Lodging immediately after July 4, 2007 storm. Managing Herbicide-Resistant Weed Populations in Grain Sorghum - 2,4-D K. L. Roozeboom and D. L. Regehr Kansas State University Department of Agronomy Introduction Many producers are searching for additional, economical options to control broadleaf weeds in sorghum. The tendency of 2,4-D to cause brittle stalks and other plant reactions is well documented. This has caused a movement away from this herbicide in recent years. The limited number of effective herbicides and the onset of triazine- and ALS-resistant amaranth has renewed interest in 2,4-D for weed control in sorghum. Objective Determine whether current hybrids differ in their response to application of 2,4-D. Methods 2005: 98 grain sorghum hybrids from 19 companies 2007: 30 hybrids from 12 companies With and without 2,4-D 2,4-D applied in 15 gal/acre at 20 psi 1.5 pints/acre of 2,4-D Amine-4 on June 25, 2005 1 pint/acre of 2,4-D Lo-vol Ester-4 on June 29, 2007 Plants at growth stage 2 (V-5 to V-6; 8" to 10") All plots weed-free pre-emergence Bicep II Magnum and hand weeding Experimental design: Three (2005) or four (2007) replications of four-row plots, 9 m in length Randomized complete block with split block and split plot Blocks split by hybrid maturity Whole plot – hybrid, four rows Sub plot – with or without 2,4-D, two rows each 21 days after treatment All hybrids had significant brace root malformation, stunting, and tiller lodging (not shown) in both years. Lodging at harvest Lodging was minimal in 2005. In 2007, 10 of 30 hybrids had significantly greater lodging with 2,4-D and had significantly lower yield with 2,4-D. The four hybrids with no yield reduction in 2007 had no significant increase in lodging with 2,4-D in either year. Days to anthesis 96 of 98 hybrids were significantly delayed in 2005. All hybrids were significantly delayed in 2007. Hybrids delayed by 3 days or more always yielded less with 2,4-D. Yield 78 of 98 hybrids were not significantly impacted by 2,4-D treatment in 2005. Only 4 of 30 hybrids were not affected by 2,4-D in 2007. Two of those four had significant yield reduction in 2005. Hybrids with greater untreated yield had the potential for greater yield reduction with application of 2,4-D. Lodging immediately after July 4, 2007 storm. Untreated Treated Notes and ratings Planted May 31, 2005 and June 11, 2007 Harvested October 12, 2005 and October 1, 2007 2005: No strong winds or storms; lodging was minimal 2007: Storm on July 4 caused significant lodging; most plants recovered but treated plants retained “goose neck” stalk appearance Visual rating of % injury (primarily stunting) caused by the 2,4-D at 7 or 8 and 14 days after treatment Visual ratings of the degree of stunting, brace root malformation, and stem and tiller lodging and goose-necking at 21 days after treatment (1 to 5 scale: 1 = little or none, 5 = severe) Plant height recorded at maturity Treated Untreated Correlations Early-season ratings were significantly correlated with each other and with differences in days to anthesis in both years (data not shown). Correlations of early-season ratings with differences in plant height, grain moisture, and test weight were either not significant or very low (data not shown). Four of five early-season ratings were significantly correlated with differences in yield in both years (Table 2). Changes in grain moisture and test weight had the strongest correlations with changes in yield with 2,4-D in both years (Table 2). Plant height and grain moisture All hybrids were shorter and had wetter grain with 2,4-D in 2005 (Table 1). Most hybrids had a significant reduction in height and increase in grain moisture in 2007. In 2007, hybrids with height reductions of >15 cm or with grain moisture increases of >30 g/kg had yield reductions with 2,4-D. Table 1. Treatment means for parameters with non-significant treatment x hybrid interaction in 2005. Treatment Plant Height (cm) Grain Moisture (g kg-1) No 2,4-D 123.4a 16.5b 2,4-D at Growth Stage 2 117.1b 17.0a Table 2. Pearson correlation coefficients for change in grain yield with early-season injury ratings and changes in hybrid performance with 2,4-D. Yield Change Injury rating 2005 2007 Change in % Injury 7 days -0.18 ns Days to Anthesis -0.17 -0.22 % Injury 14 days -0.31 -0.26 Plant Height 0.22 Stunting 21 days -0.28 -0.21 Grain Moisture -0.29 -0.49 Root injury 21 days -0.32 Test Weight 0.20 0.45 Tiller lodg. 21 days -0.13 Lodging Results and Discussion Legend for all charts, = 0.05 Seven or eight days after treatment (DAT) All hybrids showed significant injury at seven or eight days after treatment in both years. Few hybrids exhibiting the greatest amount of injury (>30%) had no yield reduction with 2,4-D. Untreated Treated Conclusions Application of 2,4-D consistently caused significant early-season injury (stunting, brace root malformation, and tiller lodging), a delay in bloom date, and a reduction in height for nearly all hybrids. Some hybrids exhibited greater effects that others, but responses were not closely related to yield reductions. Grain moisture increased and grain test weight decreased with greater yield reductions in response to application of 2,4-D in 2007. Two of the four hybrids with minimal yield change in 2007 had a significant yield reduction in 2005, indicating that yield responses to 2,4-D application may not be consistent. 14 days after treatment (DAT) Most hybrids still exhibited significant injury in 2005. All hybrids exhibited significant injury in 2007. Few hybrids with injury greater than 30% to 40% had no yield reduction with 2,4-D. Test weight 87 of 98 hybrids did not differ significantly in test weight with 2,4-D in 2005. Seven of 30 hybrids did not differ significantly in test weight with 2,4-D in 2007. In 2007, hybrids with test weight reductions >30 kg/m3 had yield reductions with 2,4-D. Other Considerations 2,4-D was applied at the “safest” sorghum growth stage, yet most hybrids exhibited a considerable adverse response. Application timing is important for maximizing weed control and minimizing sorghum response. Lowering the application rate and tank-mixing with other herbicides also may be beneficial. Altered sorghum growth and increased lodging with 2,4-D may slow or complicate harvest. 2,4-D may be more appropriate in some situations than in others. For example, 2,4-D may fit better in dryland fields with low yield potential, heavy pigweed infestation, and where cattle will graze stalks and utilize downed heads.