Introduction

Timely and effective herbicide spray programs are essential for adequate weed control in crop production (Wilcut and Askew, 1999). However, the rapid growth of many problematic broadleaf weeds influences the effectiveness of control (Horak and Loughin, 2000). A major concern in the southeastern United States is crop interference from Palmer amaranth (Amaranthus palmeri S. Wats) and common ragweed (Ambrosia artemisiifolia L.). Weeds compete with crops for light and other resources and can complicate mechanical harvest (Horak et al., 1994; Morgan et al., 2001; Young et al., 1982). Weeds must be controlled throughout the growing season to avoid yield losses in peanut (Arachis hypogaea L.) (Jordan, 2018; Wilcut et al., 1995). As the duration of interference from weeds increases, yield of peanut and other crops often decreases (Zimdahl, 2004). Minimizing contributions of weed seed to the soil seedbank is important in managing weeds in succeeding crops and addressing evolved resistance to herbicides. One of the main concerns in peanut production is interference of common ragweed and Palmer amaranth with peanut and how in-season control practices affect weed density in the following rotational crop.

Palmer amaranth, which originated in southern California and Mexico, is difficult to control in peanut and crops rotated with peanut in the United States (Leon et al., 2019). Morphological features and capacity to produce flowers at almost any growth stage makes Palmer amaranth successful in exploiting diverse environments (Sauer, 1957). In California, Keeley et al. (1987) reported that Palmer amaranth can emerge in early March when soils have reached a temperature of 18 C and continue to infest fields into October. In Georgia, Webster and Grey (2015) found that glyphosate-resistant Palmer amaranth planted in early May produced 446,000 seeds per plant, while Palmer amaranth planted six weeks later produced 50 percent fewer seed. Palmer amaranth planted 9 and 12 weeks later had 89 and 99% fewer seeds, respectively (Webster and Grey, 2015). Horak and Loughin (2000) found that under natural growing conditions and in the absence of other weed or crop species, Palmer amaranth grew 0.18 to 0.21 cm/growing degree day (GDD). Because of its rapid growth rate, the application window for optimum control of Palmer amaranth may be relatively short (Horak and Loughin, 2000). The application interval for effective control of Palmer amaranth could be as soon as one week after weed emergence based on maximum height recommended for herbicides used on peanut (Jordan, 2018).

Morgan et al. (2001) reported that when 1.1 Palmer amaranth plants/m² were present, cotton (Gossypium hirsutum L.) yield was reduced by 54%. Yield loss of field corn (Zea mays L.) ranged from 11 to 91% at 0.66 to 10 plants/m² when Palmer amaranth emerged with the corn (Massinga et al., 2001). At a density of one plant/m of row, yield loss in peanut was predicted to be 28% (Burke et al., 2007). Yield of soybean [Glycine max (L.) Merr.] was reduced 79% when 10 plants/m² were present (Bensch et al., 2003). Palmer amaranth can also reduce harvesting efficiency, which is problematic for agronomic crops that are mechanically harvested (Young et al., 1982). Smith et al. (2000) reported that when 0.33 Palmer amaranth plants/m² were present in cotton, harvest time increased by 3.5 times compared to weed-free conditions. The increased harvest time was a result of the operator having to stop the machine to dislodge stalks from moving parts (Smith et al., 2000). When Palmer amaranth density exceeds 0.66 plants/m², damage to harvest equipment can occur (Morgan et al., 2001).

Common ragweed is a monoecious annual plant native to North America and is common to the temperate regions of the United States (Oosting, 1942). Common ragweed will generally dominate in the second year if soil disturbance is halted, shading out some of the species that dominated in the first year (Oosting, 1942), which relates to the increase of common ragweed problems in no-till or reduced tillage systems in soybean and corn (Jordan et al., 2014). Common ragweed can emerge in late April as soil warms and will continue to emerge until temperatures become unfavorable sending the seeds back into dormancy (Jordan et al., 2014).

