Soybean Canopy and Tillage Effects on Emergence of Palmer Amaranth (Amaranthus palmeri) from a Natural Seed Bank

Abstract

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.

Depletion of an Artificial Seed Bank of Palmer Amaranth (<i>Amaranthus palmeri</i>) over Four Years of Burial*

“…Also reported in other small seeded weed species, shallow spring tillage (to a depth of 10 cm) stimulated seedling emergence due to increased exposure of seeds to a favorable microclimate for germination [9]. A secondary soil disturbance through hoeing to a depth of 8 to 10 cm in the spring increased redroot pigweed (Amaranthus retroflexus L.) emergence and contributed to 3-fold greater seed bank depletion compared to undisturbed soil [21].…”

“…Weed seeds in the soil undergo changes in dormancy, which are often regulated by environmental factors such as temperature, light, moisture, nutrients, and gaseous environment [6]. Agronomic practices such as presence or absence of crop, crop rotation, and tillage influence the environmental factors present at the microsite [8] [9], which implies that crop production practices influence weed seed dormancy and persistence.…”

“…Palmer amaranth is a prolific seed producer, with a single female plant producing up to 600,000 seeds [12]. It exhibits seasonal seed dormancy and an extended emergence period [9], causing season-long interference and severe yield reductions in soybean [Glycine max (L.) Merr. ], cotton (Gossypium hirsutum L.), and corn (Zea mays L.) in USA [13]- [15].…”

“…Multiple trays were used to accommodate the entire soil volume that was excavated for each burial depth. The soil was uniformly spread on the tray to a depth of 2.5 cm, which was chosen since Amaranthus species including Palmer amaranth can emerge only from shallow depths up to 2.5 cm [9]. The trays were watered to field capacity daily and emerged seedlings were identified, counted, and removed.…”

Using Herbicide Programs to Control Weeds in Corn (Zea mays L.) and Cotton (Gossypium hirsutum L.)

“…While a few late emerging Palmer amaranth plants may be considered as being harmless, previous research has reported that late season Palmer amaranth seedlings are capable of seed production within 30 days after emergence [123]. Previous research also has shown that weeds left in the field at the time of harvest have the potential to enter harvesting machinery and be distributed across the field [124].…”