Impact of Crop-Topping with a Mower on Common Lambsquarters and Potato

Common lambsquarters and potato visual injury 3 wk after application were similar in both years and were therefore combined for presentation. Mowing at early and late treatment timings had no visual impact on potato injury and minimal impact on common lambsquarters in both years (Table 2). A small increase in common lambsquarters visual injury was observed at late treatment timing compared with early timing (11% vs. 3%; Table 2). Overall, mowing resulted in minimal injury to potato and common lambsquarters.

Table 2. Effects of crop-topping with a mower or wick-applied glyphosate in two separate experiments on visual ratings of potato and common lambsquarters (CHEAL) injury at early and late treatment timings in 2020 and 2021 at Harrington PE. ^a

^a Values are means ± SE. Visual injury ratings were estimated on a 0 to 100 scale, where 0 is no plant injury and 100 is complete plant death.

There was an effect of year, mow treatment timing, and the interaction of year, cultivar, and mow treatment timing on common lambsquarters total biomass (P < 0.01), seed biomass (P < 0.01), and harvest index (P = 0.03), and data are presented separately by year (Table 3). Mowing did not affect common lambsquarters total biomass in any potato cultivar in 2020 (Figure 1 A, C, and E). Late mowing did, however, reduced common lambsquarters harvest index by 60% to 70% relative to the control in 2020 (Figure 1 A, C, and E). Results in 2021 were more variable. Early mowing decreased common lambsquarters total biomass in the upright potato cultivar (Figure 1 F) but increased total biomass in the spreading cultivar (Figure 1 D). Similarly, late mowing increased common lambsquarters total biomass in the semi-erect (Figure 1 B) and upright (Figure 1 F) potato cultivars. Despite changes to total biomass accumulation in 2021, mowing timing had minimal impact on common lambsquarters harvest index, which ranged from 12% to 25% and from 20% to 22% in early and late mow treatments, respectively (Figure 1 B, D, and F).

Figure 1. The effect of early and late crop-topping with a mower on common lambsquarters vegetative (light gray bars) and reproductive (dark gray bars) biomass allocation and harvest index (circles) in 2020 (A, C, E) and 2021 (B, D, F) in Harrington PE in semi-erect (A, B), spreading (C, D), and upright (E, F) potato cultivars. Data are least square means ± SEM of total biomass. Values not connected by the same letter are significantly different according to Tukey’s honestly significant difference (α = 0.05). Uppercase letters refer to the effect of treatment timing on total biomass (vegetative and reproductive) accumulation, and lowercase letters refer to the effect of treatment timing on harvest index.

Table 3. ANOVA table of common lambsquarters total biomass, seed biomass, harvest index, germination percentage, and thousand seed weight by year, cultivar, treatment timing, and their interaction following crop-topping with a mower or wick-applied glyphosate. ^{a, b}

^a Values are P-values. Boldface indicates that the effect was considered significant (P ≤ 0.05).

^b Abbreviations: Clt, cultivar; df/ddf, numerator degrees of freedom/denominator degrees of freedom; Time, treatment timing; TSW, thousand seed weight; Yr, year.

Like the effects on common lambsquarters biomass accumulation and allocation, germination percentage was affected by mow timing (P < 0.01), year (P < 0.01), and the interaction of year, cultivar, and treatment timing (P = 0.03) (Table 3), yet minimal differences were found between treatment timings between cultivars (Supplementary Figure 2). Despite the minimal effect on germination percentage, viable seed production was impacted by cultivar (P < 0.01), mowing timing (P < 0.01), year (P < 0.01), and the interaction of year and cultivar (P < 0.01) and of year and timing (P < 0.01) (Table 3) with consistent effects from treatments within years (Figure 2). Late mowing reduced common lambsquarters viable seed production by 72% to 91% in all cultivars in 2020 (Figure 2 A, C, and E). In contrast, regardless of timing, mowing increased viable seed production by 78% to 278% in 2021 (Figure 2 B, D, and F).

Figure 2. Effect of early and late crop-topping with a mower on common lambsquarters viable seeds plant⁻¹ (bars) and thousand seed weight (TSW) (circles) in 2020 (A, C, E) and 2021 (B, D, F) in Harrington PE in semi-erect (A, B), spreading (C, D), and upright (E, F) potato cultivars. Data are least square means ± SEM. Values not connected by the same letter are significantly different according to Tukey’s honestly significant difference (α = 0.05). Uppercase letters refer to the effects of treatment timing on viable seed plant⁻¹, and lowercase letters refer to the effects of treatment timing on TSW.