Coble et al. (1981) reported that 0.11 common ragweed/m² reduced soybean yield by 32.5 kg/ha on average. When grown in 30% shade, common ragweed has been noted to grow just as well as sweet corn (Zea mays L. var. rugosa) and dry bean (Phaseolus vulgaris Herrm) but much poorer than these crops in 73% shade (Dickerson, 1968). Predicted yield loss in peanut is 40% when 1 common ragweed is present/m² for the entire season (Clewis et al., 2001). Everman et al. (2008) reported the critical period of weed control in peanut for mixed broadleaf populations that included common ragweed of 2.6 to 8 WAP.

The 2- to 3-week period after flowering is the critical time for weed management in most of the weed species to prevent weeds from producing viable seeds (Hill et al., 2016). Most weed management studies only evaluate a single growing season failing to quantify the impact these weed management strategies may have on contributions to the soil seedbank (Inman et al., 2016). Information relative the impact of weed management in peanut on succeeding crops is limited. This study was designed to evaluate the effects that timing of weed management have on weed control, peanut yield, estimated economic returns, and weed populations and cotton lint yield in the following growing season.

MATERIALS AND METHODS

The experiments were conducted in North Carolina near Lewiston-Woodville (36.135 N, -77.177 W) and Rocky Mount (35.897 N, -77.675 W) during 2016 and 2017 growing seasons. Adjacent areas of the same fields were used for different years at both locations. Soils included a Norfolk sandy loam (fine-loamy, siliceous, thermic Typic Paleudult) at Lewiston-Woodville and an Aycock very fine sandy loam (fine-silty, siliceous, subactive, thermic Typic Paleudult) at Rocky Mount. Soil pH ranged from 5.7 to 6.1 at the two locations with soil organic matter content of 1.8% and 2.4% at these respective locations. The Lewiston-Woodville and Rocky Mount sites were naturally infested with common ragweed (129 plants/m² in 2016 and 61 plants/m² in 2017) and Palmer amaranth (54 plants/m² in 2016 and 65 plants/m² in 2017), respectively. Peanut cultivar 'Bailey' (Isleib et al., 2011) was planted into conventionally-tilled, raised seedbeds at a seeding rate designed to provide a final in-row population of 15 plants/m. Plot size was 4 rows (spaced 91 cm apart) by 9 m at both locations. Other than herbicide treatments for the experiment, peanut was managed according to North Carolina Cooperative Extension Service recommendations (Jordan et al., 2018).

Treatments were arranged in a randomized complete block design and replicated 4 times at Lewiston-Woodville and 3 times at Rocky Mount. Herbicide treatments consisted of a single application of paraquat plus bentazon at 2 weeks after planting (WAP); a single application of imazapic plus lactofen at 6 WAP; applications of paraquat plus bentazon at 2 WAP followed by bentazon plus lactofen at 4 WAP; bentazon plus lactofen at 4 WAP followed by imazapic plus lactofen at 6 WAP; and paraquat plus bentazon at 2 WAP followed by bentazon plus lactofen at 4 WAP followed by imazapic plus lactofen at 6 WAP. A NTC was included. Clethodim was applied over the entire test area 6 WAP to control annual grasses. At 2 and 4 WAP, peanut was in the vegetative growth stage while at 6 WAP peanut was in the flower (R1) growth stage (Balota, 2019; Boote, 1982). Herbicide rates and manufacturer details are listed in Table 1. Nonionic surfactant (Induce^®, Helena Chemical Co., Collierville, TN) at 0.125% (v/v) was applied with paraquat plus bentazon. Crop oil concentrate (Agri-Dex^®, Helena Chemical Co., Collierville, TN) at 1.0% (v/v) was applied with bentazon plus lactofen, imazapic plus lactofen, and clethodim. Herbicides were applied using a CO₂-pressurized backpack sprayer equipped with flat-fan nozzles (AIXR 11002 TeeJet^® Air Induction XR flat-spray nozzles, TeeJet Technologies, Wheaton, IL) calibrated to deliver 140 L/ha at 152 kPa.

Table 1.

Herbicide active ingredient, trade name, formulation, application rate, and manufacturer.

