Weed Control, Sustainability, Hazards and Risks in Sweetpotato Cropping Systems

Weed Control, Sustainability, Hazards and Risks in Sweetpotato Cropping Systems

Abstract

Sweetpotato [Ipomoea batatas (L.) Lam.] is one of the most important food crops worldwide and is used for animal feed, human consumption, and various processed products. Weed interference with the crop can cause significant yield reductions and inferior product quality. This necessitates the control of the weeds at critical stages throughout the growing season e.g. before sweetpotato transplanting and 2 to 6 weeks after transplanting. Amaranthaceae and Cyperaceae, amongst other weed families, are the most troublesome in sweetpotato and left uncontrolled can result in marketable yield reductions between 18 to 80%. The use of non-selective herbicides prior to transplanting, preemergence or preplant herbicides in combination with plowing, cultivation, and mechanical weed control have been effective weed control methods.  More efforts must focus on prevention management strategies prior to sweetpotato transplanting, early detection and containment, and the integration of various control methods such as cultivar selection and crop rotation to enhance sweetpotato’s competitive ability.

Introduction

Sweetpotato [Ipomoea batatas (L.) Lam.] ranks sixth behind rice, wheat, potato, maize and cassava as most important food crop globally (CIP International Potato Center 2017) and its production is growing in many regions of the world including the U.S. and parts of Africa.  Storage roots of sweetpotato are an important source of beta-carotene, vitamins B, C and E, and contain moderate amounts of iron and zinc.  Sweetpotato shoot tips and leaves are consumed throughout the world both raw and cooked (Bouwkamp 1985).  In addition to human consumption, sweetpotato is utilized for animal feed and processed products such as starch, flour, syrup and dye used to add pigment to food and fiber.  China is the world’s largest producer and consumer of sweetpotato, where it is used for animal feed, human food, and processing products including ethanol (USDA-FAS 2017).  In the U.S., sweetpotato is grown almost solely as human food, and storage roots are marketed nationally and internationally.  However, sweetpotato is increasingly used as an ingredient in high-end domesticated dog feed and treats.  Sweetpotato in the U.S. is grown in rotation with other agronomic crops (cotton, corn, soybean, peanut, tobacco) and vegetable crops.  Sweetpotato farmers depend on this crop for sustainability of their farming operation.

Sweetpotato production begins with shoot tip cuttings (slips) 25.4 to 30.5 cm tall, which are cut above the soil surface to prevent disease transfer to the production field.  Slips, often containing no roots, are transplanted into fields previously plowed to form 30.5 to 40.6-cm tall ridged rows that are 0.9 to 1.1 m wide (Photograph 1).  In-row plant spacing is commonly 20 to 38 cm (Schultheis et al. 1999; Stoddard et al. 2013).  Sweetpotato yield per hectare is determined by the number of sweetpotato plants per hectare, the number of storage roots per plant and the size of each storage root at harvest (Meyers et al. 2014).   These yield parameters are directly related to sweetpotato storage root initiation (14 to 30 days after transplanting), and the sizing up stage of storage roots that occurs during the last third of the growing season.  Unfavorable environmental conditions (dry soil, wet soil, high temperature, low temperature, weed competition) during storage root initiation and the sizing up stage of storage roots directly impact the number of storage roots produced per plant, the size of each storage root at harvest, and the resulting yield and quality (Gajanayake et al. 2013, 2014, 2015; Meyers et al. 2014; Pardales and Yamauchi 2003; Villordon et al. 2012).  Likewise, unfavorable environmental conditions result from weeds being present during these two critical stages of sweetpotato growth (Harrison and Jackson 2011; Jose et al. 1994; Meyers et al. 2010; Meyers and Shankle 2015a; Nedunzhiyan et al. 1998; Seem et al. 2003; Workayehu et al. 2011).

Weed interference in sweetpotato can reduce yield and quality, usually exhibited as a reduction in number one (premium) grade yield (Meyers et al. 2010; Meyers and Shankle 2015; Seem et al. 2003; Smith et al. 2017).  To prevent yield and quality reductions, weeds must be controlled at critical times during the sweetpotato growing season.  Fields are plowed to form ridged planting rows prior to sweetpotato transplanting.  The time period between plowing and transplanting can vary depending on weather or the specific grower.  As time between plowing and transplanting increases, weed emergence increases.  If weeds are near emergence or emerged, they have a competitive advantage over the crop.  It is critical to transplant sweetpotato slips in fields that are weed-free.  If weeds are either near emergence or have emerged, fields should be re-plowed to make sure all emerged weeds are controlled.  Transplants should be planted at least 12.7 cm deep and under favorable conditions to aid in their establishment, competitiveness with weeds and cultivation (Meyers et al. 2014).  To prevent sweetpotato storage root yield and quality reductions, the critical weed-free period for a mixed weed population is 2 to 6 weeks after transplanting (Harrison and Jackson 2011; Jose et al. 1994; Nedunzhiyan et al. 1998; Seem et al. 2003; Smith et al. 2017).  Weeds emerging after 6 weeks after transplanting do not usually affect sweetpotato yield and quality (Seem et al. 2003).  By six weeks after transplanting the decumbent sweetpotato vines have closed canopy, preventing light from reaching weed seeds and seedling between rows.  Sweetpotato gains a competitive advantage over weeds when planted late (higher temperature) in the recommended period for sweetpotato planting (KM Jennings, NC State University, unpublished data; Seem et al. 2003).  Thus, planting sweetpotato in fields with the highest weed density may be desirable when temperature is optimum for sweetpotato growth.

