Toxicity of Soil-Incorporated Trifluralin to Johnsongrass<i>(Sorghum halepense)</i>Rhizomes

Rhizomes of johnsongrass [Sorghum halepense(L.) Pers.], cut into 2.5-cm or 15-cm lengths, were planted in the field within and below a 7.5-cm layer of soil mixed with trifluralin(a,a,a-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) at 0, 1.7, or 3.4 kg/ha. When planted 3.8- and 10-cm deep in nontreated soil, survival was 90% and 69%, respectively, for the longer rhizomes and only 39% and 10%, respectively, for the shorter ones. When planted within trifluralin-treated soil, 95% and 100% of the shorter rhizomes and 54% and 81% of the longer rhizomes were killed at 1.7 and 3.4 kg/ha, respectively. When planted below treated soil, only 11% or 19% of the longer rhizomes were killed. The shorter rhizomes planted below the treated soil survived at less than 10%.</jats:p

Asulam for Johnsongrass Control in Sugarcane

Asulam (methyl sulfanilylcarbamate) at 3.3 or 4.5 kg/ha gave 54% and 61% control, respectively, when applied only to the initial infestation of rhizomatous johnsongrass [Sorghum halepense(L.) Pers.] in sugarcane (Saccharuminterspecific hybrids). When applied both to the initial infestation and to the regrowth, 81% and 88% control, respectively, were obtained. Both the single and dual asulam treatments produced yields of sugar that were about double the yield of an untreated check and about 90% of the yield of a hand-weeded check. Sugarcane cultivars ‘CP 52-68′, ‘CP 65-357′, ‘CP 61-37′, and ‘L 62-96’ were highly tolerant to single applications of asulam at 4.5 and 6.7 kg/ha and to dual applications of asulam at 3.4 kg/ha. Some cultivars were moderately injured by dual applications of asulam at 4.5 kg/ha.</jats:p

Tolerance of Two Sugarcane Cultivars to Terbacil, Fenac, and Dalapon

Two Louisiana sugarcane cultivars (interspecific hybrids of the genusSaccharum) CP 52–68 and L 60-25 were sprayed with herbicides commonly used for preemergence and postemergence control of johnsongrass (Sorghum halepense(L.) Pers.) and other weeds. The preemergence weed control treatments of 3-tert-butyl-5-chloro-6-methyluracil (terbacil) at 1.8 kg/ha and (2,3,6-trichlorophenyl)acetic acid (fenac) at 5 kg/ha were compared to a mixture of sodium salt of trichloroacetic acid (TCA) at 11 or 19 kg/ha plus 2-(2,4,5-trichlorophenoxy)propionic acid (silvex) at 3.4 kg/ha (control). Herbicides were applied after planting in the fall, reapplied the following spring (after sugarcane emerged), and applied again in the second spring on the stubble crop (after sugarcane emerged). In the stubble crop the sodium salt of 2,2-dichloropropionic acid (dalapon) at 0, 5, or 10 kg/ha was applied as a postemergence spray over the top of sugarcane. In the factorial arrangement each cultivar received all combinations of the preemergence x postemergence treatments.Terbacil did not injure either cultivar, but fenac injured both enough to reduce the yield of sugar in plant cane by about 10% as compared to the TCA plus silvex control. Stubble crop yields were not affected by fenac or terbacil. CP 52–68 was more tolerant to dalapon treatments than L 60–25. Both cultivars tolerated dalapon at 5 kg/ha relatively well although the yield of L 60–25 was reduced by about 5%. However at 10 kg/ha, dalapon reduced yields of CP 52–68 and L 60–25 by about 10 and 18%, respectively. Combinations of preemergence and postemergence treatments did not interact to reduce yields more than the combined effects of each treatment.</jats:p