Giant ragweed (Ambrosia trifida L.) biology, competition, and control in cotton (Gossypium hirsutum L.)

The objectives of this research were to evaluate control options and investigate the biology and competitiveness of glyphosate-resistant (GR) giant ragweed in cotton. Our results determined that glufosinate followed by glufosinate, glufosinate plus pyrithiobac, and glufosinate plus fluometuron at 0.56 or 1.12 kg ai ha^-1 resulted in the highest level of visual control and the highest yield. However, glufosinate followed by glufosinate was the only treatment that resulted in the highest yield and > 90% control of GR giant ragweed.

The development of glufosinate-tolerant, 2,4-D tolerant, and dicamba-tolerant crops may provide growers with new opportunities for difficult-to-control weeds such as GR giant ragweed. Therefore, the next objective of this research was to evaluate control options for GR giant ragweed with 2,4-D and dicamba applied alone and in combination with glufosinate or fomesafen. Results determined that tank-mix combinations with glufosinate or fomesafen that included either 2,4-D or dicamba resulted in a higher level of control of GR giant ragweed than 2,4-D or dicamba applied alone. Tank-mixing 2,4-D or dicamba with glufosinate will be a valuable approach for controlling GR giant ragweed.

The final objective was to conduct a study to determine competition of giant ragweed in cotton. Early in the growing season, treatments with 2400 or more giant ragweed plants per ha^-1 reduced cotton height when compared with the competition free control. A delay in cotton maturity was observed only with higher populations of 4800 or 9600 plants. However, the effect of giant ragweed on yield was evident with the lowest population of 600 giant ragweed plants per ha^-1 reducing lint yields by 300 kg ha^-1 when compared with a competition free control. Cotton fiber quality was not affected by giant ragweed. These results indicate that season-long giant ragweed competition can significantly reduce cotton yields.