Vol. 51—1997 - NorthEastern Weed Science Society

188
concluded at 5.1 em of simulated rain resulted in 91.5% release, and under field conditions
(northern lberta), 90.6% release resulted after 11.7 nun of rainfall. Release was virtually
complete a er 26.2 nun of cumulative rainfall.
Del ying planting I yr generally improved survival and growth of seedlings planted
into spring hexazinone treatments. Improved survival and growth resulted from lower
hexazinone exposure. Once in the soil, hexazinone persists up to I yr after application (4, II,
22), but ha no apparent negative effect on height or diameter of these bareroot seedlings
(18). In N w Brunswick, at a nearby site characterized by loam soils (47.9% sand, 32.5%
silt, and I .6% clay), aerially treated with 3.8 kg ai/ha hexazinone (VELPAR L), Feng and
Feng (4) f und 0.05 kg/ha hexazinone at 15-30 em soil depth after 361 days, and reported
DT90 = 31 days. No residues were found 453 and 537 days after application. In northern
Ontario, R y et al. (22) reported slightly higher residues for a finer-textured clay soil after
one year. pproximately 13.5% of original hexazinone residues remained 365 days after
initial soil reatment at 4 kg ailha.
Ba d upon this estimate, one might expect residues ranging from 0.2 to 0.4 kg ai/ha
after 1 yr t the present New Brunswick site, which is characterized by finer-textured soils
(mean silt nd clay content = 44.3 and 26.0%, respectively) than those studied by Feng and
Feng (4). or the 4 kg ai/ha treatment, sufficient residues evidently remained after I yr to
reduce see ling survival, since seedlings planted into this treatment 1 yr after site preparation
with hex inone continued to exhibit lower survival than control seedlings.
At a third nearby New Brunswick site (mean silt and clay content = 42.7 and 25.8%,
respective y), no difference in survival among treatments was observed for containerized
(multipot) black spruce seedlings planted 2 or 14 months after site preparation with liquid and
dry-flowa le hexazinone formulations (19, 20). Both hexazinone formulations were applied
in June I 87.
I Ontario, for boreal soils with high clay and organic matter content, Wood et a1.
(25) repo ed that both bareroot and containerized black spruce seedlings could be safely
planted 4 eeks after site preparation with hexazinone at dosages up to 2 kg ai/ha. At higher
dosages (i e., up to 4 kg ai/ha), seedling damage was avoided only by delaying planting by
about 1 y .
N rmally, soil adsorption of hexazinone increases with increased clay or organic
matter co tent, making it less available for absorption by seedlings. At high hexazinone
rates, this adsorptive capacity may be saturated, making hexazinone residues more available
for abso tion by seedlings. The high clay content of the Ontario soil may be the explanation
as to why bareroot seedlings could be safely planted I month after hexazinone treatment,
whereas ey could not in New Brunswick. However, at 4 kg ai/ha, the adsorptive capacity
of the On ario soil appeared to be exceeded. In New Brunswick, a I month difference in
planting t mes for bareroot and containerized (multipot) stock allowed for a longer
degradati n period in soils of nearly identical silt and clay content, but residues were likely
still quite high 2 months after treatment. Therefore, we speculate that poor survival of
bareroot eedlings may have also been caused by greater exposure of their roots to