Responses of the weed Digitaria Abyssinicia (A. Rich.) Stapf to selective grass herbicides in Ugandan cotton
Successful weed control is essential for economical cotton production in Uganda. Weeds compete with cotton for moisture, nutrients and light. The greatest competition usually occurs early in the growing season. Therefore, post emergence herbicides such as fluazifop-butyl and sethoxydim were used in this study to control the grass weed species which grow faster than cotton during its initial growth stage. Field trials were conducted on the sandy loam/loam soils at Namulonge Research Institute and Bukalasa Technology Verification Centre in Uganda during the 1995/96 and 1997/98 cotton seasons to investigate the control of the tropical couch grass (Digitaria abyssinica (A. Rich. ) Stapf) using the post emergence herbicides. This weed is a rhizomatous grass and difficult to control. The efficacy of the reduced dose rates of the grass selective post emergence herbicides sethoxydim and fluazifop-butyl was investigated for the control of D. abyssinica and other grass weed species in cotton so as to determine the appropriate dose rate(s). The herbicides were supplemented with two hand weedings. Hand weeding (5 times) during the growing season was included in the treatments. The results obtained from the field trials showed the potential of reduced dose rates in the control of grasses. The application of fluazifop-butyl (138,162 and 188 g a. i. ha 1) and sethoxydim (405,502 and 579 g a. i. ha 1) gave a significant density reduction of D. abyssinica and other annual grasses at 35 days after herbicide application. No significant differences were observed amongst the dose rates of both herbicides in the percentage weed control of D. abyssinica and other grass weed species in the two seasons. The percentage weed control ranged between 79-96%. The assessment showed that fresh and dry weights of D. abyssinica shoots/foliage were reduced by 70-80% irrespective of the dose rates for both herbicides at the two sites during the two seasons. Reduced dose rates below half of the full dose rates were investigated in the greenhouse in UK. A markedly reduction of D. abyssinica shoots and rhizomes was noted following the application of fluazifop-butyl (38,66,94,188 g a. i. ha 1) and sethoxydim (116,203, 290,579 g a. i. ha 1) compared to the control. An average percentage reduction of 43.2- 62% for fresh and dry shoots, and 65.9-78% for fresh and dry rhizomes was observed. Although analysis of variance indicated that there were no significant differences amongst dose rates, low percentage reduction was noted from the lowest dose rate of fluazifopbutyl (38 g a. i. ha 1). 1 As part of the plant stress assessment, results indicated that sethoxydim and fluazifopbutyl inhibited chlorophyll accumulation in the treated leaves of D. abyssinica. A reduction of 40-70% of chlorophyll content was noted. It was noted that sethoxydim had higher inhibitory effect on chlorophyll content than fluazifop-butyl. Results obtained from the measurements of fluorescence parameters showed significant reduction of Fv/Fm in the leaves of D. abyssinica due to sethoxydim and fluazifop-butyl, suggesting significant alteration of the normal fluorescence yield. Interference with the normal fluorescence of D. abyssinica was associated with the inhibition of the flow of electron transport which resulted to plant stress. Further investigation on plant stress was studied by comparing the activity levels of the intracellular proteases of D. abyssinica and cotton plants after the application of sethoxydim. The activity level of alanyl aminopeptidase was not affected by the herbicide both in D. abyssinica and cotton. It was however noted that there was significant decrease in the activity levels of argninyl aminopeptidase (arginyl-ap) (36.1%) and tripeptidyl aminopeptidase (Tap) (51.8%) in the treated plants of D. abyssinica with time after herbicide application. While in cotton, significant increase was observed in the activity level of and tripeptidyl aminopeptidase with time after herbicide application. Therefore it can be assumed that the activity levels of these enzymes in the two plant species may have an influence on their responses to the herbicide. Data on crop performance indicated that seedcotton yields realised from herbicides dose rates combined with two hand weedings were high as or higher than the yields obtained from the hand weeding (5 times). Seedcotton yields were noted high (1793-2993 kg/ha) in 1995/96 compared to 1997/98 (665.1-1184.2 kg/ha). The present study has indicated that integration of reduced dose rates with two hand weeding supplements can reduce the number of weedings and improve cotton production.