Influence of Soil Moisture on the Safening Effect of CGA-43089 in Grain Sorghum (Sorghum bicolor)

Abstract

Field and growth chamber experiments were conducted to determine the effects of surface soil moisture on the phytotoxicity of alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide] and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] to grain sorghum [Sorghum bicolor(L.) Moench. ‘Funk 623 GBR’], which was unprotected or protected with CGA-43089 {α-[(cyanomethoxy)imino] benzeneacetonitrile}. In field tests, neither alachlor nor metolachlor was phytotoxic to unprotected grain sorghum when the surface soil remained dry until the sorghum had emerged. CGA-43089 protected sorghum emerging from moist soil that had been treated with alachlor or metolachlor at rates of 2.24 or 3.36 kg/ha. Growth chamber tests showed that CGA-43089 was less effective in protecting sorghum from herbicide injury when Ships clay was continuously wet [110% field capacity (FC)] from time of planting to emergence (3 to 5 days) than when soil was wet for only 1 or 2 days prior to emergence. In contrast, if the surface soil remained dry until the coleoptile reached the soil surface, alachlor and metolachlor had little effect on sorghum even when no protectant was present. When sorghum was planted in Arenosa sand containing 5% organic matter, protected sorghum grew as well as the control even under continuous high moisture conditions. Metolachlor incorporated into the root zone at a rate of 20 ppm had no effect on either protected or unprotected sorghum. Alachlor and metolachlor were most phytotoxic when placed in the surface 1.25 cm of moist soil or when incorporated. The coleoptile was the most susceptible plant part. Thus, the key to grain sorghum response to these herbicides was in herbicide placement and availability to the coleoptile. Under conditions normally leading to phytotoxic effects from alachlor or metolachlor, grain sorghum growth was significantly better from seed protected with CGA-43089 than from unprotected seed.