Masters Theses

Date of Award


Degree Type


Degree Name

Master of Science


Plant, Soil and Environmental Sciences

Major Professor

G. Neil Rhodes Jr.

Committee Members

William Krueger, Robert Hayes, Elmer Ashburn


Field and greenhouse studies were conducted in 1987 and 1988 to determine the influence of tillage systems on the efficacy of selected preemergence herbicides in soybeans [Glycine max (L.) Merr.]. The effects were measured using visual ratings, height measurements and soybean yields.

Field studies were conducted at four locations in Tennessee. At Knoxville and Milan the cover crop was wheat (Triticum aestivum L.), while at Crossville and Spring Hill the cover crop was spring oats (Avena sativa L.). Tillage treatments at Knoxville included conventional tillage (15 cm) following wheat harvest, stubble planting following wheat harvest and planting into standing wheat which was killed at stage 9 with glyphosate. At Crossville and Spring Hill, spring oats were killed with paraquat prior to the initiation of the three tillage treatments. These treatments consisted of no-tillage (planting into the standing spring oats), shallow tillage (8 cm); and conventional tillage (15 cm). A power-driven tiller was used for all tillage operations.

Within each tillage treatment at all locations, the following preemergence treatments were included; metribuzin (0.43 kg ai/ha), chlorimuron + metribuzin (0.06 + 0.37 kg ai/ha), chlorimuron + linuron (0.04 + 0.53 kg ai/ha), imazaquin (0.14 kg ai/ha) and imazethapyr (0.11 kg ai/ha). Hand-weeded and weedy checks were also included. The experimental design used at all locations was a split-plot factorial arrangement in a randomized complete block with four replications of each treatment. The tillage treatments were assigned to the main plots and herbicide treatments were assigned to the sub-plots.

Results indicated that tillage systems may affect the activity of preemergence herbicides. All herbicide treatments with the exception of metribuzin provided good to excellent control of common cocklebur (Xanthium strumarium L.), annual morningglory (Ipomoea spp.), jimsonweed (Datura stramonium L.), yellow nutsedge (Cyperus esculentus L.) and large crabgrass (Digitaria sanguinalus L. Roth). No herbicide treatment gave satisfactory control of sicklepod (Cassia obtusifolia L.). At Knoxville and Milan, there was a trend for greater weed control with no-tillage treatments. This was not observed at Crossville or Spring Hill. There was also a trend for greater soybean injury with chlorimuron plus metribuzin, chlorimuron plus linuron, and imazaquin. Tillage systems and herbicides interacted significantly. Greater soybean injury was observed in no-tillage treatments. Soybean yields were higher in no-tillage treatments at Knoxville and Milan and equivalent to other tillage treatments at Crossville and Spring Hill. Heavy cover crop residue at Knoxville and Milan provided more favorable soil moisture levels in no-tillage treatments which compensated for greater initial soybean injury. However, this was not observed at Crossville or Spring Hill where less crop residue was present. In conclusion, weed control, soybean injury and yield increased with increasing amounts of cover crop residue.

Greenhouse studies were conducted to determine the interactions of straw mulch and selected preemergence herbicides on soybean injury. 'Asgrow 5474' soybeans were planted in 0.53 L plastic pots containing sandy loam soil. Three straw mulch treatments were established: (1) 4.5 g of straw mulch placed on the soil surface; (2) 4.5 g of straw mulch incorporated thoroughly into the soil; and (3) a no straw treatment. The straw level used was equivalent to 4480 kg/ha. Preemergence herbicide applications included: metribuzin (0.22 or 0.43 kg ai/ha), chlorimuron + metribuzin (0.03 + 0.19 or 0.06 + 0.37 kg ai/ha), chlorimuron + linuron (0.02 + 0.26 or 0.04 + 0.52 kg ai/ha), imazaquin (0.07 or 0.14 kg ai/ha) and imazethapyr (0.055 or 0.11 kg ai/ha). Control treatments with no preemergence herbicide were also included. The experimental design was a randomized complete block in the greenhouse; however it was analyzed as a split-plot factorial arrangement of treatments in a randomized complete block. This allowed for the determination of treatment interactions.

Results indicated that soybean height, injury and weight were affected by all herbicide treatments, and varied with straw treatments. Soybean injury tended to increase with increasing herbicide rates. Chlorimuron plus metribuzin and chlorimuron plus linuron affected soybean height and weight to a greater extent than did other herbicides. This was visible through internode shortening and reduced canopy formation. Root weights indicated decreased root growth with herbicide applications. Greater heights and injury was observed in the surface straw treatment compared to the other straw treatments. Significant interactions were observed between herbicide and straw treatments for all data with the exception of root dry weight. Interactions for all data may be partly explained by greater moisture in the surface straw treatment compared to the incorporated or no straw treatments.

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