Cotton yeild loss potential in response to length of Palmer amaranth (Amaranthus palmer.) interference

Authors: MACRAE, A - University Of Georgia; Webster, Theodore; SOSNOSKIE, L - University Of Georgia; CULPEPPER, A - University Of Georgia; KICHLER, J - University Of Georgia

Submitted to: Journal of Cotton Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/1/2012
Publication Date: 10/2/2013
Citation: Macrae, A.W., Webster, T.M., Sosnoskie, L.M., Culpepper, A.S., Kichler, J.M. 2013. Cotton yeild loss potential in response to length of Palmer amaranth (Amaranthus palmer.) interference. Journal of Cotton Science. 17:227-232.

Interpretive Summary: With the introduction of glyphosate-resistant cotton, many growers eliminated the use of soil-applied residual herbicides and cultivation for weed control and relied heavily on glyphosate to manage in-field weed populations. Palmer amaranth (Amaranthus palmeri) with resistance to glyphosate was found in 28 counties in Georgia as of 2010. Palmer amaranth with resistance to glyphosate has now been detected in multiple states in the southern US. Currently the only options for glyphosate resistant (GR)-Palmer amaranth control in glyphosate-resistant cotton systems are herbicides applied prior to Palmer amaranth emergence. Numerous environmental factors can lead to incomplete weed control at any stage during cotton production. Therefore, studies were initiated in Georgia to evaluate the influence of delayed emergence of Palmer amaranth relative on cotton yield. When Palmer amaranth was established at the 3- and 8-leaf cotton stages, cotton yield was reduced approximately 6% for every Palmer amaranth per 6.1 m of row, with a maximum cotton yield loss of 60%. In contrast, there was no affect of Palmer amaranth density on cotton yield when Palmer amaranth established at the 12- and 17-leaf stages of cotton. Palmer amaranth seed production per plant ranged from 61,000 for plant established at the 3-leaf stage of cotton to 14,000 seeds per plant for Palmer amaranth established at the 17-leaf stage of cotton. Palmer amaranth that established before the 12-leaf stage of cotton is likely to cause the most cotton yield loss. However, Palmer amaranth that established at the 17-leaf stage of cotton, simulating an escape from a layby application, was able to sufficiently replenishing the soil seedbank.

Technical Abstract: Field studies were conducted near Ideal, GA in 2006 and 2007 to determine the influence of simulated delayed emergence of Palmer amaranth at several densities on cotton yield and weed growth. Five densities of Palmer amaranth (ranging from 0 to 10 plants row-1) were transplanted at four time intervals defined by cotton leaf stage (3-, 8-, 12-, and 17-leaf cotton). Prior to harvest Palmer amaranth biomass was removed from the plots and quantified, Palmer amaranth seed production measured, and cotton yield determined. When Palmer amaranth was established at the 3- and 8-leaf cotton stages, cotton yield was reduced approximately 6% for every Palmer amaranth per 6.1 m of row, with a maximum cotton yield loss of 60%. In contrast, there was no affect of Palmer amaranth density on cotton yield when Palmer amaranth established at the 12- and 17-leaf stages of cotton. Maximum Palmer amaranth biomass, averaged over all densities, was achieved when Palmer amaranth was established at the 3- and 8-leaf stage of cotton, while Palmer amaranth biomass from plants established at the 12- and 17-leaf stages of cotton was reduced 73%. Palmer amaranth seed production per plant ranged from 61,000 for plant established at the 3-leaf stage of cotton to 14,000 seeds per plant for Palmer amaranth established at the 17-leaf stage of cotton. To avert cotton yield loss, Palmer amaranth must be controlled prior to the 12-leaf stage of cotton; later emerging plants did not affect cotton yields, but did replenish the soil seedbank.