1992 Res. Prog. Rep. West Soc. Weed Sci.
The site was a non-grazed south facing slope located near Kamiah, Idaho. The slope ranged from 30 to 40% and was covered with approximately 700 common crupina plants per m2. The plot design was a split plot with four replications. Herbicides used in this test were metsulfuron at 0.023, 0.035, and 0.052 kg ai/ha, picloram at 0.14, 0.28, and 0.42 kg ai/ha triclopyr at 0.14, 0.28, 0.42 kg ai/ha, clopyralid at 0.035, 0.07, and 0.14 kg ai/ha, imazapyr at 0.07, 0.14, and 0.28 kg ai/ha, UBI-C4243 at 0.14 kg ai/ha, chlorsulfuron at 0.052 kg ai/ha, dicamba at 1.12 kg ai/ha, and 2,4-D at 2.24 kg ai/ha. The herbicides were applied on May 30, 1990 using a back pack sprayer calibrated to deliver 207 l/ha. at 4 km/hr. At the time of spraying, 95% of the common crupina had bolted and flower buds were present. Plant heights ranged from 7 to 15 cm. The air temperature was 20 C with soil temperatures ranging from 15 C at surface to 17 C at a depth of 15 cm. Relative humidity was 49% and wind was 0 to 3 km/hr from varying directions. The cloud cover was 10% to 50%. There was no dew present. Other weeds present were field bindweed (Convolvulus arvensis L.), poison ivy (Rhus radicans L.), and yellow starthistle (Centaurea solstitialis L.). Plots were evaluated for chlorosis, and plant height was measured on June 25 and July 9. Seed production was estimated on July 9. The amount of live cover was estimated on June 25. Seedling produced from treated plants were counted on March 20, 1991. LSD or Duncan’s multiple range tests were used to separate the means.
In 1990, untreated plants ranged in height from 31 to 46 cm at the time of evaluation. Plants were erect, flowering and producing seed. Average seed production in check plots was over 1000 seeds per 10 by 40 ft. plot. Growth of common crupina was suppressed by the application of all herbicides, when compared to the check. Seed production was reduced 100% by all picloram rates, by the tested 2,4-D rate, and higher rates of Imazapyr. Dicamba, metsulfuron, triclopyr, and UBI-C4243 reduced seed production by about 75% or more. The vegetation remaining after herbicide treatment was largely undesirable forage for wildlife, cattle, or sheep. Surviving vegetation consisted of Japanese brome (Bromus japonicus), Palouse thistle (Cirsium brevifolium), common dogbane (Apocynum cannabinum), field bindweed (Convolvulus arvensis), yellow starthistle (Centaurea solstitialis) and poison ivy (Rhus radicans). Failure to establish desirable cover in such conditions will result in erosion until these undesirable weeds become established.
Eradication of common crupina requires preventing all seed production for at least two generations. This project has shown that common crupina seed production can be prevented for one generation by applying some herbicides as late as bud formation. In 1991, plots with significant reduction in seed in the treatment year also had lower seedling numbers the following spring. All rates of picloram and higher rates of clopyralid, imazapyr, triclopyr, UBI-C4243 and the tested rate of 2,4-D had less than one seedling per m2. Japanese brome and field bindweed tended to dominate the community after the common crupina was chemically removed. This study shows late application of some herbicides will greatly reduce seed production and subsequent seedling production of common crupina. (University of Idaho, Dept. of Plant, Soils, and Ent. Sci., Moscow, 84843).