Barnyardgrass is one of the most problematic weeds in Arkansas, and with the documentation of herbicide-resistant biotypes, there is a need to gain a detailed understanding of its ecology. In particular, knowledge on barnyardgrass seedbank size and emergence pattern is vital. An extensive seedbank survey was carried out in 2008 in 12 counties in eastern Arkansas to determine barnyardgrass seedbank size across the region. There was a great variability in seedbank size with a maximum of 215,000 seeds m−2. Among the fields surveyed, barnyardgrass seedbank was found only in 7% of the cotton fields, while it was 22 and 20%, respectively, for rice and soybean. To examine the emergence pattern of barnyardgrass, experiments were conducted in Rohwer (two sites), Stuttgart (one site), and Fayetteville (one site), Arkansas in 2008 and 2009. In each site, barnyardgrass emergence was quantified from naturally occurring seedbanks. Barnyardgrass exhibited an extended period of emergence with days to 100% emergence ranging from 99 to 165 across sites and years. Nevertheless, effective management may be achieved by targeting the peak emergence periods, which range from mid-April to mid-June in Arkansas. The four-parameter Weibull model provided a better fit to the cumulative emergence data. However, the thermal time (growing degree days, GDDs) or hydrothermal time (HTT) models did not predict barnyardgrass emergence any better than calendar days, perhaps because of the inherent variations associated with natural seedbanks. This study establishes seedbank size and general emergence pattern for barnyardgrass in Arkansas. Additionally, these results will be useful for parameterizing herbicide-resistance simulation models for barnyardgrass.

Field experiments were conducted in 2004, 2005, and 2006, at Pendleton, SC, to determine the effects of soybean canopy and tillage on Palmer amaranth emergence from sites with a uniform population of Palmer amaranth. In 2006, the effect of soybean canopy was evaluated only in no-tillage plots. Palmer amaranth emerged from May 10 through October 23, May 13 through September 2, and April 28 through August 25 in 2004, 2005, and 2006, respectively. Two to three consistent emergence periods occurred from early May through mid-July. Shallow (10-cm depth) spring tillage had minimal influence on the cumulative emergence of Palmer amaranth. Increase in light interception following soybean canopy formation was evident by early July, resulting in reduced Palmer amaranth emergence, especially in no-tillage conditions. In no-tillage plots, from 32 or 33 d after soybean emergence through senescence, Palmer amaranth emergence was reduced by 73 to 76% in plots with soybean compared with plots without soybean. Emergence of Palmer amaranth was favored by high-thermal soil amplitudes (10 to 16 C) in the absence of soybean. Of the total emergence during a season, > 90% occurred before soybean canopy closure. The seedling recruitment pattern of Palmer amaranth from this research suggests that, although Palmer amaranth exhibits an extended emergence period, cohorts during the peak emergence periods from early May to mid-July need greater attention in weed management.

Field experiments were conducted in Pendleton, SC, in 2004 and 2005, to determine the influence of tillage with or without soybean on common cocklebur emergence. Treatments included no-till/no soybean (NTNS), no-till plus soybean (NTS), tillage/no soybean (TNS), and tillage plus soybean (TS). Emergence was monitored from an artificial seed bank in 2004 and a natural seed bank in 2005. Overall, common cocklebur emerged from early May through late October and presented multiple emergence. In no-till plots with or without soybean, initial emergence was delayed 7 d in both years. In TNS plots, major emergence (daily emergence > mean emergence plus standard deviation) of common cocklebur occurred from early May to late July. In NTNS plots, major emergence occurred from late May through late August. No-till reduced total common cocklebur emergence by 59 to 69% compared with tillage. At the V5 to V6 soybean growth stage, the daily soil thermal fluctuation at 2.5 cm soil depth diminished from approximately 15 to 5 C and reduced common cocklebur emergence by 84 to 91% for the rest of the growing season. Common cocklebur emergence was higher when the mean soil temperature was > 15 C, and the daily thermal fluctuation was > 7.5 C. This study suggests that strategies that promote early crop canopy development and minimum tillage should reduce common cocklebur emergence.