Palmer amaranth (Amaranthus palmeri S. Wats.) is one of the most difficult weeds to control in soybean (Glycine max (L.) Merr.) in North Carolina. Research was conducted during 2010 and 2011 to determine if Palmer amaranth control and soybean yield were affected by soybean plant population and combinations of preemergence (PRE) herbicides followed by a single application of glufosinate postemergence (POST) versus multiple applications of glufosinate POST. Palmer amaranth was controlled more and soybean yield was greater when soybean was established at 483,000 plants in 3 of 4 experiments compared with soybean at 178,000 plants irrespective of herbicide treatments. In separate experiments, application of PRE herbicides followed by POST application of glufosinate or multiple POST applications of glufosinate provided variable Palmer amaranth control, although combinations of PRE and POST herbicides controlled Palmer amaranth the most and provided the greatest soybean yield. In 1 of 3 experiments, sequential applications of glufosinate were more effective than a single application. Yield was higher in 2 of 3 experiments when glufosinate was applied irrespective of timing of application when compared with the nontreated control. In the experiment where glufosinate was applied at various POST timings, multiple applications of the herbicide provided the best control and the greatest yield compared with single applications. 1. Introduction Resistance of Palmer amaranth to acetolactate synthase (ALS-) inhibiting herbicides and glyphosate is well documented in North Carolina [1–3]. Controlling herbicide-resistant Palmer amaranth can be challenging because alternatives to glyphosate and ALS-inhibiting herbicides require timely application [4, 5]. Row pattern and plant population can influence weed management in soybean [6, 7]. Herbicides that applied PRE or POST other than glyphosate are important in managing glyphosate-resistant Palmer amaranth in soybean [8–10]. Under weed-free conditions, soybean at populations below those established in many fields in North Carolina yield as well as soybean at higher populations considered “standard” (exceeding 300,000 plants？ha？1) (E. J. Dunphy, personal communication). Establishing soybean at lower populations can reduce production costs if yields are maintained. Results comparing the economic value of high soybean populations versus increasing the number of POST herbicides are mixed [11–16]. Additional research is needed to determine if Palmer amaranth control is more effective when soybean is established at higher
References
[1]
I. Heap, “The international survey of herbicide resistant weeds,” 2012, http://www.weedscience.org/In.asp.
[2]
J. R. Whitaker, Distribution, biology, and management of glyphosate-resistant palmer amaranth in North Carolina [PhD dissertation], North Carolina State University, 2009.
[3]
A. E. Hoffner, D. L. Jordan, and A. C. York, “Geographical distribution of herbicide resistance in Palmer amaranth (Amaranthus palmeri) populations across North Carolina,” in Proceedings of the 7th International IPM Symposium: IPM on the World Stage, pp. 27–29, Memphis, Tenn, USA, 2012.
[4]
A. S. Culpepper, A. C. York, A. W. MacRae, and J. Kichler, “Glyphosate-resistant Palmer amaranth response to weed management programs in Roundup Ready and Liberty Link Cotton,” in Proceedings of the Beltwide Cotton Conferences, National Cotton Council, Nashville, Tenn, USA, January 2008.
[5]
L. M. Sosnoskie, J. M. Kichler, R. D. Wallace, and A. S. Culpepper, “Multiple resistance in Palmer amaranth to glyphosate and pyrithiobac confirmed in Georgia,” Weed Science, vol. 59, no. 3, pp. 321–325, 2011.
[6]
D. B. Harder, C. L. Sprague, and K. A. Renner, “Effect of soybean row width and population on weeds, crop yield, and economic return,” Weed Technology, vol. 21, no. 3, pp. 744–752, 2007.
[7]
G. T. Place, S. C. Reberg-Horton, J. E. Dunphy, and A. N. Smith, “Seeding rate effects on weed control and yield for organic soybean production,” Weed Technology, vol. 23, no. 4, pp. 497–502, 2009.
[8]
T. C. Mueller, P. D. Mitchell, B. G. Young, and A. S. Culpepper, “Proactive versus reactive management of glyphosate-resistant or -tolerant weeds,” Weed Technology, vol. 19, no. 4, pp. 924–933, 2005.
[9]
M. D. K. Owen, “Evolved glyphosate-resistant weeds and weed shifts: weed species in glyphosate-resistant crops,” Pest Management Science, vol. 64, no. 4, pp. 377–387, 2008.
[10]
D. R. Shaw, M. D. Owen, P. M. Dixon et al., “Benchmark study on glyphosate-resistant cropping systems in the United States—part 1: introduction to 2006–2008,” Pest Management Science, vol. 25, pp. 183–191, 2011.
[11]
D. A. Guillermo, P. Pedersen, and R. G. Hartzler, “Soybean seeding rate effects on weed management,” Weed Technology, vol. 23, no. 1, pp. 17–22, 2009.
[12]
O. W. Howe and L. R. Oliver, “Influence of soybean (Glycine max) row spacing on pitted morningglory (Ipomoea lacunose) interference,” Weed Science, vol. 35, pp. 185–193, 1987.
[13]
G. R. W. Nice, N. W. Buehring, and D. R. Shaw, “Sicklepod (Senna obtusifolia) response to shading, soybean (Glycine max) row spacing, and population in three management systems,” Weed Technology, vol. 15, no. 1, pp. 155–162, 2001.
[14]
J. K. Norsworthy and J. R. Frederick, “Reduced seeding rate for glyphosate-resistant, drilled soybean on the southeastern Coastal Plain,” Agronomy Journal, vol. 94, no. 6, pp. 1282–1288, 2002.
[15]
J. K. Norsworthy and L. R. Oliver, “Effect of seeding rate of drilled glyphosate resistant soybean (Glycine max) on seed yield and gross profit margin,” Weed Technology, vol. 15, pp. 284–292, 2001.
[16]
R. J. Kratochvil, J. T. Pearce, and M. R. Harrison Jr., “Row spacing and seeding rate effects on glyphosate-resistant soybean for mid-Atlantic production systems,” Agronomy Journal, vol. 96, no. 4, pp. 1029–1038, 2004.
[17]
E. Coetzer, K. Al-Khatib, and D. E. Peterson, “Glufosinate efficacy on Amaranthus species in glufosinate-resistant soybean (Glycine max),” Weed Technology, vol. 16, pp. 326–331, 2002.
[18]
A. S. Culpepper, A. C. York, and L. E. Steckel, “Glyphosate-resistant Palmer amaranth management in cotton,” in Proceedings of the Southern Weed Science Society, vol. 64, p. 226, 2011.
[19]
J. D. Reed, P. A. Dotray, and J. W. Keeling, “Palmer amaranth (Amaranthus palmeri S. Wats.) management in Glytol + LibertyLink cotton,” in Proceedings of the Southern Weed Science Society, vol. 64, p. 260, 2011.
[20]
J. R. Whitaker, A. C. York, D. L. Jordan, and A. S. Culpepper, “Palmer amaranth (Amaranthus palmeri) control in soybean with glyphosate and conventional herbicide systems,” Weed Technology, vol. 24, no. 4, pp. 403–410, 2010.
[21]
Ignite 280 herbicide label. Bayer CropScience, Research Triangle Park, NC, USA, 2012.
