ON-TARGET DEPOSIT AND VERTICAL-DISTRIBUTION OF AERIALLY RELEASED HERBICIDES

As a component of the Fallingsnow Ecosystem Project, glyphosate and tr iclopyr herbicides Vision(R), Release(R)) were each applied to four ex perimental 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 th e 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 esti mates on excised natural foliage with those on proximal 2-dimensional (2D) and 3-dimensional (3D) collectors. Experimental conditions were r epresentative of difficult aerial application scenarios since the spra y plots were small (4.9 to 10.4 ha), with irregular boundaries of matu re 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 spac ing or swath offset. Excluding these points from the data analysis, re sults 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)), res pectively. A high degree of variation in deposit both within and betwe en plots demonstrate that variation in operational parameters (e.g. tr ack spacing, offset, release height and aircraft speed) as influenced by local site factors (e.g. proximity of standing timber, topographica l relief) can be important determinants in uniformity and accuracy of herbicide deposit. A consistent trend (P < 0.001) in the deposition pr ofile through tiered vegetative canopies was observed, with greatest i mpingement 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, dif ferential vertical deposition may be an important factor constraining the potential use of lower herbicide application rates, particularly w here shrub or groundcover species are important competitors. In contra st, given that only a small proportion of the spray cloud penetrates a nd impinges in the lower vegetative tiers, animals foraging or living therein may receive substantially reduced exposures, mitigating agains t any potential direct effects. In general, poor correlations (r = 0.2 2 to 0.78) in deposit estimates based on either two-dimensional or thr ee-dimensional artificial collectors as compared to excised natural fo liage were observed. Significant differences (P < 0.05) also were dete cted among deposit estimates with no consistent trend in relation to h erbicide treatment, sampler type or sampling height. These comparisons suggest that none of the artificial collector types tested accurately or consistently estimated true foliar deposit.