Evaluation of Vegetable Oils Effects on Johnsongrass (Sorghum halepense L.) Control by Sethoxydim

Weed management is one of the most important aspects of successful crop production for supplying food needed for the rising population. Johnsongrass (Sorghum halepense L.) is among the most noxious weeds in the world due to its superior biology and tremendous ecological adaptations. It causes substantial yield loss in different field crops including corn, soybean, wheat, and sorghum. Therefore, management of Johnsongrass is crucial for optimum crop production. Prevention by clean cultivation, cultural management by using weed-competitive, mechanical management by hand hoeing, biological management by applying pathogens, and eventually applying herbicide as chemical management can be used for controlling Johnsongrass. Chemical management is an effective method to control Johnsongrass. For decreasing adverse effects of herbicide and costs of production, optimizing herbicides performance is very essential. It appears that the use of adjuvants seems to be a best solution to achieve optimized herbicides performance. In spite of these advantages, some synthetic adjuvants have shown side effects on living organs. Therefore, using environmentally safe adjuvants is a key point for applying this technology. This study was conducted for evaluating vegetable oil effects on Johnsongrass control by sethoxydim herbicide.

To study the effects of vegetable oils on the performance of sethoxydim on Johnsongrass (Sorghum halepense L.) control, a factorial experiment based on completely randomized design including sethoxydim concentration at six levels (0, 46.875, 93.75, 187.5, 281.25 and 375 g ai ha-1) and vegetable oils at 8 levels (with and without corn, olive, grape seed, cumin, fennel flower, mint and sunflower oils) with four replications was conducted in the research greenhouse of college of agriculture Birjand university in 2017. For increasing seed germination and breaking seed dormancy of Johnsongrass, the seeds were treated by sulfuric acid for 3 min and then washed by tap water for 30 minutes. The seeds were then sown in potting trays (3 cm × 3 cm × 5 cm) filled with moistened peat. One week after sowing, at the one-leaf seedlings stages, they were transplanted to plastic pots with 2 liter volume and filled with a mixture of sand, clay loam soil, and peat (1:1:1; v/v/v). The pots were sub-irrigated every two days. The seedlings were thinned to four per pot at the two-leaf stage. Spraying was done at the four-leaf stage by using a chargeable sprayer equipped with an 8002 flat fan nozzle tip delivering 250 L ha-1 at 2 bar spray pressure. Four weeks after spraying, height of plant was measured and then the shoots and roots of plant were harvested and weighed immediately after the root volume was measured. The plant parts were oven-dried and reweighed.

When emulsified vegetable oils alone were sprayed against Johnsongrass, none of vegetable oils had phytotoxic effects on plant height, fresh and dry weight of shoot and root, root volume and root length of Johnsongrass (Table 2). This finding was similar to the results of Tworkoski (2002) (42) and Izadi darbandi et al (2013) (17). The ED50 parameter was estimated by dose response model based on Johnsongrass plant height, shoot and root dry and fresh weight, and root volume. All emulsifiable vegetable oils improved significantly the effectiveness of sethoxydim on Johnsongrass. Relative potency in the presence of corn, olive, grape seed, cumin, fennel flower, mint and sunflower oils was 1.30, 1.57, 1.18, 1.23, 1.27, 1.24, and 1.07 times for plant height, 3.75, 1.49, 1.59, 3.52, 2.93, 1.81, and 2.58 times for shoot fresh weight, 2.63, 1.30, 1.35, 2.57, 1.99, 1.48, and 1.86 times for shoot dry weight, 2.21, 1.27, 1.47, 2.09, 1.67, 1.82, and 1.71 times for root dry weight, and 1.74, 1.56, 1.55, 2.13, 1.58, 1.38, and 1.41 times for root volume, respectively, as compared to the condition without vegetable oils. Among emulsifiable vegetable oils, the highest effect on shoot and root dry weight was observed in corn and cumin oils while olive oil showed the lowest effect.

Our result showed that vegetable oil including corn, olive, grape seed, cumin, fennel flower, mint and sunflower oils had not only the phytotoxic effects on produced biomass by Johnsongrass, but also improved the performance of Sethoxydim for Johnsongrass control. Therefore, using vegetable oils mixed with sethoxydim can decrease the adverse impacts of this herbicide on the environment.