Total nonstructural carbohydrate trends of Texas blueweed (Helianthus ciliaris) in relation to herbicidal control with glyphosate

Texas blueweed (Hellanthus. ciliaris) is a pernicious perennial weed which is spread throughout the Southwestern United States and is a major problem in many agronomic crops. Infestation of this weed species in many crop species including cotton (Gossvplum hirsutum) and grain sorghum (Sorghum vulgare) has been shown to exert a negative effect on both yield and quality of the crop.

This study was conducted at two locations over a two-year period from June 1984 to June 1986. The plot areas consisted of a cultivated dryland site and a rangeland site each season and in each case was located in Lubbock County. The soils involved in the .study ranged from a fine sandy loam to a clay loam in texture and fell into the Amariilo, Acuff, and Olton series of soil classification.

The specific objectives were: (1) to evaluate the environmental influences of soil moisture and/or soil temperature on the total nonstructural carbohydrate (TNC) concentrations of Texas blueweed; (2) to determine the response of blueweed to treatment with four levels of glyphosate (N-phosphonomethyl glycine) at different times over the growing season; (3) to evaluate the seasonal TNC concentration patterns as related to plant stage of development and its relation to herbicidal control. Field measurements of soil moisture and soil temperature, and collection of untreated plant samples were made monthly. Above- and belowground plant parts were analyzed for TNC by use of NIRS spectrophotometry, calibrated by the method of acid extraction with 0,2N HCL.

The patterns of TNC concentrations and influence of soil moisture and/or soil temperature were found to be highly correlated at all locations in both years of the experiment. Under favorable growing conditions (good soil moisture, etc.) carbohydrates were found to be translocated to all parts of the plant, however, under moisture stress no translocation to the root tissue occurred until the stress was relieved.

Application of glyphosate to Texas blueweed at three rates, (0,54, 1.12 and 2,24 kg/ha) indicated that effectiveness at the two lower concentrations was less effective when rapid growth occurred. The most effective control of blueweed was achieved at 2.24 kg/ha when applied in June, July, or August.