Common lambsquarters TSW was impacted by mowing timing (P < 0.01), year (P < 0.01), and their interaction (P < 0.01) (Table 3). No differences in TSW across mowing timings or potato cultivars were found in 2020 (Figure 2 A, C, and E). In contrast, common lambsquarters TSW was increased by early mowing in the semi-erect (Figure 2 B) and upright (Figure 2 F) potato cultivars compared with the control in 2021. The common lambsquarters germination percentage was positively correlated with TSW and increased as TSW increased (Figure 3).

Figure 3. Relationship between thousand seed weight (TSW) and germination percentage of common lambsquarters seed following crop-topping with a mower or wick-applied glyphosate.

Like our study, other authors have found inconsistent results when mowing common lambsquarters. For example, in controlled conditions, Butler et al. (Reference Butler, Brouder, Johnson and Gibson2013) observed that common lambsquarters cut once near anthesis recovered 97% of its inflorescence dry weight and that repeated cuts throughout the season were required to reduce seed production. In contrast, the greater reduction of common lambsquarters seed production by delaying mowing to after flowering (BBCH 69) in our study is consistent with the findings of Hill et al. (Reference Hill, Renner, VanGessel, Bellinder and Scott2016), who found that mowing common lambsquarters at flowering (BBCH 60) or when immature seeds were present (BBCH 81) decreased seed shed by up to 99% in soybean. Weed seed size and weight can be highly plastic and affected by the growing environment of the parent plant (Winn Reference Winn1985). We observed a contrasting trade-off across years in response to mowing. In 2020, overall investment in common lambsquarters seed production dropped and viable seed decreased when mowed late. In contrast, in 2021, mowing increased investment in individual seed, and viable seed production increased. Several studies have documented rapid declines in seed number per plant and maintenance of seed weight in common lambsquarters in response to crowding stress (Colquhoun et al. Reference Colquhoun, Boerboom, Binning, Stoltenberg and Norman2001) and decreased light intensity and nitrogen availability (Mahoney and Swanton Reference Mahoney and Swanton2008). Stress periods during later development are known to contribute to declines in seed number while maintaining seed weight and viability in highly plastic species like soybean (McKenzie-Gopsill et al. Reference McKenzie-Gopsill, Lukens, Lee and Swanton2016). Low overall resource availability in 2020 due to an extended period of drought (AAFC 2023) may have contributed to declines in viable seed production of common lambsquarters when mowed late. The contrasting response of increased viable seed production by mowing in 2021 may be due to greater resource availability in that year (AAFC 2023), the R-selected life history strategy of common lambsquarters, and the breaking of apical dominance allowing for an increased number of inflorescences and increased seed production. Together, these results suggest that crop-topping with a mower postflowering can contribute to declines in viable seed production when resources are not sufficient to allow common lambsquarters to recover, thus likely limiting the efficacy of this technique to years with minimal precipitation only.

Potato marketable yield (P < 0.01), cull yield (P < 0.01), and percentage of yield lost to culls (P < 0.01) were affected by the interaction of cultivar and year, yet were not impacted by mowing timing (Table 4). In 2020, marketable yield was 23.5, 20.5, and 14.4 T ha⁻¹ in the semi-erect, spreading, and upright cultivar, respectively (Figure 4 A). Furthermore, in 2020, cull yield varied from a low of 0.5 T ha⁻¹ in the upright cultivar to 1.3 T ha⁻¹ in the semi-erect cultivar (Figure 4 A). The percentage of total yield lost to culls ranged from 3% to 6% in 2020 and was highest in the spreading (6%) and semi-erect (5%) potato cultivars (Figure 4 A). In contrast, marketable yield was higher in 2021 than in 2020 (Figure 4) and was 36.1, 32.0, and 31.3 T ha⁻¹ in the semi-erect, spreading, and upright cultivar, respectively. The increase in marketable yield in 2021 was accompanied by increased cull yield and the percentage of yield lost to culls, which ranged from 4% to 9% (Figure 4 B). Similar to 2020, the percentage lost to culls was highest in the spreading cultivar (9%) compared with the semi-erect and upright cultivars (Figure 4 B).

Table 4. ANOVA table of potato marketable yield, cull yield, and percentage of yield lost to culls by year, cultivar, treatment timing, and their interaction following crop-topping with a mower or wick-applied glyphosate. ^{a, b}

^a Values are P-values. Boldface indicates that the effect was considered significant (P ≤ 0.05).