Weeds in Sweetpotato (major weeds, weed life cycle, weed strengths and weaknesses, impact on yield loss)

Annual and perennial weeds threaten sweetpotato fields worldwide.  Because sweetpotato vines grow along the soil and canopy height is often less than 0.5 m tall, weeds that grow up through and above the sweetpotato canopy are considered the most competitive.  Pigweed species like Palmer amaranth (Amaranthus palmeri L.) (Photograph 2) and sedges like yellow nutsedge (Cyperus esculentus L.) (Photograph 3) are so aggressive that they can compete and reduce sweetpotato yield and quality drastically without any other weed species present (Meyers et al. 2010, Meyers and Shankle 2015a).  Other weeds, even low growing annual weeds, can compete effectively with sweetpotato when they are present in mixed populations (Seem et al. 2003).  Vining weeds like morningglory species (Ipomoea spp.) are low growing like sweetpotato, but have the ability to grow to the top of the sweetpotato canopy where they compete with sweetpotato for light resources (J.R. Schultheis, Sweetpotato Specialist, NC State University, personal communication).  Within the U.S. some weed species are common to fields in all states, while other weed species in sweetpotato are specific to fields in an individual state (Table 1).  The following information is focused on the most troublesome weeds in U.S. sweetpotato.

Amaranthaceae – The pigweeds.  A number of annual species of the Amaranthaceae are troublesome weeds in sweetpotato including Palmer amaranth, redroot pigweed (Amaranthus retroflexus L.), spiny amaranth (A. spinosus L.), and smooth pigweed (A. hybridus L.) (Table 1).  Amaranthus spp. grow rapidly, are capable of growth under varying environmental conditions including hot dry conditions, easily exceed the sweetpotato canopy height within two to three weeks after transplanting (Smith et al. 2017), and are prolific seed producers.  The height of Amaranthus spp. from tallest to shortest is Palmer amaranth (> 2 m) > redroot pigweed = smooth pigweed > spiny amaranth (1.5 m or less) (Sellers et al. 2003).  Seed production for these species ranges from 100,000 to 1 million seeds per plant with seed production from most to least as Palmer amaranth > redroot pigweed = smooth pigweed > spiny amaranth (Sellers et al. 2003; Sosnoskie et al. 2014; A.C. York, NC State University unpublished data).  Members of this family have demonstrated resistance to many herbicide sites of action including EPSP synthase inhibitors, acetolactate synthase inhibitors (imidazolinones and sulfonylureas), HPPD inhibitors, microtubule inhibitors, protoporphyrinogen oxidase inhibitors, and triazines (Heap 2017; Ward et al. 2013).

Uncontrolled smooth pigweed in mixed populations of weed species resulted in a 40 to 50% reduction in marketable sweetpotato yield (Seem et al. 2003).  Uncontrolled Palmer amaranth at 0.5 to 6.5 plants per m of row resulted in 36 to 81% marketable sweetpotato yield reduction (Meyers et al. 2010).  Palmer amaranth can grow more than 5 cm per day and produce viable seeds 30 days after germinating (K.M. Jennings unpublished data; Legleiter and Johnson 2013).  In North Carolina when Palmer amaranth is allowed to compete with the sweetpotato crop from sweetpotato transplanting until 3 weeks after transplanting total yield loss was 10%.  If Palmer amaranth is not controlled until 6 weeks after sweetpotato transplanting total yield loss was 70% and loss of the premium number one grade was 90% (Smith et al. 2017).  Amaranthus spp. are controlled using plowing or non-selective herbicides prior to transplanting, preemergence herbicides preplant or after planting if available, cultivation during the season (usually 2 to 3 shallow cultivations), hand-removal and/or hoeing when small, and wicking with herbicide (glyphosate).  Hand-removal of spiny amaranth is especially cumbersome due to sharp 5 to 10 mm long spines that appear at nodes along the stem.  Escaped Amaranthus spp. should be rouged and removed from fields to prevent seed dispersal.  Some Amaranthus spp. have the ability to produce roots and re-establish if left in the field.  These plants should be cut at the soil surface to limit seed production (LM Sosnoskie, University of Georgia, unpublished data).   In contrast when Amaranthus spp. are cut above the soil surface they are capable of regrowing rapidly and producing many seeds.

Cyperaceae – The sedges.  Yellow nutsedge, purple nutsedge (Cyperus rotundus L.), annual sedge (Cyperus compressus L.) and rice flatsedge (Cyperus iria L.) are among the most troublesome weeds in sweetpotato (Table 1) (Meyers and Shankle 2015b; Webster 2014).  Yellow and purple nutsedge are perennial weeds that spread by underground rhizomes and reproduce vegetatively by tubers (Meyers and Shankle 2015b).  A single yellow nutsedge plant from a spouted tuber can form a compact, densely  populated patch (210 shoots/0.18 m²) after 6 months of growth (Webster 2005), and 3,000 shoots and 19,000 tubers in one year (Ransom et al. 2009).  Meyers and Shankle (2015a) reported that yellow nutsedge shoot density in sweetpotato increased by 2.3 to 7.6 times in a single four month growing season, further documenting the ability of yellow nutsedge to expand rapidly by vegetative reproduction.   Meyers and Shankle (2015a) reported marketable sweetpotato yield losses of 18 to 80% as yellow nutsedge density increased from 5 to 90 shoots per m².  In studies by Webster (2005), purple nutsedge formed larger patches with less shoot density but had the ability to spread and distribute further than yellow nutsedge. Because of its high potential for vegetative growth and reproduction, management strategies for nutsedge should be focused on prevention (control prior to transplanting sweetpotato), early detection and containment, early treatment (cultivation) and integration of control strategies (optimum sweetpotato growth, herbicides, cultivation, and crop rotation) to reduce it competitiveness and spread (Meyers and Shankle 2015a, 2015b; Ransom et al. 2009).  Additionally, equipment sanitation to prevent spread of tubers from field to field is critical (Meyers and Shankle 2015b).  Annual sedge and rice flatsedge are annuals and not generally as competitive as the perennial nutsedges.  However, in some sweetpotato producing areas, annual sedge and rice flatsedge occur in high density and/or with other weeds resulting in a highly competitive weed population with sweetpotato (Table 1).

