Nicosulfuron and primisulfuron activiy, absorption, translocation, and metabolism in broadleaf signalgrass (Brachiaria platyphylla)

Field and greenhouse experiments were conducted to examine the effect of application rate and timing of Post nicosulfuron and primisulfuron to control several grass species. Under greenhouse conditions, the susceptibility to nicosulfuron was (from most to least) green foxtail = giant foxtail = barnyardgrass = johnsongrass = yellow foxtail = broadleaf signalgrass ≥ large crabgrass = fall panicum ≥ annual ryegrass. The susceptibility to primisulfuron was (from most to least) johnsongrass = yellow foxtail = giant foxtail ≥ green foxtail = large crabgrass = barnyardgrass = broadleaf signalgrass = annual ryegrass ≥ fall panicum. Nicosulfuron (≥ 32 g ai ha-1) reduced shoot fresh weight of broadleaf signalgrass and barnyardgrass by 70% to 80%, while primisulfioron (≥ 36 g ai ha-1) reduced fresh weight of these species only 55%. Otherwise, the two herbicides performed similarly with respect to herbicide rate and grass species. Nicosulfuron Post at 32 to 64 g ai ha-1, controlled small (1 to 3-leaf stage) broadleaf signalgrass 80% to 90%. Under field conditions nicosulfuron at 32 to 64 g ai ha-1 controlled broadleaf signalgrass between 60% and 70%, 40% and 60%, and 30% when applied at the 2 to 3, 3 to 4, and 4 to 5-leaf stages, respectively. Primisulfuron did not reduce broadleaf signalgrass fresh weight at any application timing or application rate. The normal use rate of nicosulfuron (32 g ai ha-1) and primisulfuron (36 g ai ha-1) and twice the normal use rates (2X) reduced johnsongrass fresh weight by 70% to 80% when applied at either the 3 to 4-leaf or 4 to 5-leaf stage . Johnsongrass fresh weight reduction was no greater than 65% when these herbicides were applied at the 5-leaf stage. Nicosulfuron at 1.5X and 2X rates applied to 2 to 3-leaf large crabgrass reduced fresh weight 70% to 80%, while primisulfuron (2X rate) reduced fresh weight reduction only 67% at the 2 to 3-Ieaf stage. Large crabgrass control diminished rapidly for both herbicides at the 3 to 4-leaf and 4 to 5-leaf application timings. Research was conducted to determine the effect of environment (high light/temperature = High E versus Low light/temperature = Low E) on the absorption, translocation and metabolism of nicosulfuron and primisulfuron in broadleaf signalgrass and com. Nicosulfuron and primisulfuron absorption in com was rapid, 60% to 85% within 72 hours after treatment (HAT) depending on environmental treatment. Nicosulfuron and primisulfuron translocation out of the treated leaf was less than 4.5% of herbicide absorbed through 72 HAT. Com rapidly metabolized both herbicides in both environments. However, primisulfuron was metabolized more rapidly (High = 99%, Low = 92%) than nicosulfuron (High = 95%, Low = 78%). Nicosulfuron absorption was up to 20% greater than primisulfuron absorption in broadleaf signalgrass through 72 HAT. Nicosulfliron translocation out of the treated leaf in broadleaf signalgrass was ≤ 15% of absorbed through 72 hours, while primisulfuron translocation was ≤ 4% during the same time period. Primisulfuron metabolism was more rapid than nicosulfuron in broadleaf signalgrass. During the first 4 HAT broadleaf signalgrass metabolized more than 20X the amotmt of primisulfuron than nicosulfuron. By 72 HAT, broadleaf signalgrass had metabolized nearly 90% of the primisulfuron absorbed by the plant (High E) while it had metabolized no more than 7% of the nicosulfliron in the same time period. These results suggest that differential activity of nicosulfuron and primisulfuron on broadleaf signalgrass is likely based on differential rates of metabolism to nonphytotoxic compounds. Additionally, environment has the potential to affect rates of sulfonylurea absorption, translocation, and metabolism