^b Abbreviations: Clt, cultivar; df/ddf, numerator degrees of freedom/denominator degrees of freedom; Time, treatment timing; Yr, year.

Figure 4. Effect of crop-topping with a mower on yield of marketable tubers (black bars) and culled tubers (gray bars) in 2020 (A) and 2021 (B) of semi-erect, spreading, and upright potato cultivars. Data are least square means ± SEM. Values above bars indicate percentage of yield lost to culls, and letters refer to the effects of treatment timing on percentage of yield lost to culls. Values not connected by the same letter are significantly different according to Tukey’s honestly significant difference (α = 0.05).

The minimal impact of mowing on potato marketable yield in our study contrasts with several studies that have documented improvements to yields in tuber crops following crop-topping. Meyers et al. (Reference Meyers, Jennings, Schultheis and Monks2016) found that crop-topping with a mower in sweet potato at or beyond mid-fruit ripening (BBCH 85/47) decreased cull yield while improving marketable yield. This treatment timing coincided with sweet potato tuber bulking, whereby water, nutrients, and carbohydrates shifted from aboveground vegetation production to belowground reproductive development. By removing the fruits of a tuber-producing crop, resources required for sexual reproduction can be reallocated to the reproductive structures, thus improving tuber quality and marketable yields. Such a response has been observed in several potato cultivars (Jansky and Thompson Reference Jansky and Thompson1990) and wild tuber–forming plants (Van Drunen and Dorken Reference Van Drunen and Dorken2012). Among potato cultivars in this study, however, the trade-off between sexual and asexual reproduction appears to be dependent on environment. Minimal injury to potato reported from early and late mowing, combined with strong competition from common lambsquarters, may have contributed to minimal shifts in reproductive allocation and the incongruences with previous studies. Together our results suggest that potato marketable yields are dependent on cultivar selection and environment and that improved understanding of shifts in biomass allocation in response to crop-topping with a mower in potato is required to elucidate the potential positive effects of mowing on yield.

Impact of Wick Weeding Timing on Common Lambsquarters and Potato

Potato visual injury and common lambsquarters visual injury 3 wk after application were similar across both years and averaged for presentation (Table 2). Potato injury increased from 33% to 49% from early and late wick-applied glyphosate, respectively (Table 2). Potato visual injury symptoms included leaf chlorosis and formation of necrotic tissue along the leaf margins and at the growing point (Supplementary Figure 1 B). In contrast, common lambsquarters visual injury did not differ between early and late treatment timings and was 32% overall (Table 2). Visual injury symptoms included wilting of the main stem and severe necrosis of leaves and reproductive tissue (Supplementary Figure 1 C and D).

Common lambsquarters total biomass (P < 0.01), seed biomass (P < 0.01), and harvest index (P < 0.01) were affected by the interaction of wick weeding timing, cultivar, and year (Table 3). Seed biomass was increased in late treatments compared with early in 2020. However, the late treatment did not differ from the control (Figure 5 A, C, and E). The increase in seed biomass in 2020 resulted in an increase in the harvest index in control–late and late treatments compared with early treatments and ranged from 31% to 40% (Figure 5 A, C, and E). In contrast, early treatment in 2021 increased common lambsquarters total biomass compared with the control in the spreading and upright potato cultivars (Figure 5 D and F) yet did not differ in the semi-erect potato cultivar (Figure 5 B). Similarly, late treatment increased common lambsquarters total biomass over the control in the upright potato cultivar (Figure 5 F). These changes to biomass accumulation in 2021, however, had minimal impact on common lambsquarters harvest index. In general, harvest index increased in the late treatments compared with the early treatments. However, no differences were found between late wick treatment timing and controls (Figure 5 B, D, and F). Whereas common lambsquarters total biomass increased from early wick treatment in the upright cultivar, the harvest index drastically declined from 31% in the control to 1% following the early wick-applied glyphosate treatment (Figure 5 F).

Figure 5. Effect of early and late crop-topping with wick-applied glyphosate on common lambsquarters vegetative (light gray bars) and reproductive (dark gray bars) biomass allocation and harvest index (circles) in 2020 (A, C, E) and 2021 (B, D, F) in Harrington PE in semi-erect (A, B), spreading (C, D), and upright (E, F) potato cultivars. Data are least square means ± SEM of harvest index. Values not connected by the same letter are significantly different according to Tukey’s honestly significant difference (α = 0.05). Uppercase letters refer to the effect of treatment timing on total biomass accumulation, and lowercase letters refer to the effect of treatment timing on harvest index.