Prior to the first herbicide application, initial weed densities were recorded from 15 randomly determined sections across the entire test area using a 0.3 by 0.3 m square. After herbicides were applied, weed control was visually estimated 7, 10, and 20 WAP using a 0 to 100% scale where 0 was equal to no control and 100 to complete control. Foliar chlorosis, necrosis, and density were considered when recording the visual estimates. Peanut pods were dug and vines inverted at optimum maturity based on pod mesocarp color (Williams and Drexler, 1981). Within two weeks prior to digging and vine inversion, weeds were mowed above the canopy to improve efficiency of digging and vine inversion.

During the year following peanut, cotton cultivar DP 1522 (Monsanto Co., St. Louis, MO) was planted into the previous year's plots on 4 May and 11 May during 2017 at Lewiston-Woodville and Rocky Mount, respectively. In 2018, the cotton cultivar DP 1646 (Monsanto Co., St. Louis, MO) was planted 10 May and 15 May at Lewiston-Woodville and Rocky Mount, respectively. Cotton was planted in conventionally-tilled, raised seedbeds. Within each plot, density of common ragweed at Lewiston-Woodville and density of Palmer amaranth at Rocky Mount were determined 3, 7, and 20 WAP. Density at 3 and 7 WAP was measured using a 0.3 by 0.3 m square in three randomly determined sections of each plot and then the average of each plot was converted to a m² basis. Due to reduced weed densities at 20 WAP, populations were determined by counting common ragweed or Palmer amaranth present in the center two rows of the entire plot area and then converted to a m² unit. After initial weed densities were recorded 3 and 7 WAP in cotton, glyphosate was applied at Lewiston-Woodville. Glyphosate plus dicamba was applied 3 WAP followed by glyphosate only 7 WAP at Rocky Mount. These herbicides were applied over the entire test area.

Economic returns were estimated using a modified North Carolina State University enterprise budget for conventional Virginia market type peanut to accommodate a 400-ha operation in North Carolina (Washburn, 2019). Costs were calculated for the different treatments with respect to varying costs of herbicides and adjuvants quoted from major suppliers in the region. Costs associated with hauling, drying and cleaning peanut, state check-off fee, and national assessment costs based on yield varied by treatment. A base cost of $1502/ha for peanut production before these respective inputs was used. A ten-year average price of $0.57/kg received for Virginia market type peanuts in the United States was used for estimated economic return analysis (USDA-NASS, 2018).

Data for common ragweed and Palmer amaranth control, peanut yield, estimated economic return, broadleaf densities in cotton the following year, and cotton lint yield were subjected to analysis of variance using PROC GLIMMIX and PROC CORR procedures of SAS (version 9.4; SAS Institute INC., Cary, NC). Data for each location with the same broadleaf weeds were also subjected to ANOVA for a 2 (year) by 6 (timing of herbicide application) factorial arrangement of treatments. Timing of herbicide application treatments and locations were fixed factors whereas replications were treated as random. Means for significant main effects and interactions were separated using Fisher's Protected LSD at 𝜶 = 0.05.

RESULTS AND DISCUSSION

The interaction of year by timing of herbicide application was not significant for common ragweed control 7, 10, and 20 WAP; pod yield; and estimated economic return at Lewiston-Woodville or for Palmer amaranth control 10 and 20 WAP; pod yield; and estimated economic return at Rocky Mount (Tables 2 and 3). The interaction of year by timing of herbicide application was significant for Palmer amaranth control 7 WAP. Therefore, data for common ragweed control 7, 10, and 20 WAP, pod yield, and estimated economic return at Lewiston-Woodville (Table 4) and Palmer amaranth control 10 and 20 WAP, pod yield, and estimated economic return at Rocky Mount (Table 5) are presented pooled over years. Palmer amaranth control 7 WAP was analyzed by year at Rocky Mount.

Table 2.

Analysis of variance (F statistic) for common ragweed control 7, 10, and 20 WAP; pod yield; and estimated economic return as influenced by timing of herbicide applications in peanut at Lewiston-Woodville.

Table 3.

Analysis of variance (F statistic) for Palmer amaranth control 7, 10, and 20 WAP; pod yield; and estimated economic return as influenced by timing of herbicide applications in peanut at Rocky Mount.

Table 4.

