/ha) of picloram, alone and with 2,4-D at 1.12 kg ae/ha, can prevent seed production when applied to yellow starthistle at bud or flower stage. Picloram applied at bud stage curtailed both seed production and germination, reducing seed production by 42 to 86% and viability by 80 to 99%. Neither the picloram rate nor the addition of 2,4-D to the spray solution affected the percentage of nonviable seeds. The addition of 2,4-D further decreased germination of developed seeds only at the lowest picloram rate. At flower stage, picloram and 2,4-D neither killed mature plants nor consistently reduced the quantity and quality (viability) of seeds. Bud stage was the phenological limit for effective reduction of viable seed by picloram, which caused both bud abortion and lower seed germination.