The adoption of conservation tillage systems has been challenged by concerns potential weed species shifts. A 9-yr study from 1988 to 1996 was conducted Delhi, Ontario, on a loamy sand soil to evaluate the effect of tillage systems (conventional [CT] and no-till [NT]), cover crop Secale cereale, and nitrogen (N) rate (0, 50, 100, 125, 150, and 200 kg N ha−1) on monocrop Zea mays L. (corn) yield and changes in the composition of the weed flora. CT consisted of spring moldboard plowing followed by cultivation with a tooth cultivator. Weed counts were taken in the last 3 yr of the study (1994, 1995, and 1996) prior to postemergence herbicide application and then again 2 to 3 wk after herbicide treatment. Composition or the weed flora was analyzed by canonical discriminant analysis (CDA). The relationship between weed density and tillage system was not consistent. Weed species composition differed between CT and NT systems. Chenopodium album and Amaranthus retroflexus were associated with CT and Digitaria sanguinalis with NT. N rate and cover crop did not affect weed density or species composition. Proper management of weeds with herbicides appeared to minimize any long-term effect on the weed flora resulting from varying N rates. Zea mays yields did not differ between CT and NT systems but were greater in both systems with a cover crop at the higher N rates. Disturbance caused by tillage was more important than N rate and cover crop as a mechanism influencing composition of the weed flora.

Clomazone was evaluated preplant incorporated (PPI) or post-transplant (PT) for weed control in sweet potatoes. No crop injury by clomazone was detected. Clomazone controlled (≥90%). broadleaf signalgrass, large crabgrass, barnyardgrass, and prickly sida at all rates and methods of application (MOA). Common cocklebur was controlled (≥93%) by clomazone at 1.1 kg ai ha-1 applied PT or at 3.4 kg ai ha-1 PPI in 1988 and at all rates and MOA in 1989. Clomazone did not control carpetweed.

The control of puncturevine in southern peas with imazethapyr and imazaquin and the potential for carryover of these herbicides to spinach was evaluated in Arkansas. Imazethapyr (0.07 kg ai ha−1) and imazaquin (0.14 kg ha−1) applied to southern peas either PPI tank-mixed with trifluralin (0.84 kg ha−1) or PRE or POST following trifluralin controlled puncturevine at least 70%. Imazaquin and imazethapyr remaining in soil after use caused injury and yield reduction to spinach planted 3 to 4 mo after herbicide application in the fall after southern pea harvest. In spring spinach planted 8 to 9 mo after herbicide application, the only injury observed was from soil residues of imazethapyr applied POST in one of two years, with no yield reduction. In rate by timing experiments, imazethapyr at 0.07 kg ha−1 applied PPI, PRE, and 3 and 6 d after puncturevine emergence controlled puncturevine at least 95% at the 2-wk rating. At the 4-wk rating, imazethapyr at 0.07 kg ha−1 applied PRE provided this level of control. Imazaquin at 0.07 and 0.14 kg ha−1 controlled puncturevine greater than 93% when applied up to 12 d after emergence. Puncturevine control was less with both herbicides at lower rates and as application timing increased.

Transplanted cabbage was grown in conventional-tillage (100% cultivated) and strip-tillage (25% cultivated and 75% residue) production systems with various herbicide treatments to evaluate weed control and cabbage yield. DCPA at 9 kg ai/ha, napropamide at 1.1 kg ai/ha, oxyfluorfen at 0.3 kg ai/ha, and oxyfluorfen at 0.2 kg ai/ha plus napropamide at 1.1 kg ai/ha provided commercially acceptable (> 80%) control of carpetweed, swinecress, large crabgrass, redroot pigweed, and common lambsquarters under both tillage systems. However, only oxyfluorfen plus napropamide provided control of eclipta. At a second location, all herbicides controlled common purslane and barnyardgrass and all but DCPA controlled hairy galinsoga. Overall, cabbage yields were the same between conventional and strip-tillage systems, and among herbicide treatments when average cabbage head weights across tillage method were pooled.

Total weed density increased after 1 yr of no-tillage and after 2 yr of conventional tillage in a 4-yr experiment with repeated assignment of the same treatment to the same plots. Large crabgrass, goosegrass, and carpetweed densities were higher in the no-tillage compared with the conventional-tillage treatment in at least 1 yr whereas common lambsquarters density was greater in the conventional-tillage treatment the last year of the experiment. Within the no-tillage treatment, rye or hairy vetch residue reduced total weed density an average of 78% compared to the treatment without cover crop when cover crop biomass exceeded 300 g m–2 and when residue covered more than 90% of the soil. Goosegrass, stinkgrass, and carpetweed densities were reduced by cover crop residue in at least 1 yr whereas large crabgrass was unaffected. Common lambsquarters density increased where rye was grown as a cover crop prior to conventional tillage. Despite differences in weed density among treatments, weed biomass was equivalent in all treatments during the last 2 yr.

Studies were initiated at two different planting dates and conducted at two different locations in 2001 to determine the critical weed-free period for certain populations of weeds in organically produced ‘Beauregard’ sweetpotato. Naturally occurring weed populations were used, and they included sicklepod, redroot pigweed, and yellow nutsedge. Treatments included allowing weeds to grow for 2, 4, 6, or 8 wk after transplanting (WAT) sweetpotato before weed removal and maintaining the sweetpotato weed-free for 2, 4, 6, or 8 WAT. Weedy and weed-free checks were also included in the study. These treatments were used to determine the length of time weeds can compete with sweetpotato without reducing yield and the length of time sweetpotato must grow before yield is no longer affected by newly emerging weeds. Yield of number one grade sweetpotato roots best fit a quadratic plateau curve for the grow-back treatments and a logistic curve for the removal treatments. Yields in weed-free plots of sweetpotato were higher at the early planting date, whereas yields in plots of weedy sweetpotato were higher at the late planting date. Weed biomass was lower in the grow-back treatments at the late planting date. Data indicate that sweetpotato may gain a competitive advantage over weeds when planted at a later date. At both planting dates, a critical weed-free period of 2 to 6 WAT was observed.