L.L. Lyon, J.W. Keeling, B.W. Bean, M.W. Rowland, and P.A. Dotray; Texas Agricultural Experiment Station, Lubbock and Texas Cooperative Extension, Bushland.
The Texas High and Low Plains accounted for approximately 4.6 million of the 6 million acres of upland cotton planted in Texas in 2001, and 3.3 million of the 3.5 million acres of sorghum. Approximately 60% of Texas cotton is glyphosate-tolerant. Sorghum is often planted as a primary crop or in a crop failure situation, often adjacent to cotton fields, in regions where cotton is the predominant crop. Wet or windy conditions occasionally cause the postemergence-topical application window for glyphosate-tolerant cotton to narrow, and in an attempt to cover large acreages, applications are made aerially, or under less than ideal conditions suggested on the herbicide label. Therefore the objective of this study was to determine the effects of low rates of glyphosate (similar to drift) on sorghum.
Experiments were conducted at Lubbock, TX and Bushland, TX in 2002. At the Lubbock location, DK 44 was planted and Pioneer 8699 was planted at Bushland. Glyphosate (Roundup UltraMax) was applied at 0.38 lb ae/A, 0.19 lb ae/A, 0.094 lb ae/A, 0.047 lb ae/A, and 0.023 lb ae/A (1/2X, 1/4X, 1/8X, 1/16X, and 1/32X of 0.75 lb ae/A, respectively) postemergence-topical to sorghum at the 4-inch, 12-inch, boot, and bloom growth stages. Sorghum visual injury ratings were taken at 14 and 28 days after treatment (DAT) and grain yields were determined.
At Lubbock, glyphosate applied early season (4- and 12-inch growth stages) injured sorghum 5 to 100% 14 DAT. By 28 DAT, rates > 0.05 lb ae/A caused 30 to 100% injury. Rates > 0.09 lb ae/A applied later in the season (boot and bloom growth stages) injured sorghum 12 to 100% 14 DAT. By 28 DAT, rates > 0.05 lb ae/A applied at boot injured sorghum 5 to 100%, but only 0.19 and 0.38 lb ae/A applied at bloom showed injury (20 to 90%). Glyphosate applied at 0.05 lb ae/A at 4-inches reduced yield approximately 30%, while no yield was produced when rates > 0.09 lb ae/A were applied. Yield was reduced 25% from 0.02 lb ae/A applied at 12-inches, while rates of 0.05, 0.09, and 0.19 lb ae/A at 12-inches stunted and delayed plant maturity, so harvest was not possible. Rates > 0.05 lb ae/A at boot caused blasted heads, which reduced sorghum yield. Only 0.19 and 0.38 lb ae/A applied at bloom reduced yield, primarily due to blasted heads and plant lodging.
At Bushland, glyphosate at > 0.09 lb ae/A on 4-inch sorghum caused injury 20 to 100% 14 DAT, and this injury did not change by 28 DAT. Injury ranged from 10 to 100% from rates > 0.05 lb ae/A applied on 12-inch sorghum, with similar injury observed at 28 DAT. Glyphosate > 0.09 lb ae/A applied at boot injured sorghum 15 to 80%; however, only the 0.19 and 0.38 lb ae/A rates applied at bloom caused injury (3 to 5%). Similar injury was observed at both application timings at 28 DAT. Glyphosate at 0.09 lb ae/A applied early season reduced yield 20 to 25%, while higher rates caused > 85% yield reduction. Yield was reduced from boot applications of glyphosate > 0.05 lb ae/A (31 to 100%), while only 0.38 lb ae/A applied at bloom reduced yield (51%).
At both locations, increasing glyphosate rate increased sorghum injury
, especially early season. Young sorghum was more susceptible to lower rates of glyphosate than older sorghum, although higher glyphosate rates on older sorghum interfered with grain production. Glyphosate rates as low as 1/16X of 0.75 lb ae/A reduced sorghum yield at the 4-inch, 12-inch, and boot stages, while sorghum tolerated rates up to 1/4X at bloom. Sorghum injury and recovery varied by location and may be dependent on the growing season and crop conditions.