2014 ASHS Annual Conference

Commercial lettuce producers struggle with weed control in direct-seeded fields. Pre-emergent herbicides with the ability to control weeds without injuring the crop would be beneficial to producers. In this study, ‘Green Salad Bowl’ leaf lettuce was seeded into plots arranged in a randomized complete block with 4 replications three days prior to pre-emergent herbicide application. Precision single row push seeders were used to direct seed the crop. Plot size was 4 rows totaling 16 ft x 20 ft. The 2 center rows were used to collect data.  Eight treatments were evaluated for their potential weed control without suppression of lettuce growth. Treatments included: Pronamide at 1lb/A and 2lb/A rates. Pronamide is currently labelled for use in leaf lettuce production. Non-labeled herbicides were also evaluated, including Imazapic at 2oz/A, 4oz/A  and 6oz/A rates and Imazethapyr at 6 fluid oz/A rate. An untreated weed-free check was maintained by weekly cultivation and an untreated check receiving no cultivation served as a control treatment. Herbicides were sprayed at the listed rates with a CO₂ backpack sprayer delivering 15 GPA.  Throughout the 78d study, lettuce germination rates and heights were recorded. Final lettuce fresh and dry weight was collected and statistically analyzed. Data were subjected to analysis of variance (P=0.05) and means were separated using Fisher’s LSD. Plots treated with Pronamide at the 1lb/A rate and 2lb/A rate and the untreated weed-free check produced significantly more fresh and dry tissue weight than all other treatments. Although lettuce germinated and grew in the Imazapic and Imazethapyr treated plots, growth was severely stunted. Preliminary results suggest that Imazapic and Imazethapyr not be considered for pre-emergent weed control in direct seeded leaf lettuce as weed control was sufficient but lettuce growth was severely stunted. Additional research in a second season is warranted.