HEXAZINONE TOXICITY IN RED PINE AND JACK PINE

Experiments were conducted under controlled environmental conditions t o determine the toxicity of hexazinone ylamino)-1-methyl-1,3,5-triazin e-2,4(1H,3H)-dione) to red pine (Pinus resinosa Ait.) and jack pine (P . banksiana Lamb.) during their period of rapid shoot elongation in sp ring and after initiation of overwintering buds in August. Red pine wa s a minimum of 10- and 12-fold more tolerant to soil-applied hexazinon e than was jack pine on the basis of effective concentration values (E C50, the herbicide concentration that results in a 50% growth reductio n compared with the control) for root dry weight and shoot fresh weigh t, respectively. In the spring experiment, EC50 values were approximat ely 0.062 and 1.75 kg a.i. (active ingredient).ha-1 for jack pine and red pine shoot fresh weights, respectively. For root dry weight, EC50 values were roughly 0.04 and 2.0 kg a.i..ha-1 for jack pine and red pi ne, respectively. Except for a somewhat reduced sensitivity in jack pi ne, similar results were obtained with soil applications of hexazinone after bud initiation in August.