The effect of time of day (TOD) on the activity of six common POST herbicides was investigated in field trials from 2007 to 2009 at two locations in southwestern Ontario. Percentage weed control was assessed following application of bentazon, chlorimuron-ethyl, fomesafen, glyphosate, imazethapyr, or quizalofop-p-ethyl applied at 3-h intervals from 6:00 A.M. to midnight, when weeds averaged 15 cm tall. The effect of time of day varied with weed species, but weed control was generally reduced when herbicides were applied at 6:00 A.M., 9:00 P.M., and midnight. Herbicide activity on velvetleaf was most frequently reduced, especially for chlorimuron-ethyl, glyphosate, and imazethapyr. Control of common ragweed with glyphosate and imazethapyr was also affected by the timing of application, and pigweed species only showed an effect with glyphosate. Variation in temperature, relative humidity, and dew presence/absence at different times of the day, as well as morphological/physiological characteristics such as weed size at time of application and diurnal leaf movement in response to light intensity, may account for the variation in weed control at different times of the day. Significant soybean yield loss was not observed in this study, but may occur if herbicide efficacy is severely reduced by application at inappropriate times of day. These results provide valuable information for growers, and suggest that POST herbicides are most effective when applied midday, rather than in the early morning or late evening.

Stopps, G. J., White, S. N., Clements, D. R. and Upadhyaya, M. K. 2011. The Biology of Canadian weeds. 149. Rumex acetosella L. Can. J. Plant Sci. 91: 1037–1052. Rumex acetosella L., sheep sorrel, is a perennial herb originating from Europe and southwestern Asia, now found in at least 70 countries worldwide. A common weed, it appears in grasslands, pastures, rangelands, waste areas, and along roadsides across North America. It frequently occurs as a weed in lowbush blueberry fields in Atlantic Canada, particularly in Nova Scotia. Rumex acetosella thrives on poor, acidic, and disturbed soils across a broad range of climatic conditions. A persistent seed bank and vegetative reproduction from creeping roots contribute to the ability of R. acetosella to tolerate various methods of control including certain herbicides, fire, and tillage. Tart-tasting leaves, rich in vitamin C have led to the use of R. acetosella in European and North American cultures as both a source of food and a component of herbal medicines. Excessive consumption of the leaves, which contain oxalic acid, can be poisonous to horses and sheep. In addition, R. acetosella is a possible host of the tomato spotted wilt virus, which is detrimental to a variety of vegetable crops.

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