Annual Report of Progress
to the
MISSISSIPPI SOYBEAN PROMOTION BOARD
for 1996

Objectives

1. Determine the potential of Roundup-, Liberty-, and sulfonylurea-tolerant soybean cultivars as weed management tools in Mississippi soybeans.
2. Develop weed management systems for Mississippi soybeans utilizing herbicide-tolerant cultivars exhibiting the greatest yield potential.

Publications

Arnold, J. C., D. R. Shaw, and S. M. Schraer. 1997. Influence of application timing on efficacy of glyphosate in Roundup Ready soybean. Proc. South. Weed Sci. Soc. 50:in press.

Askew, S.D., D. R. Shaw, and J. C. Arnold. 1997. Weed control in Liberty-Link soybean. Proc. South. Weed Sci. Soc. 50:in press.

Palmer, E. W., D. R. Shaw, and R. C. Scott. 1997. Weed control programs in Roundup Ready soybean. Proc. South. Weed Sci. Soc. 50:in press.

Smith, M. C., D. R. Shaw, and C. L. Brommer. 1997. Weed control with sequential Roundup applications in Roundup Ready soybean. Proc. South. Weed Sci. Soc. 50:in press.

Significant Accomplishments

This project was funded by the Soybean Promotion Board in 1995 and 1996 with Dr. Bill Barrentine. However, in March of 1996 Dr. Barrentine retired, and Dr. Shaw was given responsibility for this project in early-June of 1996. Therefore, several of the exp eriments were initiated somewhat later than would be considered optimum. However, substantial research was still established.

Experiments were conducted at the Coastal Plain Branch Experiment Station near Newton, a producer farm near Greenwood, and the Plant Science Research Center near Starkville, MS, to compare single and sequential glyphosate (Roundup Ultra^TM) applications at various rates to standard herbicide treatments for weed control in Roundup Ready soybean. The first postemergence treatments (POST1) were applied at the V3 soybean stage, and the second postemergence applications (POST2 ) followed 2 weeks later. Sicklepod [Senna obtusifolia (L.) Irwin and Barney], pitted morningglory (Ipomea lacunosa L.), and broadleaf signalgrass [Brachiaria platyphylla (Griseb.) Nash] densities at Newton were from 43 to 54 plants/rn . Purple nutsedge (Cvnerus rotundus L.) density in a separate test at this location was 75 to 86 plants/m².

At 8 weeks after POST2, 0.84 kg ai/ha glyphosate POST1 followed by (fb) 0.56 kg/ha glyphosate POST2 controlled broadleaf signalgrass, sicklepod, and pitted morningglory at least 95% and weed control did not change with increased glyphosate rates. Single POST1 applications of 0.84 to 1.7 kg/ha glyphosate controlled broadleaf signalgrass equal to the sequential treatments, but sicklepod and pitted morningglory control decreased by approximately 10 and 15%, respectively, with the single application. Sequent ial POST1 fb POST2 applications of 9 g/ha chlorimuron in combination with 70 g/ha quizalofop-P POST1 controlled broadleaf signalgrass control 85% at 8 weeks after POST2. However, sicklepod and pitted morningglory control with this treatment was approximately 38%. Dimethenamid at 0.84 kg/ha +0.36 kg/ha metribuzin + 6 g/ha chlorimuron, PRE, fb 140 g/ ha imazaquin POST1 controlled sicklepod and pitted momingglory 39 and 59%, respectively. Soybean yields with sequential glyphosate applications ranged from 2,000 to 2,150 kg/ha, but decreased with other herbicide treatments.

At Greenwood, the primary weed was large crabgrass (Digitaria sanguinalis L.) at 32 plants/m², which was not completely controlled with secondary tillage before planting. Sequential glyphosate applications or treatments that included 70 g/ha quizalofop-P POST2 controlled large crabgrass at least 95%, but control greatly decrea sed with single glyphosate applications, regardless of rate.

The primary weed at Starkville was barnyardgrass [Echinochloa crus-galli (L.) Beauv.] at a density of 32 plants/m² . Sequential applications of 0.56 kg/ha glyphosate (POST1 fb POST2) controlled barnyardgrass 95% 3 weeks after POST2. However, control decreased to approximately 45% with single applications of glyphosate with rates up to 1.7 kg/ha. P endimethalin at 0.84 kg/ha + 140 g/ha imazaquin, PRE controlled barnyardgrass only 65%, but when these soil-applied herbicides where followed with 0.56 kg/ha glyphosate POST1 control increased to 95%.

These studies demonstrated that annual grass, pitted morningglory, sicklepod, and purple nutsedge control was often greatest with sequential applications of 0.84 fb 0.56 kg/ha glyphosate. Increasing the rate of glyphosate in the sequential treatments did not improve weed control or soybean yield. Single POST1 glyphosate applications with rates up to 1.7 kg/ha, or herbicide treatments that did not include glyphosate as a component, generally provided lower weed control and soybeans yield when compared to sequential glyphosate applications.

Field studies were established in 1996 at Greenwood, MS, to evaluate glyphosate rates and application timings in Roundup-Ready soybean. Weed species evaluated were large crabgrass [Digitaria sanguinalis (L.) Scop.] and prickly sida (Sida spinosa L.). Glyphosate was applied at 560, 840, and 1120 g ai/ha at 0-4, 5-9, and 10-14 days after emergence (DAE). Each rate was applied at each application timing.

Control of large crabgrass and prickly sida was not substantially influenced by rate, but application timing did affect season long control. Glyphosate applied 5-14 days after emergence controlled prickly sida and large crabgrass no less than 73% and 90%, respectively, regardless of rate, 6 weeks after planting.

At 8 weeks after planting, the 0-4 DAE applications controlled large crabgrass and prickly sida less than 38%, regardless of rate. The 5-9 DAE applications controlled large crabgrass and prickly sida no more than 73% and 53%, respectively. The 10-14 DAE a pplications controlled large crabgrass 70% and 90% at 560 and 1120 g /ha, respectively. Prickly sida control was 53% and 68% at 840 and 1120 g /ha, respectively. Lower control of prickly sida was due to regrowth and later emergence.

The study suggests that, in order to maximize weed control with a single application, glyphosate should be applied at least 5-14 days after emergence. Earlier applications allowed for germination of additional flushes of these problem weeds.

Soybeans in Mississippi Mississippi Agricultural and Forestry Experiment Station  Mississippi State University Extension Service Division of Agriculture, Forestry and Veterinary Medicine

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