Cold air temperatures at the time of herbicide application are generally thought to decrease efficacy of systemic herbicides. Contact herbicides such as carfentrazone and sulfentrazone combined with systemic herbicides may provide an increased level of weed control when applied at cold air temperatures compared to products containing only systemic ones. Identical herbicide treatments were made at air temperatures of approximately 7 C (45 F) and 18 C (65 F) in four different trials. Control of ivyleaf speedwell, common chickweed, henbit, purple deadnettle, and white clover was evaluated. In general, warm temperature applications initially provided better weed control during the first week after treatment (WAT). By 2 to 3 WAT though, air temperature at the time of application generally did not affect overall weed control. In study 1, at 56 days after treatment (DAT) regardless of air temperature at application, all tested herbicides provided 86% or greater control of ivyleaf speedwell (Veronica hederifolia L.), and 98% or greater control of henbit (Lamium amplexicaule L.), with the exception of Trimec Classic, which did not provide acceptable control of henbit. Common chickweed (Stellaria media L.) control with combination treatments containing carfentrazone or sulfentrazone was similar at both temperature regimes, with control of 95% or greater, averaged across application air temperature. In study 2, after higher initial control with warm temperature treatments during the first WAT, Powerzone and Speedzone provided the highest level of control of ivyleaf speedwell and purple deadnettle at 35 DAT with no difference noted between air temperatures at application. Control of ivyleaf speedwell and purple deadnettle (Lamium purpureum L.) with Powerzone was 85 and 82%, respectively, averaged across temperature applications, while Speedzone controlled these two weed species 88 and 92%, respectively. In general, herbicides evaluated provided similar weed control at approximately 50 DAT when applied at warm or cold temperatures. Only for common chickweed control in trial 3 was there a significant interaction between temperature and herbicide treatment when evaluated 50 DAT. In that study, Speedzone and Trimec Classic gave greater control at warm compared to cold temperature application. Herbicides containing carfentrazone plus ester forms of 2,4-D or MCPA provided the highest levels of control.
Air temperatures in Central Amazonia. III. - Vertical Temperature Distribution on a Clearcut Area and in a Secondary Forest near Manaus (Cold Front Conditions July 10 th. 1969)()