Poaceae – The grasses.  Grasses infest sweetpotato worldwide.  Annual grasses are of greater concern than perennial grasses as the latter do no frequently persist as a result of tillage operations utilized for sweetpotato production.  In the U.S., some of the common annual grasses that infest sweetpoato are large crabgrass [Digitaria sanguinalis (L.) Scop.], goosegrass [Eleusine indica (L.) Gaertn.] , barnyardgrass [Echinochloa crus-galli (L.) Beauv.], broadleaf signalgrass [Urochloa platyphylla (Nash) R.D. Webster], and fall panicum (Panicum dichotomiflorum Michx.).  The specific annual grass species that emerge in sweetpotato are dependent on history of weeds in that field, environment (temperature, rainfall) for that specific year, and transplanting date.  These weeds typically emerge in the period of time between field preparation and the early growing season during crop establishment.  Although the annual grasses have the ability to grow above the sweetpotato canopy, to do so they must establish early in the season before extensive sweetpotato vining has occurred.  Grasses not controlled early in the season must be controlled prior to the last third of the crop growing season when sweetpotato storage roots are sizing up otherwise competition of annual grasses with sweetpotato will result in reduced crop vigor and yield.  Members of Poaceae have demonstrated resistance to ALS inhibitors, ACCase inhibitors, cellulose inhibitors, EPSP synthase inhibitors, glutamine synthase inhibitors, lipid inhibitors, long chain fatty acid inhibitors, microtubule inhibitors, PSI Electron diverter, synthetic auxins and Photosystem II inhibitors (Heap 2017).

Convolvulaceae – The annual morningglories.   Members of the morninglory family that are troublesome in sweetpotato are annuals and include entireleaf morningglory [Ipomoea hederacea (L.) Jacq.], ivyleaf morningglory (I. hederacea var. integriuscula Gray), pitted morningglory (I. lacunosa L.), tall morningglory [(I. purpurea (L.) Roth], and smallflower morningglory [Jacquemontia tamnifolia (L.) Griseb].  These weeds are low growing vines with fibrous roots from a taproot and can grow to 3 m long (Bryson and DeFelice 2010; DeFelice 2001).  They intertwine with sweetpotato and can compete with the crop for light, nutrients and water.  Morningglory species often not only grow throughout the sweetpotato canopy, they preferentially grow toward and on upright weeds where they can capture more sunlight resulting in growth and seed production (Price and Wilcut 2007).  Per plant seed production of morningglories is 2,000 to 5,800 for ivyleaf morningglory (Gomes et al. 1978; Price and Wilcut 2007); 14,600 for entireleaf morningglory (Gomes et al. 1978); 15,400 for pitted morningglory (Gomes et al. 1978), and 26,000 for tall morningglory (DeFelice 2001). Morningglory seeds have a hard seed coat which contributes to their ability to remain viable in the soil for many years (DeFelice 2001; Elmore et al. 1990).  Removal or control is difficult as seeds of these weeds are large allowing them to emerge from soil quickly and making them difficult to control with preemergence herbicides.  As they become established and intertwined with sweetpotato, they become increasingly difficult to cultivate or remove by hand.

Portulacaceae – The purslanes.  Common purslane (Portulaca oleracea L.) and pink purslane (P. pilosa L.) are predominately prostrate-growing annual plants with a thick taproot with many fibrous roots, stems up to 50 cm long and whole plant canopy diameter of 60 cm (Bryson and DeFelice 2001).  However, height is dependent on the amount of light it receives.  Common purslane plants grown under conditions in which light is limited such as when competing with a crop, will grow taller than plants grown in a noncompetitive environment.  The purslanes have succulent stems and leaves which contribute to their drought resistance.  They can reproduce from seeds (greater than 100,000 per plant) or from fragmented stems with a node (Holm et al. 1977; Proctor 2013; Proctor et al. 2011).  Purslane seeds have reportedly remained viable for as long as 40 years (Darlington 1941).  Under favorable conditions (moist, 30 to 40 C), seeds begin to germinate in 12 hours and emergence is complete in 24 hours.  Rapid growth occurs at about 2 weeks and seed production increases rapidly at 4 to 6 weeks after emergence (Haar and Fennimore 2003; Holm et al. 1977).  To prevent seed production, cultivation or hand hoeing of common purslane should occur before 3 weeks after emergence or 125 growing degree days (single sine method of calculating growing degree days was used, and the lower and upper temperature thresholds were 10 and 42°C) (Haar and Fennimore 2003; University of California 1990).

Pennsylvania smartweed (Polygonum pensylvanicum L.).  Pennsylvania smartweed is an annual weed and can grow 1.2 to 1.8 m tall (Bryson and DeFelice 2010, Lorenzi and Jeffery 1987). It can exceed the height of sweetpotato canopy (0.5 m) by 40 and 60 days after emergence (Askew and Wilcut 2002).  It prefers wet, poorly drained soils and those high in nitrogen or phosphorus but does not tolerate dry weather. Once established, this weed is adapted to a wide range of environments and its extensive root system in the upper and lower soil horizons allows for maximum nutrient uptake (Parrish and Bazzaz 1976). This weed reproduces by seed with as many as 20,000 seeds per plant (Anonymous 2017a).  Its seeds are contained in buoyant achenes which move toward wetter regions of fields (Pickett and Bazzaz 1978).  Seeds are viable for as many as 26 years in the soil (Anonymous 2017a).  Members of this family have demonstrated resistance to ALS inhibitors and photosystem II inhibitors (Heap 2017).  Pennsylvania smartweed should be controlled before exceeding 6.35 mm in height (Anonymous 2017a). Tillage at night can reduce smartweed emergence by 30 to 50% (Anonymous 2017a).

