Field experiments were conducted in Georgia in 2004 and 2005 to evaluate the effects of S-metolachlor on summer squash fruit yield. Main treatment effects included summer squash cultivar (yellow or zucchini), planting method (seeded or transplanted) and herbicide program (nontreated control, S-metolachlor applied at planting and prior to transplanting [PRE] at 0.5 and 1.0 kg ai/ha, S-metolachlor applied postemergence [POST] 3 wk after planting [WAP] at 0.5 and 1.0 kg/ha, and S-metolachlor applied PRE at 0.5 kg/ha followed by POST at 0.5 kg/ha [PRE fb POST]). Fruit number and weight were measured 12 times during each growing season and the harvests combined into early (harvests 1 to 4), middle (harvests 5 to 8), late (harvests 9 to 12), and cumulative (harvests 1 to 12) yield categories. Mixed-models analyses were used to evaluate the effects of herbicide rate and timing, squash cultivar, and planting method on squash yield for each harvest period. Summer squash cultivar and planting method did not affect squash response to S-metolachlor. Averaged over squash cultivar and planting method, S-metolachlor applied PRE and PRE fb POST reduced fruit number and weight at the early harvest between 35 and 60%, middle harvest between 14 and 30%, and cumulative harvest between 14 and 22%. S-metolachlor applied POST at 0.5 kg/ha did not impact squash yield compared to the nontreated control at any harvest period, whereas 1.0 kg/ha reduced fruit number and weight at the middle harvest 14 and 20%, respectively. We propose that POST applications of S-metolachlor at 0.5 kg/ha or lower can be adopted for use in summer squash production in Georgia.

Nomenclature: S-metolachlor, summer squash, Cucurbita pepo L. ‘Enterprise’ and ‘Payroll’