Pigweed seeds have multiple dormancy mechanisms, so that seeds produced in a given season germinate at different times over the next several years, thereby enhancing the weed's long-term persistence (Egley, 1986). Newly shed pigweed seeds are mostly dormant, and become less so by the following spring. Germination is promoted by high temperatures (95 °F), diurnally fluctuating temperatures (e.g., 85–95 °F day,
Pigweeds are highly responsive to nutrients, especially the nitrate form of nitrogen (N) (Blackshaw and Brandt, 2008; Teyker et al., 1991). Fertilization enhances both weed biomass and seed production. In addition, nitrate can stimulate pigweed seed germination (Egley, 1986). A mulch of legume cover crop residues has been observed to enhance pigweed emergence in some years (Fig. 7), likely as a result of rapid mineralization of legume N (Teasdale and Mohler, 2000).
Pigweeds are shade intolerant, and the growth and reproduction of individuals that emerge under a heavy crop canopy are substantially reduced. However, rapid stem elongation allows pigweeds to escape shading in many cropping situations. Late-season pigweeds that break through established cucurbit, tomato, pepper, and other vegetables can promote crop disease by reducing air circulation, interfere with harvest, and set many thousands of seeds (Fig. 8).
Reported time intervals from pollination to formation of viable seeds range from 7–12 days in waterhemp (Bell and Tranel , 2010) to 6 weeks in field populations of redroot pigweed in Ontario (Shrestha and Swanton, 2007). In California, Palmer amaranth formed viable seeds 2–6 weeks after flowering (Keeley et al., 1987). Seeds become viable at about the same time that they develop their mature dark brown or black color.
Figure 9. The amaranth flea beetle feeds on pigweed foliage, and has been observed to cause substantial defoliation and reduce weed vigor in some parts of Virginia. This insect can occasionally become a pest in beet and chard by feeding on seedlings. Photo credit: Mark Schonbeck, Virginia Association for Biological Farming.
ANOVA for soybean height and yield along with A. palmeri biological characteristics (i.e., height, aboveground biomass, seed production, and flowering) was performed using JMP Pro v. 13.1.0 software (SAS Institute, Cary, NC). ANOVA for the height and flowering of A. palmeri throughout the growing season was performed separately for each sampling date. No differences in crop stand were found between treatments by year (unpublished data). Nevertheless, due to interactions between A. palmeri establishment time and year for soybean yield, the statistical analysis was done separately by year. Amaranthus palmeri biological and phenological characteristics were analyzed using weed establishment time and distance from the crop as fixed effects.
The efficiency of light interception by the crop canopy was estimated by measurements of light transmittance through the crop canopy using an AccuPAR/LAI Sunfleck Ceptometer Model LP-80 (Meter Group). Light-transmittance recordings were taken above and below the crop canopy from the same two predetermined points used for canopy closure photographs, under uniform sky conditions between 1100 and 1400 hours. Two measurements perpendicular to the crop row were taken from each plot; these were averaged, and the extinction coefficient, based on light transmittance, was estimated (Equation 2) (Wolf et al. Reference Wolf, Carson and Brown 1972).
Research was conducted, therefore, to investigate whether various A. palmeri establishment times and distances from soybean affect the performance of the weed by evaluating: (1) A. palmeri biological characteristics (i.e., height, dry matter production, seed production); (2) A. palmeri phenology (flowering); and (3) the effects these variations in A. palmeri plantings have on soybean yield.