Florida pusley (Richardia scabra L).  Florida pusley is a very persistent summer annual weed that can be prostrate or upright with stems 15 to 50 cm long.  This weed reproduces by seed with as many as 2,297 seeds per plant (Brewer and Oliver 2007).  The germination and growing season for this weed is consistent with that of sweetpotato (Biswas et al. 1975).

Common Lambsquarters (Chenopodium album L.).  A member of the Chenopodiaceae, this summer annual begins to emerge prior to sweetpotato transplanting and emergence continues through the early transplanting season when temperatures tend to be cooler.  This emergence period may be extended when above average rainfall or irrigation occurs during the season.  Common lambsquarters has a short-branched taproot and is capable of growing 2 m tall (Bryson and DeFelice 2010).  It is a prolific seed producer (as many as 70,000 seeds per plant) and can reach reproductive maturity six weeks after emergence (Curran et al 2007).  Members of this family have demonstrated resistance to ALS inhibitors, photosystem II inhibitors, and synthetic auxins (Heap 2017).  Transplanting sweetpotato fields with a known history of this weed in the last half of the recommended transplanting season will aid in its management.

Malvaceae – The mallows. Prickly sida (Sida spinosaL.)is an erect, branched annual weed that can grow 1 m tall (Bryson and DeFelice 2010).  A related species, arrowleaf sida (S. rhombifolia L.), has a branching tap root with fibrous roots.  Prickly sida seeds can germinate under limited soil moisture (Hoveland and Buchanan 1973) and germination is encouraged by high temperature and cycles of wet-dry soil moisture, and shifting of colder temperatures to a higher temperature regime (Baskin and Baskin 1984).  Resistance of prickly sida to the ALS inhibitor herbicides has been reported (WSSA).  Prickly sida can be suppressed by highly competitive crops (Green 2016).

Solanaceae – The black nightshade complex and groundcherries.  Several Solanum species make up the black nightshade complex.  They include black nightshade (Solanum nigrum L.), American black nightshade (S. americanum Mill.), and Eastern black nightshade (S. ptycanthum Dun.).  Black nightshade and American black nightshade are low growing spreading, sometimes upright annual to short-lived perennials growing to 1 to 1.5 m in height.  Eastern black nightshade is an annual weed (Thomson and Witt 1987).  Nightshade species have a fibrous root system with a shallow taproot.  Plants in the black nightshade complex can produce 30,000 (American black nightshade) to 100,000 (Eastern black nightshade) to 178,000 (black nightshade) seeds per plant (Holm 1977; Keeley and Thullen 1983; Werner et al. 1998).  Tillage at night in the dark reduces Eastern black nightshade emergence by 50 to 75% (Anonymous 2017b).  Members of this family have demonstrated resistance to ALS inhibitors, photosystem II inhibitors, and PSI electron diverter.

Clammy (Physalis heterophylla) and smooth (P. subglabrata) groundcherries grow 0.3 to 0.9 m tall and have an upright, branching growth habit.  Groundcherry plants have deep penetrating and creeping roots.  The plants can produce as many as 30,000 seeds and can reproduce by root fragments.  Seeds are contained in berries, each covered by a bladder-like husk that looks similar to a paper lantern.  Root fragments that are moved to the soil surface do not usually survive (Anonymous 2017c).  A related annual species, cutleaf groundcherry (P. angulata L.), can produce up to 4,200 seeds per plant and exceed 0.5 m by 30 to 40 days after emergence (Thomson and Witt 1987; Travlos 2012).

Hophornbeam copperleaf (Acalypha ostryifolia Riddell).  A member of the Euphorbiaceae or spurge family, this summer annual can emerge over a wide range of environmental conditions and throughout the sweetpotato growing season after each rain.  Hophornbeam copperleaf is an erect plant that can reach 1 m or more, reproduces by seed, and can produce as many as 12,500 seeds per plant (Harak et al. 1998; Steckel 2006). It can exceed the height of the sweetpotato canopy by 6 to 8 weeks after emergence (Harak et al. 1998).  Management programs must continue all season because of this weed’s prolonged germination throughout the growing season.

Fabaceae– The bean family.  Sicklepod [Senna obtusifolia (L.) Irwin and Barnaby] is an upright, summer annual that grows to over 1.5 m and begins flowering at 50 to 84 days after germination followed by seed production (Retzinger 1984).  It is a troublesome weed in crops like sweetpotato because of its extensive seed production (over 16,000 seeds per plant), ability to germinate under varying environmental conditions, and hard seed coat that contributes to seed dormancy (Nice et al. 2001).  By 30 to 40 days after emergence, its height can exceed the canopy of sweetpotato (Smith 1992).  Seeds of sicklepod can remain viable in the soil for a long period of time ( Bararpour and Oliver 1998).

Florida beggarweed [Desmodium tortuosum (Sweet) DC.] germinates throughout the sweetpotato growing season (late May through September).  It is an erect plant that can exceed the height of the sweetpotato canopy by 45 days after emergence and grow to 3.5 m tall (Cardina and Brecke 1991; Webster and Cardina 2004).  Prior to exceeding the height of the crop, this weed remains unbranched and as it exceeds the crop canopy it begins rapid growth and produces branches with leaves that effectively shade the low-growing sweetpotato by intercepting 30% of ambient sunlight.  Flowering begins as early as 67 days after emergence and viable seeds can be produced within 10 days after flowering.  Seeds can germinate and emerge throughout the growing season when soil is disturbed and sufficient soil moisture is present.  Seeds have a hard seed coat that contributes to its persistence in the soil where it can remain viable for 5 years or more (Cardina and Brecke 1991; Webster and Cardina 2004).  Late emerging Florida beggarweed is not very competitive with low-growing crops like sweetpotato (Cardina and Brecke 1991).  Rotation with tall growing crops like corn, repeated shallow tillage/cultivation to deplete the soil seed bank, and had removal are effective methods for controlling this weed.