Common ragweed control 7, 10, and 20 WAP; pod yield; and estimated economic return as influenced by timing of herbicide applications in peanut at Lewiston-Woodville.^a

Table 5.

Palmer amaranth control 7, 10, and 20 weeks after planting (WAP); pod yield; and estimated economic return as influenced by timing of herbicide applications in peanut at Rocky Mount^a.

Common ragweed control at Lewiston-Woodville and peanut pod yield as well as common ragweed control and estimated economic returns were positively correlated (p ≤ 0.0001; R = 0.60 to 0.68) regardless of when the visual estimates of weed control were recorded (Table 6). As was noted for the experiments at Lewiston-Woodville for common ragweed, peanut yield and estimated economic returns were positively correlated with Palmer amaranth control 7, 10, and 20 WAP at Rocky Mount (p ≤ 0.0001, R = 0.60 to 0.68) (Table 7).

Table 6.

Pearson correlation coefficients for common ragweed control, peanut pod yield, estimated economic returns, common ragweed densities 3 weeks after planting (WAP) in cotton, and cotton lint yield at Lewiston-Woodville.

Table 7.

Pearson correlation coefficients for Palmer amaranth control, peanut pod yield, estimated economic returns, Palmer amaranth densities 3 weeks after planting (WAP) in cotton, and cotton lint yield at Rocky Mount.

Weed control

At Lewiston-Woodville, common ragweed control 7 WAP was at least 97% when 2 or more herbicide applications were administered or at least one application was made at 6 WAP (Table 4). Common ragweed control 10 WAP was 98% or greater when 2 or more herbicide applications were made. A single herbicide application 2 WAP was approximately 52% less effective as a single application 6 WAP. Control of common ragweed was optimum at 20 WAP when at least 2 herbicide applications were made. Common ragweed 20 WAP was controlled less effectively when a single herbicide application was made 2 WAP compared to a single application made 6 WAP.

Palmer amaranth control 7 WAP was at least 82% when herbicides were applied at least twice, regardless the timing in 2016 and 2017 (Table 5). In 2016, Palmer amaranth control from a single herbicide application 6 WAP was greater than control from a single application 2 WAP, while in 2017 the opposite occurred. In 2016, Palmer amaranth control was greater when any herbicide application was administered compared to the NTC. In 2017, Palmer amaranth control was similar following a single herbicide application 6 WAP or when no herbicides were applied. At 10 WAP, Palmer amaranth control was at least 89% when herbicide applications were administered at least twice regardless of timing. Palmer amaranth control at 10 WAP was at least 40% greater when a single application was administered compared to the NTC.

Peanut yield

At Lewiston-Woodville, peanut yield was at least 3,280 kg/ha when herbicide applications were made three times at 2, 4, and 6 WAP or when applications were made twice at 4 and 6 WAP (Table 4). Peanut yield following a single application 6 WAP or two applications at 2 and 4 or 4 and 6 WAP was similar. Yields were greater any time herbicides were applied compared to the NTC.

At Rocky Mount, a yield of 5,030 kg/ha was achieved when herbicides were applied three times at 2, 4, and 6 WAP (Table 5). Yield was greater when at least two applications were administered compared to when a single application was made 2 WAP only. Yields were similar when a single herbicide application was made regardless of the timing. Yields were also similar when two herbicide applications were made irrespective of the timing.

In these experiments the NTC reflected interference from broadleaf weeds only because clethodim was applied across the entire experiment, but there was approximately 6 weeks of interference in some plots. For mixed weed populations in peanut, Everman et al. (2008) found that the critical timing of weed removal was 3.1 WAP in order to produce at least 95% of the maximum yield. They also reported the critical-weed free period in peanut under interference with combined grass and broadleaf weeds, mixed grass species, and mixed broadleaf species to be 3.1 to 7.5 WAP, 4.3 to 9 WAP, and 2.6 to 8 WAP, respectively. Hill and Santleman (1969) found that when large crabgrass [Digitaria sanguinalis (L.) Scop.] was removed 3 WAP and control persisted for at least 6 WAP, peanut yield was not reduced compared to weed-free peanut. Based on percent of maximum yield noted for the herbicide programs in these experiments, the percent yield loss from season-long interference by common ragweed and Palmer amaranth was 77% and 78%, respectively. These yield loss assessments compare relatively well with yield loss predictions associated with the decision tool WebHADSS (WebHADSS, 2019). At similar average densities for each site year, yield loss based on WebHADSS was 80% and 79% for common ragweed at Lewiston-Woodville in 2016 and 2017, respectively. Yield loss based on WebHADSS was 79% in both 2016 and 2017 from Palmer amaranth interference at Rocky Mount.

