In majority of the instances in 2012, both daytime and night-time spray applications were more effective than dawn application (Table 3). ERs for both daytime and night-time applications ranked better than the dawn time application for 75% of the time (three of the four herbicides) in Trial 1 and 100% of the time (all four herbicides) in Trial 2. Similarly, in 75% of the occurrences, WBRs for both day- and night-time applications ranked better than the dawn time application in both trials.
Results were different, however, in 2013 compared to 2012, particularly in trial 1, where dawn time application ER and WBR ranked better than both day- and night-time in 75% of the instances (Table 4). However, similar to the 2012 results, both ER and WBR for daytime application in trial 2 ranked better than both night-time and dawn applications in 50% and 75% of the instances, respectively. While ER and WBR for the dawn time application on the average scored better than the day and night-time applications in trial 1, results from trial 2 were similar to 2012 favoring the daytime application over the night-time and dawn applications (Table 4).
On average, PSBD with the daytime application of the selected herbicides was more effective than the Dawn application in 75% of the instances when examined using ERs and about 67% of the time based on the WBRs over the three years, 2012, 2013 and 2014. However, when averaged over 2012 and 2014, the ERs and WBRs values increased from 75% and 67%, respectively, to 94% and 75%, which indicated a substantial increase in the number of instances in which PSBD with the daytime application of the selected herbicide performed better than the dawn time application.
The yearly differences in treatment efficiencies can be linked to prevailing weather conditions at the time of herbicide application. A close examination of the weather data in 2013 clearly showed low soil moisture and precipitation shortly after the spray application occurred around the dates of the trials 1 and 2, respectively. Only 24 mm of precipitation had accumulated between April 1 and May 10, 2013, (52% below normal; Figure 1b), indicating a very dry period at the time of the herbicides' application in trial-1 (May 9 and 10, 2013) compared to the 85 mm between April 1 and May 7 (79% above normal) in trial-1 in 2012 (Figure 1a) and 60 mm between April 1 and May 8 (23% above normal) for trial-1 in 2014 (Figure 1c). The impaired performance of all four herbicides due to the dry conditions in 2013 led to WBRs similar to Control (100%) in trial 1 indicating almost no weed kill, and loss of herbicide efficacy.
|Figure 1: Daily instantaneous, accumulated and long term normal rainfall for April and May, 2014, at the Lethbridge Demo Farm Lethbridge in a) 2012, b) 2013 and c) 2014. Source: Agro-Climatic Information Services (ACIS), Government of Alberta: http://agriculture.alberta.ca/acis/about.jsp
In-crop trials: The results at Lethbridge and Falher sites indicated that the daytime spray applications generally performed better than the other timings for all three years of the study. The results at the third site, LARA, were however mixed. The three application timings did not show any advantage over each other for both oats and mustard. herbicides TM Muster+Select and TM Axial+Infinity turned out to be the most effective ones in Canola (LL) and Wheat, respectively. In addition to some apparent weather conditions, such as, temperature inversions and heavy dew on leaves (Enz et al., 2014), reduced interception of herbicides due to the vertical position of leaves at night could also cause substantial decrease in herbicides' efficacies at night and dawn, especially, in the broadleaf weeds exhibiting diurnal leaf movement (Stopps et al., 2013; Mohr et al., 2007).
In a study examining the effect of the application time-of-day on glyphosate efficacy on velvetleaf, Mohr et al (2007) indicated that leaf angle and time of application accounted for 82 and 18%, respectively, of the biomass change. In a diurnal cycle, plants keep leaves horizontal relative to stem during the daylight with the maximum leaf surface area exposed to the sunlight and fold them in vertical position parallel to stem during the night. Accordingly, efficacy of an herbicide applied during the day time would be higher because plants with the maximum leaf surface area exposed to the sunlight are more likely to intercept greater amounts of herbicide during the day compared to the night application. However, the diurnal leaf movement phenomenon is not as prevalent in grassy weeds (Mohr et al., 2007).
Our results, especially at Lethbridge site, are in agreement with the aforementioned studies showing the effect of diurnal leaf movement (daytime effect) on the herbicide efficacies. When the ER and WBR values were averaged over all herbicides and crops for mustard, a broad leaf weed, the daytime application performed better than the night and dawn timings for all four site years (Table 3). However, for oats, a grassy weed, the daytime application showed better performance than the night and dawn timings in 50 and 75% of the instances for ER and WBR, respectively (Table 4).