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management with herbicides has become more complex. Weed management options
have to be planned prior to the growing season.
Tolerance of Sweet Sorghum to Metolachlor and Mesotrione Herbicides in Non-
Irrigated Conditions.
P. Sapkota*, N.R Burgos, L. Estorninos, Jr., E.A.L. Alcober, T. M. Tseng, Department of Crop,
Soil, and Environmental Sciences, University of Arkansas, Fayetteville, AR.
Sweet sorghum (Sorghum vulgare) is grown on very small acres in Arkansas for
molasses, but is being considered as an alternative biofuel crop that can fit into existing
cropping systems in the Delta. A study was conducted to evaluate the tolerance of sweet
sorghum to metolachlor and mesotrione under non-irrigated condition. The experimental
units were arranged in a randomized complete block design with four replications, with
the treatments being combinations of metolachlor and mesotrione at different rates. Sweet
sorghum cv. ‗Dale‘ was planted in plots with 4 rows, 20 ft long, 40 in apart. The
experiment was conducted at the Main Agricultural Research and Experiment Station,
Fayetteville, AR in 2007, 2008 and 2009. Mesotrione at 0, 0.094 and 0.375 lb ai/A and
metolachlor at 0, 0.48, 0.96, 1.43 and 1.91 lb ai/A were applied preemergence, separately
and as mixtures. A combination of metolachlor (1.91 lb/A) + mesotrione (0.375 lb/A) was
included as a ‗standard treatment‘ for comparison of weed control efficacy and crop
tolerance.
In 2007, 13% injury was observed on sweet sorghum treated with metolachlor +
mesotrione (1.43 lb ai/A + 0.094 lb ai/A), metolachlor + mesotrione (0.48 lb ai/A + 0.094
lb ai/A) and metolachlor alone (0.96 lb ai/A). Plots treated with metolachor alone (0.48 lb
ai/A), metolachor + mesotrione (0.96 lb ai/A + 0.094 lb ai/A), mesotrione alone (0.375 lb
ai/A) and metolachor + mesotrione (1.91 lb ai/A + 0.375 lb ai/A) had less crop stand than
the non-treated plots. Average fresh biomass of 18.93 mt/acre as recorded where none of
the herbicide caused any significant reduction in fresh biomass.
In 2008, mesotrione (0.094 lb ai/A) caused 14% crop injury; metolachlor (0.48 lb ai/A),
23%; and metolachlor + mesotrione (0.48 + 0. 094 lb a.i./A), 29% at 21 d after
application. Mesotrione alone (0.094 lb ai/A) had the highest stand count, followed by
metolachor + mesotrione (1.91 lb ai/A + 0.094 lb ai/A), metolachor (0.48 lb ai/A) and
metolachor + mesotrione (0.48 + 0. 094 lb a.i./A) treatments. Sweet sorghum sprayed
with metolachor + mesotrione (1.91 lb a.i./A + 0.094 lb ai/A) had the highest biomass
yield (28.935 mt/acre) comparable to other treatments.
In 2009, of the 17 treatments, the combination of mesotrione and metolachor (0.094 +
0.48 lb ai/A), and metolachlor alone at 0.96 and 1.43 lb ai/A caused the least stand loss or
early-season crop injury. Sweet sorghum in plots sprayed with metolachlor at 1.43 lb ai/A
showed the highest biomass of 34.45 mt/acre.
Therefore we conclude that a combination of mesotrione and metolachlor at
(0.094 lb ai/A + 0.48 lb ai/A) is safe for sweet sorghum. Higher rates of metolachlor can
be used alone without biomass yield loss.