Estimated economic return

At Lewiston-Woodville, estimated economic returns were similar when a single herbicide application was made irrespective of the timing or when two applications were made at 2 and 4 WAP (Table 4). A single herbicide application 6 WAP produced similar estimated economic returns as herbicides applied at least twice, either at 2 and 4 WAP or 4 and 6 WAP. Also, no difference was noted when herbicide was applied 6 WAP compared with three applications. Estimated economic returns were greater when two applications were made 4 and 6 WAP or when three applications were made 2, 4, and 6 WAP compared to a single application 2 WAP. Greater estimated economic returns were observed when any herbicide application sequence was made compared to non-treated peanut.

At Rocky Mount, estimated economic return was $349/ha when herbicides were applied 2, 4, and 6 WAP (Table 5). Estimated economic returns were similar when two herbicide applications were made irrespective of the timing. Two herbicide applications made at 4 and 6 WAP produced similar estimated economic returns as a single herbicide application made 2 WAP. Estimated economic returns were greater when at least two herbicide applications were made irrespective of the timing compared to when a single herbicide application was made 6 WAP.

Weed density the year following peanut

The interaction of year by timing of herbicide application was not significant for common ragweed densities at 3 and 20 WAP at Lewiston-Woodville (Table 8) or for Palmer amaranth densities at 3, 7, and 20 WAP at Rocky Mount (Table 9). The main effect of herbicide application timing was not significant for common ragweed densities at 20 WAP at Lewiston-Woodville or for Palmer amaranth densities at 3, 7, and 20 WAP at Rocky Mount. Therefore, common ragweed densities at 3 and 20 WAP are presented pooled over years while common ragweed densities at 7 WAP are presented by year (Table 10). Palmer amaranth densities at 3, 7, and 20 WAP are presented pooled over years (Table 11).

Table 8.

Analysis of variance (F statistic) for common ragweed density 3, 7, and 20 weeks after planting (WAP); and cotton lint yield as influenced by timing of herbicide applications in peanut at Lewiston-Woodville.

Table 9.

Analysis of variance (F statistic) for Palmer amaranth density 3, 7, and 20 weeks after planting (WAP); and cotton lint yield as influenced by timing of herbicide applications in peanut at Rocky Mount.

Table 10.

Common ragweed density 3, 7, and 20 weeks after planting (WAP) in cotton as influenced by timing of herbicide applications in peanut at Lewiston-Woodville^a.

Table 11.

Palmer amaranth density 3, 7, and 20 weeks after planting (WAP) as influenced by timing of herbicide applications in peanut at Rocky Mount^a.

Common ragweed control in peanut and subsequent weed density in cotton the following year were negatively correlated (p ≤ 0.05, R = -0.40 to -0.47) regardless of when the visual estimates of percent control were recorded in peanut (Table 6). Peanut yield was a predictor of common ragweed density 3 WAP in cotton the following year (p ≤ 0.0001, R = -0.73). This is not surprising because weed control is a relatively good predictor of peanut yield, hence lowering the contributions of seed to the soil seedbank. Palmer amaranth control, regardless of timing, was not a good predictor of Palmer amaranth densities in cotton the following year (Table 7). However, peanut yield was a marginal predictor of Palmer amaranth density 3 WAP in cotton the following year.