Common lambsquarters plants were significantly larger with greater biomass in 2021 compared with 2020, which may have contributed to differences in response to wick-applied glyphosate. Schuster et al. (Reference Schuster, Shoup and Al-Khatib2007) noted that glyphosate absorption was similar between common lambsquarters plants with heights of 2.5, 7.5, and 15 cm, yet noted variation in tolerance across populations regardless of size. Larger plants, however, translocated glyphosate more rapidly through basipetal movement, which can be attributed to their larger carbohydrate sinks (Schuster et al. Reference Schuster, Shoup and Al-Khatib2007). We observed that early treatments in 2021 increased biomass production of common lambsquarters. Hormetic effects, including stimulation of biomass productivity by low doses of glyphosate, have been reported in several crop and weed species (Brito et al. Reference Brito, Tropaldi, Carbonari and Velini2018), including common lambsquarters (Nadeem et al. Reference Nadeem, Abbas, Tanveer, Maqbool, Zohaib and Shehzad2016). Nadeem et al. observed a stimulatory effect of glyphosate on common lambsquarters seedling shoot length and seedling biomass up to a concentration of 16 g ae ha⁻¹. Despite the high concentration of glyphosate used in the present study (180 g ae L⁻¹), visual injury on common lambsquarters was generally confined to the upper portion of the plant. Therefore the high tolerance of common lambsquarters, the sublethal dose, and possible hormetic effects of glyphosate may provide an explanation for improved biomass productivity by early wick treatments.

Common lambsquarters germination percentage (P < 0.01) and TSW (P = 0.04) were impacted by the interaction of wick application timing, potato cultivar, and year (Table 3). The overall common lambsquarters germination percentage was highly variable and ranged from 10.5% to 50.4% across treatments and years (Supplementary Figure 3). In general, the common lambsquarters germination percentage declined (19% to 55%) with early and late treatment in the semi-erect and upright potato cultivars in 2020; however, differences were not significant between treatments and controls (Supplementary Figure 3 A and E). In 2021, the common lambsquarters germination percentage was decreased by 82% and 75% in the spreading (Supplementary Figure 3 D) and upright cultivar (Supplementary Figure 3 F), respectively, following wick-applied glyphosate.

Common lambsquarters viable seed production was impacted by wick application timing (P < 0.01) and the interaction of timing and year (P = 0.03) (Table 3). In 2020, late treatment with wick-applied glyphosate decreased viable seed production in the semi-erect cultivar (36%), yet increased viable seed production (85%) in the upright cultivar (Figure 6 A and F). In contrast, viable seed production declined (95%) following early wick-applied glyphosate in the spreading and upright cultivars in 2021 (Figure 6 D and F). Common lambsquarters TSW did not differ by wick application timing across cultivars in 2020 (Figure 6 A, C, and E). Similarly, common lambsquarters TSW did not differ by wick weeding timing in 2021, except for late wick weeding in the semi-erect cultivar, which decreased compared with the control (Figure 6 B, D, and F).

Figure 6. Effect of early and late crop-topping with wick-applied glyphosate on common lambsquarters viable seeds plant⁻¹ (bars) and thousand seed weight (TSW) (circles) in 2020 (A, C, E) and 2021 (B, D, F) in Harrington PE in semi-erect (A, B), spreading (C, D), and upright (E, F) potato cultivars. Data are least square means ± SEM. Values not connected by the same letter are significantly different according to Tukey’s honestly significant difference (α = 0.05). Uppercase letters refer to the effects of treatment timing on viable seeds plant⁻¹, and lowercase letters refer to the effects of treatment timing on TSW.

Wick-applied glyphosate at the early timing reduced common lambsquarters seed production in 2021 only. The impact of glyphosate on reducing viable weed seed production has been reported in a variety of species, including pitted morning glory (Ipomoea lacunosa L.), velvetleaf (Abutilon theophrasti Medik.), and giant foxtail (Setaria faberi Herrm.) (Biniak and Aldrich Reference Biniak and Aldrich1986; Bennett and Shaw Reference Bennett and Shaw2000; Culpepper and York Reference Culpepper and York2000). Walker and Oliver (Reference Walker and Oliver2008) observed that a single glyphosate application at flowering reduced seed production of numerous weed species by greater than 83%. Glyphosate accumulates in the plant reproductive sinks, including floral buds, where it disrupts embryo and endosperm development (Pline et al. Reference Pline, Price, Wilcut, Edmisten and Wells2001). Reduced seed production from early wick application made as flowering was initiated may be due to increased glyphosate absorption to sink tissues and disruption of subsequent flower and seed production. It is unclear why this effect was so dramatic in 2021 compared with 2020, but it may have been due to low biomass accumulation overall in 2020 and an extended drought period limiting glyphosate translocation (AAFC 2023). Therefore, although wick-applied glyphosate may provide a hormetic effect on common lambsquarters biomass production, doses can be sufficient to disrupt seed development and seed viability, and further studies with varying glyphosate doses are warranted. If integrated into weed management practices, wick-applied glyphosate could reduce common lambsquarters seed production and aid in significantly reducing seedbank stores.