Common cocklebur (Xanthium strumarium L.).  A member of the Asteraceae, common cocklebur has a tall growth habit (up to 1.5 m) and large canopy, and an extensive root system that gives it an advantage in nutrient and water uptake (Anonymous 2017d; Crooks et al. 2005).  Common cocklebur has potential to exceed the sweetpotato canopy within 30 to 40 days of emergence (Crooks et al. 2005).  It is a summer annual weed that reproduces by flowering in late summer to early fall, and produces burs (fruit with prickles on outside) that contain seeds.  Each bur contains two seeds that can survive up to 3 years in soil.  One seed has the capacity to germinate the following year, whereas germination of the second seed is delayed at least two years.  Members of this family have demonstrated resistance to ALS inhibitors, and nucleic acid inhibitors, (Heap 2017).  Common cocklebur can thrive in varying soils and moisture conditions but will not tolerate shade.

 Wild radish (Raphanus raphanistrum L.).  A member of the Brassicaceae, this winter annual or summer annual can emerge anytime during the year when moisture is sufficient for germination.  Wild radish forms a basal rosette with upright, leafy inflorescences up to 1.5 m tall.  It has an extensive fibrous root system capable of spreading 80 cm in all directions and a taproot capable of growing 1 m deep.  As a result, it grows quickly and survives under varying environmental conditions.  It reproduces by seeds (over 700 seeds per plant) that can remain viable in soil for more than 20 years (Anonymous 2015; Eslami et al. 2006; Peltzer and Douglas 2017).  Wild radish seeds germinate in soils between 5 and 35C.  Members of this family have demonstrated resistance to ALS inhibitors, carotenoid biosynthesis inhibitors, ESP synthase inhibitors, and synthetic auxins (Heap 2017).

Table 1. Most troublesome weeds in sweetpotato in U.S. states.

Methods for weed control:

Weed management in sweetpotato relies on the integration of multiple control methods including mechanical, chemical, and cultural.  Implementation of each method will vary by location, as access to labor, equipment, and registered herbicides vary from country-to-country and farm-to-farm.  Below is an overview of control methods utilized by sweetpotato producers in the Southeast United States.

Mechanical control: In the spring, land intended for sweetpotato production is cultivated with a disc or similar implement to remove winter annual weeds and loosen soil prior to ridged row formation.  After transplanting, between-row cultivation is utilized.  In the U.S., an implement consisting of rolling cultivators followed by soil sweeps is used to removed emerged weeds between rows and throw soil from between-row spaces onto the ridged row to bury small emerging weeds within the row.  Between-row cultivation can be used until vine closer, typically three to four weeks after transplanting.  Timely cultivation should target small emerging weeds.  Between-row cultivation often fails to completely remove weeds in the planted row.

Weeds that escape cultivation are often removed by hand.  In the U.S. most commercial sweetpotato fields receive two hand-weeding events per growing season.  Hand-removal of weeds is labor intensive and, depending upon location, can be expensive.  This practice is most appropriate for upright, annual weeds.  Hand-removal of grasses and perennial sedges is difficult.

Mowing weeds in sweetpotato is practiced, but provides limited control.  Meyers et al. (2017) reported that Palmer amaranth mowed above the sweetpotato canopy branches below the cut and results in a dense canopy of weedy vegetation just above the sweetpotato canopy.  Mowing may be used as a salvage effort to reduce weed seed set, but is not a stand-alone control method.

Cultural weed control:

Multiple cultural weed control methods have been investigated by researchers, but few are implemented on a commercial scale.  Allelopathic sweetpotato cultivars, those that exude chemical substances capable of hindering weed growth and development, have been documented but often lack commercially desirable traits to be grown on a large scale (Harrison and Peterson 1986, 1991).  La Bonte et al. (1999) reported that 11 sweetpotato clones with architecturally different canopies demonstrated a 2-fold and 3-fold difference in percent canopy closure 42 days after transplanting and at harvest, respectively.  The authors further identified five weed-tolerant clones, three of which were bunch or medium-internode types and concluded that more research was required to understand the interactions of canopy architecture and other production practices (row spacing, crop fertility) as well as investigations into competitive ability and tolerance to individual weeds or classes of weeds.

Crop rotation is used to manage pests including weeds.  Producers can rotate to crops that are more competitive with weeds and those that have registered and efficacious herbicides for the target weed species.  In Mississippi, producers rotate sweetpotato fields invested with yellow nutsedge to corn (Zea mays L.) or soybean [Glycine max (L.) Merr.].  Both have more upright growth and compete for light resources more than the decumbent growing sweetpotato.  Additionally both corn and soybean have registered and efficacious herbicides that will control yellow nutsedge.  A minimum of two growing seasons is often required to significantly reduce weed pressure.  Of special consideration when utilizing crop rotation is plant-back interval, the required amount of time between an herbicide application and when sweetpotato may be safely transplanted.  Plant-back restrictions for sweetpotato often err on the side of caution due to a lack of research to support reduced intervals.

Weed propagules (rhizomes and seeds) move with sweetpotato farming implements.  Researchers recommend that implements such as discs, mechanical transplanters, and diggers have soil removed when being moved between fields.  It is unclear what percentage of producers currently follow this recommendation.

