Submitted to: World Cotton Research Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: September 10, 2007
Publication Date: September 14, 2007
Citation: Mahan, J.R. 2007. Glufosinate-toleranct cotton: What have we learned in three years of commerical use on the Texas Southern High Plans?[abstract]. World Cotton Research Conference. Lubbock, Texas. September 10-14, 2007. Technical Abstract: The development of herbicide-tolerant crops increase weed management options in these systems. Herbicide-tolerant crops have been developed by selection and bioengineering. Tolerance to glufosinate-ammonium has been bioengineered into cotton (Gossypium hirsutum L.) by Agrobacterium tumefaciens through the expression of a gene encoding the enzyme phosphinothricin acetyl transferase. This enzyme inactivates glufosinate by transferring the acetyl group of acetyl-coenzyme A to the free amino group of glufosinate. Laboratory studies conducted to determine thermal limitations on herbicide activity in bioengineered cotton concluded that glufosinate-tolerant cotton should not be damaged over a wide range of application temperatures. Additional field and growth chamber studies have confirmed that transgenic glufosinate-tolerant cotton has high levels of glufosinate tolerance (up to 8X labeled rates). Glufosinate-tolerant (LibertyLink) cotton was commercially available in the United States in 2004 and Australia in 2006, with commercial availability in Brazil expected in 2008. In 2006, LibertyLink cotton was planted on 3.5% of U.S. cotton hectares. Texas planted 7.7% of its cotton hectares to this technology, and planted hectares in other states are expected to increase as adapted varieties become available across the cotton belt. Glufosinate-ammonium is classified as a non-selective, amino acid inhibitor that is foliarly applied to control many annual and perennial weeds. It is a non-volatile, photo- and thermostable herbicide that is not readily translocated in plants. The current glufosinate label (Ignite 280) indicates that 0.47 to 0.58 kg ai/ha may be used per application and up to 1.18 kg ai/ha may be used per growing season. To achieve effective weed control with glufosinate, weed size and spray coverage are critical for maximum herbicidal activity. The glufosinate-ammonium label (Ignite 280) recommends a maximum weed height of 8 cm for Palmer amaranth (Amaranthus palmeri) and 5 cm for devil’s-claw (Proboscidea louisianica) when applying the 0.47 kg ai/ha rate. If weed size exceeds label recommendations, glufosinate efficacy will likely decrease. To accomplish season-long weed control, sequential glufosinate applications were necessary throughout the growing season. The use of residual herbicides in a LibertyLink cotton system improved weed control in most instances. Glufosinate plus pyrithiobac (Staple) applied early-postemergence (EP) followed by (fb) glufosinate plus pyrithiobac applied mid-postemergence (MP), and pyrithiobac applied preemergence (PRE) fb sequential glufosinate applications (EP fb MP) controlled Palmer amaranth and devil’s-claw at least 95%. Although sequential glufosinate applications made to small weeds have been effective, the need for timely applications on multiple weed flushes make control with a glufosinate-only system very challenging. The objective of this research was to evaluate Palmer amaranth and devil’s-claw control following glufosinate applications at different rates, timings, and in sequential combinations when weed size exceeded label recommendations. Early-, mid-, and late-postemergence (LP) applications were made on June 7, June 14, and June 21, respectively, in a field naturally infested with Palmer amaranth and devil’s-claw. Palmer amaranth and devil’s-claw size at the EP, MP, and LP timing were 10 to 25 cm and 5 to 15 cm, 15 to 30 and 15 to 20 cm, and 30 to 51 and 20 to 25 cm, respectively. Glufosinate at 0.87 kg ai/ha controlled Palmer amaranth 75% following the EP application. This control was more effective than glufosinate at rates of 0.58 (53%) and 0.45 kg ai/ha (29%). Palmer amaranth was controlled 47% (0.45 fb 0.45 kg ai/A) to 90% (0.58 fb 0.87 kg ai/A) following EP fb MP sequential applications. Glufosinate applied LP did not effectively control Palmer amaranth (<60%) regardless of rate. Three applications of glufosinate (EP fb MP fb LP) controlled Palmer amaranth 73 to 86%. More effective Palmer amaranth control was observed when the EP application contained glufosinate at 0.87 kg ai/A compared to glufosinate at 0.45 kg ai/ha. Glufosinate at 0.45 kg ai/ha applied EP controlled devil’s-claw 90%. If the glufosinate rate increased to 0.58 or 0.87 kg ai/ha, devil’s-claw control increased (96%). Two glufosinate applications (EP fb MP) controlled devil’s-claw at least 99% regardless of rate. Devil’s-claw control ranged from 20 to 58% with a single LP application. Devil’s-claw control following sequential glufosinate applications (EP fb MP fb LP) did not improve from the addition of prometryn PRE in 2006. These data indicate the importance of weed size at application. The initial application timing followed by sequential applications appear to be the key for successful weed management when using glufosinate. Delayed glufosinate applications made to weeds that exceed the size recommendations on the label will likely result in less than effective control even when using the maximum labeled rate per application per season. Once Palmer amaranth size exceeds the label recommendation, it will be difficult to achieve effective weed control.