Sampling and Data Collection
The effects of the distance from the crop on A. palmeri flowering time were not significant in both years, although the greater the distance of A. palmeri from the crop, the greater the number of plants that flowered (unpublished data). Flowering was significantly (P<0.001) affected by establishment time independent of distance from the crop (Figure 6). The shorter the weed establishment time, the greater the number of A. palmeri plants that flowered. More particularly, flowering for A. palmeri plants established at 8 WAE was 33% and 59% lower than for plants established at 0 WAE for 2014 and 2015, respectively (Figure 6). Not surprisingly, the earliest established A. palmeri plants flowered within 7 WAE in 2014 (Figure 6). Likewise, the earliest established A. palmeri reached full flowering between 9.8 and 11.1 WAE (Figure 6) for 2014 and 2015, respectively, probably facilitated by taller A. palmeri plants (Sosnoskie et al. Reference Sosnoskie, Culpepper, Grey and Webster 2012). Taller A. palmeri plants were able to avoid shading caused by crop canopy due to dense ground cover as indicated by the relatively strong relationship between ground cover and extinction coefficient around these dates, particularly at 10.6 and 8.4 WAE for 2014 and 2015, respectively (Figures 7 and 8). The greater the ground cover, the higher the (absolute) value of the extinction coefficient, indicating a high light interception.
A digital camera (Sony DSC-W570, 16.1 megapixels, 25-mm wide-angle lens, 2.7 LCD screen Sony, New York, NY) was used to obtain photographs of crop canopy from two predetermined marked positions in each experimental plot as described in Bell et al. ( Reference Bell, Norsworthy and Scott 2015). Photographs from each plot were analyzed individually using SigmaScan Pro v. 5.0 (Systat Software, San Jose, CA) and Turf Analyzer (https://www.turfanalyzer.com) for the determination of canopy closure, described as “ground cover” (Purcell Reference Purcell 2000; Richardson et al. Reference Richardson, Karcher and Purcell 2001), immediately before each A. palmeri transplanting treatment.
Soybean crop establishment was evaluated at first trifoliate (V1) growth stage, and it was recorded as >95% for both years. In addition, for 20 randomly selected plants from the middle 2 rows plot −1 , soybean height (from ground level to the apical meristem) was recorded six and eight times (i.e., sampling occasions) starting at 2.4 and 1.8 WAE for 2014 and 2015, respectively. In parallel to soybean height recordings, height for each A. palmeri plant (18 and 14 plants plot −1 for 2014 and 2015, respectively, to adjust for plot size changes between years) from ground level to the apical meristem was also recorded, along with the number of A. palmeri flowering plants (i.e., flowering initiation and cumulative number of flowering plants thereafter).
Among the abiotic factors regulating a habitat’s suitability for a species, shade is considered one of the most pertinent. Characterizing the response of A. palmeri to crop canopy shade is important for improved understanding of crop–weed interference and weed population dynamics (Jha et al. Reference Jha, Norsworthy, Bridges and Riley 2008; Korres and Norsworthy Reference Korres and Norsworthy 2017; Korres et al. Reference Korres, Norsworthy, FitzSimons, Trent and Oosterhuis 2017b). Light regime affects A. palmeri biomass production, leaf number, partitioning of dry weight to stem tissue, stem elongation, specific leaf area, and photosynthesis (Korres et al. Reference Korres, Norsworthy, FitzSimons, Trent and Oosterhuis 2017b). It has been reported that shading by the soybean [Glycine max (L.) Merr.] canopy is directly related to row spacing, an important factor to determine not only soybean yield (Bradley Reference Bradley 2006), but also A. palmeri performance and cultural weed management methods (Jha et al. Reference Jha, Norsworthy, Bridges and Riley 2008; Yelverton and Coble Reference Yelverton and Coble 1991). The cosmopolitan nature of this species (EPPO 2018), the threat imposed by this species (Korres et al. Reference Korres, Norsworthy, Brye, Skinner and Mauromoustakos 2017a), and its ability to develop herbicide resistance to various herbicide mechanisms of action in many countries (Heap Reference Heap 2018) justify the present work. Development of efficient weed control strategies and cropping systems that enhance soybean competitiveness against A. palmeri need to be exploited more aggressively and under various crop–weed interference scenarios. In addition, results of this work can be used in population dynamic models that depend on data involving weed biological characteristics such as height, biomass production, and fecundity.