Chemical weed control:

Limited herbicides are registered for use in sweetpotato in the United States.  Flumioxazin, a protoporphyrinogen oxidase inhibitor, is applied pre-transplanting to prepared production fields and provides preemergence control of numerous broadleaf weeds and suppression of grasses (Kelly et al. 2006).  S-metolachlor is a soil-applied chloroacetamide herbicide that inhibits the biosynthesis of fatty acids, lipids, proteins, isoprenoids, and flavonoids in susceptible plant species.  It is broadcast-applied after transplanting for preemergence control of small-seeded broadleaf weeds, grasses, and yellow nutsedge (Meyers and Shankle 2017).  Clomazone, a carotenoid biosynthesis inhibitor, is broadcast-applied after transplanting for the control of grasses and select broadleaf weeds.  Fluazifop, sethoxydim, and clethodim are applied post-emergence to selectively control grass weed species.  These graminicides are applied with either a crop-based oil or non-ionic surfactant to improve efficacy.  Glyphosate and carfentrazone-ethyl are used pre-plant to “burn-down” existing weedy vegetation.  Both can be applied with a directed application between rows for postemergence weed control.  Napropamide and DCPA are both registered for use in sweetpotato in the U.S., but are rarely utilized as they are highly dependent upon rainfall- or soil-incorporation and efficacy is often variable.

Herbicide wick and wiper applicators are used by a limited number of sweetpotato producers.  Many models are available, but all have a similar function.  A concentrated herbicide solution is placed into a reservoir and is soaked into canvas sleeves or absorbent ropes.  The absorbent material is placed in contact with weeds that grow above the sweetpotato canopy.  The systemic herbicide is translocated throughout the contacted weed.  This application method has some limitations.  As it is selective based on a weed-crop height differential, the target weeds must exceed the sweetpotato canopy, and in doing so compete with the crop before and between applications (Meyers et al. 2017).

Table 2. Suggestions for species specific weed control methods in sweetpotato

¹Weeds emerge after field bedding but prior to transplanting sweetpotato.  Control of small emerged weeds with tillage or Post (non-selective) herbicide.

²Preemergence herbicide applied preplant (flumioxazin marketed under the trade name Valor herbicide by Valent or clomazone marketed under the trade name Command herbicide by FMC Corporation) to sweetpotato or Postplant preemergence herbicide (S-metolachlor marketed under the trade name Dual Magnum herbicide by Syngenta or clomazone marketed under the trade name Command herbicide by FMC Corporation) applied after sweetpotato planting. Note: Valor is registered to only apply prior to planting sweetpotato.  Dual Magnum is registered to apply only after sweetpotato transplanting.

³May require multiple tillage events or multiple applications of nonselective postemergence herbicide(s).

⁴Suppression only.

Conclusions

Weed control in sweetpotato, one of the most important crops worldwide, requires an integrated approach, particularly as the development of herbicide resistant weeds is accelerating. Amaranths, sedges, various grasses and morningglories can cause remarkable yield reductions of up to 80%.  Sweetpotato fields should start weed-free and remain weed-free through canopy closure, approximately six weeks after transplanting.  Weeds that establish after canopy closure are not likely to reduce yield but, depending on the weed species, may need to be removed to prevent weed seed production.

References

Anonymous (2015) Raphanus raphanistrum (wild radish) datasheet. CABI. www.cabi.org/isc/datasheet/46795

Anonymous (2017a) Michigan’s Worst Weeds:  Pennsylvania smartweeds and ladysthumb. Michigan State University. http://www.msuweeds.com/worst-weeds/smartweeds/

Anonymous (2017b) Michigan’s Worst Weeds: Eastern black nightshade (Solanum ptycanthum Dun.) Michigan State University. http://www.msuweeds.com/worst-weeds/eastern-black-nightshade/

Anonymous (2017c) Ohio perennial and biennial weed guide – groundcherries (Physalis spp.).  The Ohio State University.  http://www.oardc.ohio-state.edu/weedguide/single_weed.php?id=50

Anonymous (2017d) Xanthium stumarium, common cocklebur. http://articles.extension.org/pages/65827/xanthium-strumarium-common-cocklebur

Askew SD, Wilcut JW (2002) Pennsylvania smartweed interference and achene production in cotton.  Weed Science 50:350-356.

Bararpour MT, Oliver LR (1998) Effect of tillage and interference on common cocklebur (Xanthium strumarium) and sicklepod (Senna obtusifolia) population, seed production and seedbank.  Weed Science 46:424-431.

Baskin JM, Baskin CC (1984) Environmental conditions required for germination of prickly sida (Sida spinosa).  Weed Science 32:786-791.

Biswas PK, Bell PD, Crayton JL, Paul KB (1975) Germination behavior of Florida pusley seeds. I. Effects of storage, light, temperature and planting depth on germination.  Weed Science 23:400-403.

Bouwkamp JC, Ed. (1985) Sweet Potato Products: A Natural Resource for the Tropics. CRC Press, Inc.: Boca Raton, FL. Pp. 175-183.

Brewer CE, Oliver LR (2007) Reducing weed seed rain with late-season glyphosate applications.  Weed Technology 21:753-758.

Bryson CT, DeFelice MS (2010) Weeds of the Midwestern United States & Central Canada.  University of Georgia Press, Athens, GA, 427 pp.

Cardina J, Brecke BJ (1991) Florida beggarweed (Desmodium toruosum) growth and development in peanuts (Arachis hypogaea).  Weed Technology 5:147-153.

