On-target deposit and vertical distribution of aerially released herbicides
Authors: Thompson Dean G. , Pitt Douglas G. , Staznik Bozena , Payne Nicholas J. , Jaipersaid David , Lautenschlager R. A. , Bell F. Wayne 
Published: February 2014

As a component of the Fallingsnow Ecosystem Project, glyphosate and triclopyr herbicides (Vision®, Release®) were each applied to four experimental spray plots at nominal rates of 1.5 and 1.9 kg a.e. ha−1 respectively. Empirical studies were undertaken on these plots with the objectives of; a) quantifying mean on-target deposit and variability b) assessing the vertical distribution of active ingredient deposits through the vegetative complex and c) comparing herbicide deposit estimates on excised natural foliage with those on proximal 2-dimensional (2D) and 3-dimensional (3D) collectors. Experimental conditions were representative of difficult aerial application scenarios since the spray plots were small (4.9 to 10.4 ha), with irregular boundaries of mature timber, and in some cases substantial topographical relief. Deposit analysis confirmed that, in some circumstances, locations well within target areas were missed completely owing to inappropriate track spacing or swath offset. Excluding these points from the data analysis, results demonstrated overall mean deposition (mean ± SE) of glyphosate and triclopyr on aspen foliage equating to 68.45 ± 6.13 and 50.28 ± 6.01% of the nominal application rates (1.5 and 1.9 kg ha−1), respectively. A high degree of variation in deposit both within and between plots demonstrate that variation in operational parameters (e.g. track spacing, offset, release height and aircraft speed) as influenced by local site factors (e.g. proximity of standing timber, topographical relief) can be important determinants in uniformity and accuracy of herbicide deposit. A consistent trend (P < 0.001) in the deposition profile through tiered vegetative canopies was observed, with greatest impingement of the spray in the upper target canopy as noted above, and average 25% and 12% in the shrub and ground-level tiers respectively. Results suggest that for sites characterized by complex canopies, differential vertical deposition may be an important factor constraining the potential use of lower herbicide application rates, particularly where shrub or groundcover species are important competitors. In contrast, given that only a small proportion of the spray cloud penetrates and impinges in the lower vegetative tiers, animals foraging or living therein may receive substantially reduced exposures, mitigating against any potential direct effects. In general, poor correlations (r = 0.22 to 0.78) in deposit estimates based on either two-dimensional or three-dimensional artificial collectors as compared to excised natural foliage were observed. Significant differences (P < 0.05) also were detected among deposit estimates with no consistent trend in relation to herbicide treatment, sampler type or sampling height. These comparisons suggest that none of the artificial collector types tested accurately or consistently estimated true foliar deposit. Key words: alternative conifer release treatments, Fallingsnow Ecosystem Project, glyphosate, herbicides, herbicide deposit, tending, triclopyr, vegetation management, vertical distribution