Abstract/Contents: | "Quinclorac efficacy on smooth crabgrass [Digitaria ischaemum (Schreb. ex Schweig) Schreb. ex Muhl] has been shown to be affected by several factors such as drought, irrigation schedule, spray coverage, and smooth crabgrass biotype and growth stage. This year, smooth crabgrass seed source and growth stage effects were further examined in greenhouse and field tests. In greenhouse experiments, three seed sources, collected in Ithaca, New York, Marysville, Ohio, and Kingston, Rhode Island, were compared in three growth stages, young seedling (2- to 3-leaf), early tillering (1- to 3-tillers), and mid-tillering (5- to 6-tillers) stage. In two field tests in Ithaca, New York, native crabgrass was treated at the same growth stages. Quinclorac (75% DF) was sprayed at rates of 0, 120, 250, 500, 750, and 1,000 g ai ha⁻¹ using a CO₂-pressurized backpack sprayer calibrated to deliver 280 L ha⁻¹ in both greenhouse and field experiments. In the greenhouse experiment, shoot fresh weight was measured 2 and 4 weeks after treatment (WAT). In the field experiments, herbicidal efficacy was visually evaluated at one week intervals. The final visual evaluation was made in early October, after the first frost. Using regression analysis, rates to achieve 50 and 90% control (I₅₀ and I₉₀) were calculated. In the greenhouse tests, I₅₀s were similar for all three biotypes at all the growth stages tested. For the Ohio and Rhode Island biotypes, I₉₀s also tended to be stable across growth stages. In contrast, the I₉₀ for the New York biotype increased as growth stage advanced. In summary, greenhouse experiments showed that susceptibility of three biotypes was similar up to early tillering stage, and that New York biotype was somewhat more tolerant than others at mid-tillering stage. In the field, the New York biotype showed stable I₅₀s and increasing I₉₀s at advanced growth stages up to 6 WAT, similar to the results from the greenhouse experiments. At 6 WAT, I₅₀s were 120, 170, and 170 g ai ha⁻¹, and I₉₀s were 330, 470, and >1,000 g ai ha⁻¹ for young seedling, early tillering, and mid-tillering stage, respectively. Tolerance increased remarkably between early and mid-tillering stage. At final evaluation, I₅₀s were 260, 130, and 170 g ai ha⁻¹, and I₉₀s were 640, 410, and >1,000 g ai ha⁻¹ for young seedling, early tillering, and mid-tillering stage, respectively. Differences in the results between two evaluation times were most pronounced in young seedling stage treatment. In conclusion, the New York biotype tends to be more tolerant at advanced growth stages both in the greenhouse and field, whereas other biotypes are not. In practical perspectives, early tillering stage treatment provides better long-term control. Young seedling stage treatment does not provide good control through the season possibly due to late germination or poor spray coverage of young seedlings within a dense turf canopy. Late treatment did not provide adequate control." |