CIP International Potato Center (2017) Sweetpotato facts and figures. cipotato.org/sweetpotato/facts-2/

Crooks HL, Burton MG, York AC, Brownie C (2005) Vegetative growth and competiveness of common cocklebur resistant and susceptible to acetolactate synthase-inhibiting herbicides. Journal of Cotton Science 9:229-237.

Curran B, Sprague C, Stachler J, Loux M (2007) Biology and management of common lambsquarters. Purdue Extension Publication GWC-112.

Darlington HT (1941) The sixty-year period for Dr. Beal’s seed viability experiment.  American Journal of Botany 28:271-273.

DeFelice MS (2001) Tall morningglory, Ipomoea purpurea (L.) Roth- flower or foe?  Weed Technology 15:601-606.

Elmore CD, Hurst HR, Austin DF (1990) Biology and control of morningglories (Ipomoea spp.). Reviews of Weed Science 5:83-224.

Eslami SV, Gill GS, Belloti B, McDonald G (2006) Weed radish (Raphanus raphanistrum L.) interference in wheat.  Weed Science 54:749-756.

Gajanayake B, Reddy KR, Shankle (2015) Quantifying growth and development responses of sweetpotato to mid- and late-season temperature. Agronomy Journal 107:1854-1862.

Gajanayake B, Reddy KR, Shankle MW, Arancibia RA, Villordon AO (2014) Quantifying storage root initiation, growth, and development responses of sweetpotato to early season temperature.  Agronomy Journal 106:1795-1804.

Gajanayake B, Reddy KR, Shankle MW, Arancibia RA (2013) Early-season soil moisture deficit reduces sweetpotato storage root initiation and development.  HortScience 48:1457-1462.

Gilbert, CA (2006) American black nightshade (Solanum americanum MILL.) interference in watermelon (Citrullus lanatus L.).  University of Florida M.S. thesis.  70 pp.

Gomes LF, Chandler JM, Vaughan CE (1978) Aspects of germination, emergence, and seed production of three Ipomoea taxa.  Weed Science 26:245-248.

Green JD (2016) Prickly Sida (Teaweed) Management in Soybean, USB Take Action: Herbicide-resistance management series, http://takeactiononweeds.com/

Haar MJ, Fennimore SA (2003) Evaluation of integrated practices for common purslane (Portulaca oleracea) management in lettuce (Latuca sativa).  Weed Technology 17:229-233.

Harak MT, Gao Z, Peterson DE, Maddux LD (1998) Hophornbeam copperleaf (Acalypha ostryifolia) biology and control.  Weed Technology 12:515-521.

Harrison HF, Jackson DM (2011) Response of two sweet potato cultivars to weed interference. Crop Protection 30:1291-1296.

Harrison Jr HF, Peterson JK (1986) Allelopathic effects of sweet potatoes (Ipomoea batatas) on yellow nutsedge (Cyperus esculentus) and alfalfa (Medicago sativa). Weed Science 34:623-627.

Harrison Jr HF, Peterson JK (1991) Evidence that sweet potato (Ipomoea batatas) is allelopathic to yellow nutsedge (Cyperus esculentus). Weed Science 39:308-312.

Heap I (2017) The international survey of herbicide resistant weeds. Online. Internet. http://www.weedscience.org/default.aspx

Holm LG, Pluchnett DL, Pancho JV, Herberger JP (1977) The worlds worst weeds: distribution and biology.  The University Press of Hawaii, Honolulu. 609pp.

Hoveland CS, Buchanan GA (1973) Weed seed germination under simulated drought.  Weed Science 21:322-324.

Jose J, Marcano A, Omar D, Colmenarez G, Florencio Paredes G (1994) Periodo critico de competencia por maleza en el cultivo de batata (Ipomoea batatas (L) Lam) cultivar UCV-7. Bioagro 6:86-94.

Keeley PE, Thullen RJ (1983) Influence of planting date on the growth of black nightshade (Solanum nigrum L.).  Weed Science 31:180-184.

Kelly, ST, Shankle MW, Miller DK (2006) Efficacy and tolerance of flumioxazin on sweetpotato (Ipomoea batatas).  Weed Technol. 20:334-339.

Korres NE, Norsworthy JK. (2017) Palmer amaranth (Amaranthus palmeri L.) demographic and biological characteristics in wide row soybean. Weed Science. (In press)

La Bonte DR, Harrison HF, Motsenbocker CE (1999) Sweetpotato clone tolerance to weed interference. HortScience 34:229-232.

Legleiter T, Johnson B (2013) Palmer amaranth biology, identification, and management.Purdue University Extension Publication WS-51.

Lorenzi HJ, Jeffery LS (1987) Weeds of the United States and their control.Van Nostrand Reinhold Co., NY.  355 pp.

Meyers SL, Arancibia RA, Shankle MW, Main J, Reddy RK (2014) Sweet potato storage root initiation.  Mississippi State University Extension Publication 2809:1-4.

Meyers SL, Jennings KM, Schultheis JR, Monks DW (2010) Interference of Palmer amaranth (Amaranthus palmer) in sweetpotato.  Weed Science 58:199-203.

Meyers SL, Jennings KM, Schultheis JR, Monks DW (2017) Evaluation of wick-applied glyphosate for Palmer amaranth (Amaranthus palmeri) control in sweetpotato. Weed Technology 30:765-772.

Meyers SL, Shankle MW (2015b) Nutsedge management in Mississippi sweetpotatoes. Mississippi State University Extension Publication 2909.

Meyers SL, Shankle MW (2015) Interference of yellow nutsedge (Cyperus esculentus) in ‘Beauregard’ sweet potato (Ipomoea batatas). Weed Technology 29:854-860.

Meyers, SL, Shankle MW (2017) An evaluation of pre-emergence metam-potassium and S-metolachlor for yellow nutsedge (Cyperus esculentus) management in sweetpotato. Weed Technol. (In Press).