The different response of common lambsquarters to wick-applied glyphosate treatments between potato cultivars may be related to morphological features of each cultivar. Many potato cultivars do not vary in their ability to suppress weeds (Colquhoun et al. Reference Colquhoun, Konieczka and Rittmeyer2009; Conley et al. Reference Conley, Binning and Connell2001). On the other hand, early-emerging, short stature, and highly branched cultivars have been shown to provide superior weed suppression over erect, less leafy cultivars (Barbaś et al. Reference Barbaś, Sawicka, Marczak and Pszczółkowski2020). Although our results do not show a consistent response, environmental differences between years may influence potato emergence and early canopy development across cultivars. These differences may in turn have affected wick treatment interception and subsequently common lambsquarters growth and seed production.

Potato marketable yield (P < 0.01) was impacted by the interaction of year and wick timing, yet cull yield (P < 0.01) and percentage of yield lost to culls (P < 0.01) differed by year and wick timing, but not their interaction (Table 4). Overall, potato marketable yield was 71% greater (P < 0.01) in 2021 than in 2020 and averaged 33.1 and 19.4 T ha⁻¹, respectively (Figure 7). Cull yield was 70% greater (P < 0.01) in 2021 than in 2020 and averaged 2.91 and 9.53 T ha⁻¹, respectively (Figure 7). Regardless of wick application timing or year, wick-applied glyphosate consistently increased the percentage of yield lost to culls by 14% to 15% (Figure 7). These results suggest that wick weeding negatively impacts potato yield and quality. The timing of wick application did not influence this effect. Yield losses following wick treatments were likely due to interception with the wick bar and subsequent phytotoxic effects of glyphosate across potato cultivars (Bailey et al. Reference Bailey, Wilson and Hines2002; Friesen and Wall Reference Friesen and Wall1984; Hutchison et al. Reference Hutchison, Eberlein and Tonks2017). Felix et al. (Reference Felix, Boydston and Burke2011) found that potato yields were most sensitive to glyphosate injury when applied at the hooking (BBCH 51/40) and tuber initiation (BBCH 57/41) stages, potentially due to disruptions in sucrose shunting. Similarly, Meyers et al. (Reference Meyers, Jennings, Schultheis and Monks2016) found that sweet potato tolerance to glyphosate drip from a wick bar increased as the plants matured. This highlights the importance of potato cultivar growth pattern as an important consideration for crop-topping with a wick bar and of possible evaluation of a range of wiper designs and changes to the physical properties of the herbicide solution to reduce risk of glyphosate drip onto potato plants (Harrington and Ghanizadeh Reference Harrington and Ghanizadeh2017).

Figure 7. Impact of early and late crop-topping with wick-applied glyphosate on yield of potato marketable tubers (black bars) and culled tubers (gray bars) in 2020 (A) and 2021 (B). Data are least square means ± SEM. Values above bars indicate percentage of yield lost to culls, and letters refer to the effects of treatment timing on percentage of yield lost to culls. Values not connected by the same letter are significantly different according to Tukey’s honestly significant difference (α = 0.05).

Our results demonstrate that crop-topping in potato production systems can be a viable method of late-season common lambsquarters control and can be incorporated into an integrated weed management program to target the weed seedbank. Further research into treatment applications made at additional potato and common lambsquarters phenological stages and using a range of glyphosate rates should be considered. Elucidation of the critical period of weed seed control (Geddes and Davis Reference Geddes and Davis2021) in common lambsquarters and timing crop-topping applications to this period may provide greater reductions in viable seed return. Furthermore, combining wick or mow treatments together with potato fungicidal spray application schedules could provide a low-cost integration of herbicide-resistant weed management strategies to potato production systems. Additional research into sequential and bidirectional wick and mow treatments and other crop-topping methods, such as electricity, may provide further avenues to improve consistency in reducing common lambsquarters seed production without compromising yield.