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item Koger Iii, Clifford
item Reddy, Krishna

Submitted to: Weed Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/29/2003
Publication Date: 1/1/2004
Citation: Koger III, C.H., Poston, D.H., Reddy, K.N. 2004. Effect of glyphosate spray coverage on control of pitted morningglory (Ipomoea lacunosa). Weed Technology. 18:124-130.

Interpretive Summary: Pitted morningglory is consistently listed as one of the most common and difficult-to-control weed species in crop production systems of the southern United States. It is also difficult to control with glyphosate in glyphosate-tolerant crops, which have been widely adopted by United States farmers. However, little information as to why this vining weed species is so difficult to control with glyphosate is available. Thus, greenhouse and field field studies were conducted at the Southern Weed Science Research Unit, Stoneville, MS to investigate the effect that glyphosate rate and degree of glyphosate spray coverage have on control of pitted morningglory. Control of pitted morningglory increased as glyphosate rate increased and as plant size decreased. Increasing the amount of leaf surface exposed to glyphosate spray had little effect on control of pitted morningglory. These results are important because they show that pitted morningglory tolerance to glyphosate increases with increasing plant size regardless of level of glyphosate spray coverage.

Technical Abstract: Greenhouse and field experiments were conducted to investigate the effect of glyphosate rate and degree of glyphosate spray coverage on pitted morningglory control. Pitted morningglory in the two-, four-, and six-leaf growth stages were treated with glyphosate at 0.28, 0.56, 0.84, 1.12, 1.40, and 1.68 kg ai/ha. Two- and four-leaf plants were controlled 98% and six-leaf plants 68% with 1.68 kg/ha glyphosate. In a separate greenhouse study, four-leaf pitted morningglory plants with 0, 33, 66, or 100% of their total leaf area exposed to spray were treated with 0.84, 1.68, or 3.36 kg/ha glyphosate. Increasing glyphosate rate from 0.84 kg/ha to 3.36 kg/ha increased control from 36 to 88%. Increasing percent leaf exposure to glyphosate from 0 to 100% increased control from 57 to 75%. In a field study, glyphosate spray coverage decreased from 85 to 40% as plant density increased from 1 to 32 plants/m2. Control decreased only 11% (90 to 79%) between the highest and lowest levels of glyphosate spray coverage. These results demonstrated that pitted morningglory is more tolerant to glyphosate with increasing plant size regardless of the degree of glyphosate spray coverage.