At Lewiston-Woodville in cotton 3 WAP, common ragweed densities ranged from 95 to 189 plants/m² (Table 10). Common ragweed densities were at least 47% lower when herbicides were applied 2, 4, and 6 WAP compared to when herbicides were not applied or when a single herbicide application was made 2 WAP only. Common ragweed densities were lower when herbicides were applied at least twice the previous year or when a single application was made 6 WAP compared to when no herbicides were applied or when a single application was made 2 WAP. Densities recorded 3 WAP most likely are the best indicator of how weed management in peanut the previous season affected weed populations the following season in cotton. However, comparing weed emergence during the following year may not reflect fully the impact of weed management on the soil seedbank. In addition to seed fecundity, dormancy could impact the amount of weed escapes over a longer period of time than a single season. However, this measurement was recorded prior to any weed control actions were administered in cotton during the subsequent season.

At 7 WAP, common ragweed densities ranged from 1 to 18 plants/m² in 2017 and 0 to 5 plants/m² in 2018 (Table 10). In 2017, Common ragweed densities following a single herbicide application 6 WAP, 2 applications 4 and 6 WAP, or 3 herbicide applications the previous year were lower than densities following no herbicides or a single application 2 WAP. In 2018, common ragweed densities were similar following no herbicide applications, a single herbicide application 2 or 6 WAP, applications 2 and 4 WAP, or applications 2, 4, and 6 WAP.

By 20 WAP, common ragweed populations were less than 1 plant/m² and timing of weed management in peanut the previous season had no effect on densities (Table 10). In some cases, secondary dormancy of seeds can occur when the non-dormant seeds are exposed to favorable temperature and moisture, but an overriding factor inhibits the seed from germinating (Willemsen, 1975). Baskin and Baskin (1980) reported common ragweed seed viability of 39 years once in the seedbank.

At Rocky Mount the year after peanut, Palmer amaranth densities ranged from 66 to 107 plants/m², 6 to 13 plants/m², and less than 1 plant/m² 3, 7, and 20 WAP, respectively (Table 11). Weed management in peanut the previous season did not have an effect on Palmer amaranth densities in cotton 3, 7, or 20 WAP.

Menges (1987) reported that it may take up to 6 years of weed-free Palmer amaranth management to reduce the seedbank by 98%. Sosnoskie et al. (2013) reported that 36 months were required for Palmer amaranth seed burial to deplete the seedbank. Therefore, a two-year study may not have been long enough to observe the complete effect of weed management due to the abundance of seeds present in the seedbank.

Cotton lint yield the year following peanut

Yield of peanut negatively affected cotton yield the following year at Lewiston-Woodville (p = 0.0005, R = -0.49) (Table 6). Yield of peanut at Rocky Mount the previous year was not a good predictor of cotton yield the following year (Table 7).

At Lewiston-Woodville, cotton lint yield ranged from 1540 to 1690 kg/ha (Table 10), while at Rocky Mount cotton lint yield ranged from 920 to 1090 kg/ha (Table 11). Timing of weed management in peanut the previous year did not influence cotton lint yield at either location. Applying effective herbicides to cotton 3 WAP may have masked the potential impact of weed management the previous year on cotton yield. Weeds in cotton were controlled within the critical weed-free period for cotton in presence of Palmer amaranth and common ragweed, the predominant broadleaf weeds in this experiment. Glyphosate was applied at Lewiston-Woodville for each timing of application in cotton. Glyphosate-resistant biotypes of common ragweed were not present at Lewiston-Woodville. The combination of glyphosate plus dicamba was used at Rocky Mount because these populations of Palmer amaranth were resistant to glyphosate (Inman et al., 2016).

CONCLUSIONS

The presented research reinforces the importance of timely and effective weed management in peanut. Effective control of common ragweed and Palmer amaranth can be achieved with proper timing of herbicide applications. Usually, the more intensive herbicide programs generated the greatest weed control, yield, and estimated economic returns. In the case of common ragweed, an intensive weed management approach in peanut the previous season showed a reduction in weed emergence the following year in cotton. Due to the abundance of Palmer amaranth seed in the seedbank, multiple years of implementation are likely needed to observe the full benefits of an intensive weed management program.

This research was funded by the North Carolina Peanut Growers Association, Inc. The authors would like to thank the staff members of the Peanut Belt Research Station and the Upper Coastal Plains Research Station for technical assistance.

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