Field studies were conducted at 35 sites throughout the north-central United States in 1998 and 1999 to determine the effect of postemergence glyphosate application timing on weed control and grain yield in glyphosate-resistant corn. Glyphosate was applied at various timings based on the height of the most dominant weed species. Weed control and corn grain yields were considerably more variable when glyphosate was applied only once. The most effective and consistent season-long annual grass and broadleaf weed control occurred when a single glyphosate application was delayed until weeds were 15 cm or taller. Two glyphosate applications provided more consistent weed control when weeds were 10 cm tall or less and higher corn grain yields when weeds were 5 cm tall or less, compared with a single application. Weed control averaged at least 94 and 97% across all sites in 1998 and 1999, respectively, with two glyphosate applications but was occasionally less than 70% because of late emergence of annual grass and Amaranthus spp. or reduced control of Ipomoea spp. With a single application of glyphosate, corn grain yield was most often reduced when the application was delayed until weeds were 23 cm or taller. Averaged across all sites in 1998 and 1999, corn grain yields from a single glyphosate application at the 5-, 10-, 15-, 23-, and 30-cm timings were 93, 94, 93, 91, and 79% of the weed-free control, respectively. There was a significant effect of herbicide treatment on corn grain yield in 23 of the 35 sites when weed reinfestation was prevented with a second glyphosate application. When weed reinfestation was prevented, corn grain yield at the 5-, 10-, and 15-cm application timings was 101, 97, and 93% of the weed-free control, respectively, averaged across all sites. Results of this study suggested that the optimum timing for initial glyphosate application to avoid corn grain yield loss was when weeds were less than 10 cm in height, no more than 23 d after corn planting, and when corn growth was not more advanced than the V4 stage.
Field experiments were conducted in 11 states to evaluate broadleaf weed management programs in dicamba‐resistant soybeans which involved the use of preemergence and postemergence dicamba. Preemergence (PRE) dicamba at 0.25 lb ae/acre provided less than 60% control of smooth pigweed, giant ragweed, velvetleaf, palmer amaranth, waterhemp, and morningglory spp., but 97% control of common lambsquarters and horseweed at 3 weeks after treatment (WAT). Preemergence flumioxazin plus chlorimuron or sulfentrazone plus cloransulam provided 66 to 100% control of these weeds. Use of dicamba postemergence (POST) improved uniformity of control of velvetleaf, smooth pigweed, morningglory, and glyphosate‐susceptible waterhemp. However, combining dicamba at 0.25 lb/acre with glyphosate resulted in 30% to 65% greater control of glyphosate‐resistant palmer amaranth, glyphosate‐resistant common waterhemp, glyphosate‐resistant horseweed, and glyphosate‐resistant giant ragweed compared to sequentially applied glyphosate.
Field trials were conducted in 2002 and 2003 at seven sites to determine the optimum rates of mesotrione and atrazine applied PRE for minimal crop injury and control of common lambsquarters, velvetleaf, Pennsylvania smartweed, common ragweed, giant ragweed, ivyleaf morningglory, and common cocklebur. All rates of each herbicide resulted in greater than 95% control of triazine-susceptible common lambsquarters. Mesotrione at 105 g ai/ha resulted in greater than 90% control of triazine-resistant common lambsquarters, velvetleaf, and Pennsylvania smartweed. Control of common ragweed was 90% or greater from mesotrione at 158 g/ha in combination with atrazine at 280 g/ha or greater. In addition, mesotrione at 210 g/ha combined with any rate of atrazine provided at least 92% control of common ragweed. Combinations of mesotrione and atrazine only suppressed, and did not effectively control, giant ragweed, common cocklebur, and ivyleaf morningglory.
Field studies on the persistence of simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] showed less persistence under no-tillage corn(Zea maysL. ‘Pioneer 3369A’) culture than conventionally tilled corn. Bioassay studies conducted in the greenhouse using oats(Avena sativaL. ‘Compact’) as indicator crop showed the lowest level of simazine remaining in soils of low pH (5.4 and lower). Persistence of simazine in the soil increased with increasing soil pH.
Trumpetcreeper [Campsis radicans(L.) Seem.] grown from 10-cm root sections was more susceptible to foliar applications of 2,4-D [(2,4-dichlorophenoxy)acetic acid] and dicamba (3,6-dichloro-o-anisic acid) than trumpetcreeper grown from 45-cm root sections. Dicamba controlled trumpetcreeper better than did 2,4-D, particularly trumpetcreeper grown from 45-cm root sections. More14C-dicamba than14C-2,4-D was absorbed through the foliage and translocated to the roots of trumpetcreeper grown from 10-cm roots. No14C-2,4-D was detected in the 45-cm roots; however,14C-dicamba was recovered from both the upper and the lower sections of the 45-cm roots.
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