Significance of herbicide order in sequential applications to target weeds in a sunn hemp living mulch

Striking a balance between the weed control capacity of living mulches and their competition with the main crop is complex. At rates that avoid severe injury to living mulch, herbicides may reduce their vigor while simultaneously contributing to weed control. In a 2-yr field study carried out in Freeville, NY, we evaluated the effects of various combinations consisting of two herbicides, applied sequentially at reduced rates, on the growth of a sunn hemp living mulch and weeds (including common lambsquarters, common purslane, hairy galinsoga, and Powell amaranth). When a herbicide with primarily POST activity (Type 1; e.g., rimsulfuron, 0.005 to 0.007 kg ai ha−1) was applied first, performance of sunn hemp (1700 to 3900 kg ha−1 dry biomass; 10% to 88% groundcover) was poor and weed growth (25% to 62% groundcover) was high, likely because sunn hemp was severely injured at a young growth stage and was outcompeted by weeds. A follow-up application (approximately 2 wk later) of a herbicide with primarily PRE and residual activities (Type 2; e.g., metribuzin, 0.05 to 0.15 kg ai ha−1), with a surfactant to enhance its POST activity, had little effect on established weeds. However, because sunn hemp was already 20 cm tall at weed emergence, applying a Type 2 herbicide first did not cause severe injury to sunn hemp and reduced weed pressure, thereby also enhancing sunn hemp performance (3,800 to 6,100 kg ha−1 dry biomass; 85% to 94% groundcover). Moreover, the follow-up application of a Type 1 herbicide affected the smaller weeds more (4% to 21% groundcover) than the better-established sunn hemp. Our results demonstrate that an appropriate sequence of herbicides at reduced rates may be important to control weeds while maintaining a healthy living mulch stand.