Nedunzhiyan M, Varma SP, Ray RC (1998) Estimation of critical period of crop-weed competition in sweet potato (Ipomoea batatas L.).  Adv. Hort. Sci. 12:101-104.

Nice GRW, Buehring NW, Shaw DR (2001) Sicklepod (Senna obtusifolia) response to shading soybean (Glycine max) row spacing, and population in three management systems.  Weed Technology 15:155-162.

Pardales JR, Yamauchi A (2003) Regulation of root development in sweetpotato and cassava by soil moisture during their establishment period.  Plant and Soil 255:201-208.

Parrish JAD, Bazzaz FA (1976) Underground niche separation in successional plants. Ecology 57:1281–1288.

Peltzer S, Douglas A (2017) Wild radish. Government of Western Australia Department of Agriculture and Food. https://www.agric.wa.gov.au/grains-research-development/wild-radish?page=0%2C0

Pickett STA, Bazzaz FA (1978) Organization of an assemblage of early successional species of a soil moisture gradient. Ecology 59:1248– 1255.

Price AJ, Wilcut JW (2007) Response of ivyleaf morningglory (Ipomoea hederaceae) to  neighboring plants and objects.  Weed Technology 21:922-927.

Proctor C (2013) Biology and control of common purslane (Portulaca oleracea L.).  University of Nebraska – Lincoln,  PhD thesis, 67 pp.

Proctor CA, Gaussoin RE, Reicher ZJ (2011) Vegetative reproduction potential of common purslane (Portulaca oleracea). Weed Technology 25:694-697.

Ransom CV, Rice CA, Shock CC (2009) Yellow nutsedge (Cyperus esculentus) growth and reproduction in response to nitrogen and irrigation.  Weed Science 57:21-25.

Retzinger EJ (1984) Growth and development of sicklepod (Cassia obtusifolia) selections.  Weed Science 32:608-611.

Schultheis JR, Walters SA, Adams DE, Estes EA (1999) In-row plant spacing and date of harvest of ‘Beauregard’ sweetpotato affect yield and return on investment.  HortScience 34:1220-1233.

Seem JE, Creamer NG, Monks DW (2003) Critical weed-free period for ‘Beauregard’ sweetpotato (Ipomoea batatas).  Weed Science 17: 686-695.

Sellers BA, Smeda RJ, Johnson WG, Kendig JA, Ellersieck MR (2003) Comparative growth of six Amaranthus species in Missouri. Weed Science 51:329-333.

Smith S, Jennings KM, Monks DW (2017) Timing of Palmer amaranth control on sweetpotato yield and quality.  SRASHS meeting vegetable section abstracts.

Smith JE (1992) Shoot growth and form of Senna obtusifolia in response to soybean and intraspecific competition.  LSU PhD dissertation.  101pp.

Sosnoskie LM, Webster TM, Culpepper AS, Kichler J (2014) The biology and ecology of Palmer amaranth: implications for control.  UGA Extension Circular 1000. http://extension.uga.edu/publications/files/pdf/C%201000_2.PDF

Steckel L (2006) Hophornbeam copperleaf.  University of Tennessee Extension publication https://extension.tennessee.edu/publications/Documents/W120.pdf

Stoddard CS, Davis RM, Cantwell M (2013) Sweetpotato production in California.  University of California Agriculture and Natural Resources Publication 7237.

Thomson CE, Witt WW (1987) Germination of cutleaf groundcherry (Physalis angulata), smooth groundcherry (Physalis virginiana) and eastern black nightshade (Solanum ptycanthum).  Weed Science 35:58-62.

Travlos IS (2012) Invasiveness of cut-leaf ground-cherry (P. angulata L.) populations and impact of soil water and nutrient availability. Chilean Journal of Agricultural Research 72 (3):358-363.

[USDA-FAS] US Department of Agriculture-Foreign Agricultural Service (2017) Global Agriculture Information Network Report CH16067: Peoples Republic of China annual biofuels- biofuels demand expands, supply uncertain. https://gain.fas.usda.gov/Recent%20GAIN%20Publications/Biofuels%20Annual_Beijing_China%20-%20Peoples%20Republic%20of_1-18-2017.pdf

University of California. 1990. Degree-Day Utility User’s Guide Version 2.0. University of California Integrated Pest Management 9: Web page: http://www.ipm.ucdavis.edu/IPMPROJECT/software.html.

Villordon A, LaBonte D, Solis J, Firon N (2012) Characterization of lateral root development at the onset of storage root initiation in ‘Beauregard’ sweetpotato adventitious roots. HortScience 47:961-968.

Ward SM, Webster TM, Steckel LE (2013) Palmer amaranth (Amaranthus palmeri): a review. Weed Technology 27:12-27.

Webster TM (2014) Weed survey – southern states. Pages 292 in Proceedings of the 67^th Southern Weed Science Society.  Birmingham, AL: Southern Weed Science Society.

Webster TM (2005) Patch expansion of purple nutsedge (Cyperus rotundus) and yellow nutsedge (Cyperus esculentus) with and without polyethylene mulch. Weed Science 53(6):839-845.

Webster TM, Cardina J (2004) A review of the biology and ecology of Florida beggarweed (Desmodium tortuosum).  Weed Science 52:185-200.

Werner EL, Curran WS, Lingenfelter DD (1998) Management of Eastern black nightshade in agronomic crops: an integrated approach.  Penn State Extension Agronomy Facts 58:1-6.

Workayehu T, Mazengia W, Hidoto L (2011) Growth habit, plant density and weed control on weed and root yield of sweet potato (Ipomoea batatas L.) Areka, Southern Ethiopia.  Journal of Horticulture and Forestry 3:251-258.