IntroductionNorth Carolina produces 40% of the nation’s sweetpotato hectarage (USDA-

NASS 2009); worth $141 million to the state (NCDA&CS 2009).

Sweetpotato yield and quality are greatly threatened by Palmer amaranth

(Amaranthus palmeri S. Watson), the most troublesome weed in N.C. crops

(Webster 2006). Palmer amaranth can grow in excess of 2 m tall (Sellers et

al. 2003) and produce as many as half a million seeds per plant (Keeley et al.

1987). Palmer amaranth interference studies have been conducted in

agronomic crops, yet the parameters governing competitive interactions

between Palmer amaranth and sweetpotato have never been reported.

Modeling these interactions, particularly as they pertain to sweetpotato yield

and quality loss, is critical in guiding decision making processes of N.C.

growers in implementing a control strategy for this weed. Understanding the

dynamic interactions between Palmer amaranth and sweetpotato will allow

N.C. sweetpotato growers to establish Palmer amaranth density thresholds

that optimize yield, quality, and income potential.

Objective and Hypothesis

Objective: Research was conducted to determine and model the influence of

Palmer amaranth interference on sweetpotato yield and quality.

Hypothesis: Increasing Palmer amaranth densities will contribute to a

reduction in sweetpotato yield and quality.

Results and Discussion

-Palmer amaranth height and shoot dry biomass m-1 displayed a quadratic relationship

with Palmer amaranth density (Figure 1B, 1C) and increased from 177 to 197 cm and 0.6

to 2.2 kg m-1, respectively at 0.5 to 4.1 plants m-1 and decreased from 197 to 188 cm and

2.2 to 1.5 kg m-1, respectively at 4.1 to 6.5 plants m-1.

-Palmer amaranth width and shoot dry biomass plant-1 decreased linearly from 145 to 69

cm and 1.1 to 0.2 kg plant-1, respectively as density increased from 0.5 to 6.5 plants m-1

(Figure 1B, 1C). Massinga et al. (2003) also observed that Palmer amaranth at lower

densities were more branched and those at higher densities grew more upright.

-Sweetpotato yield loss ranged from 56 to 94% for jumbo, 30 to 85% for one, and 36 to

81% for marketable roots as Palmer amaranth densities increased from 0.5 to 6.5 plants

m-1 (Figure 2B). The I parameter estimate for marketable sweetpotato (121%) indicates

the crop is less competitive with Palmer amaranth than peanut- 39% (Burke et al. 2007),

corn- 62% (Massinga 2001), and soybean- 12 to 105% (Bensch et al. 2003).

-Palmer amaranth light interception 6, 10, and 13 wk after planting ranged from 47 to

68%, 46 to 82%, and 42 to 71% as density increased from 0.5 to 6.5 plants m-1 (Figure

3B).

-Sweetpotato yield loss was due primarily to interference by the Palmer amaranth canopy

(Figure 3C) which limited available light for sweetpotato vines to develop and form

storage roots.

Implications for N.C. and Future Research

A Palmer amaranth density of < 0.09 m-1 (1 plant 11 m-1) is required to limit marketable

sweeetpotato yield loss to < 10%. Many N.C. production fields have densities greater than

this threshold. Future research should focus on preemergence control of Palmer amaranth

to limit interference by the weed and maximize sweetpotato yield and quality.Materials and Methods

In 2007 and 2008 ‘Covington’ and ‘Beauregard’ sweetpotato slips (stem cuttings)

were transplanted 31 cm apart into five fields with historically high Palmer

amaranth densities. Palmer amaranth was established at densities of 0 (weed-free

check), 0.5, 1.1, 1.6, 3.3, and 6.5 plants m-1 of crop row and remained season-

long. Palmer amaranth light interception (the difference between ambient light

and light reaching the sweetpotato canopy) was measured using a LiCor line

quantum sensor 6, 10, and 13 weeks after transplanting (WAP). Near the

conclusion of the study, Palmer amaranth height, canopy width, and shoot dry

biomass were measured. Sweetpotatoes were harvested 110 to 121 days after

planting and graded into jumbo, one, canner, and marketable (all grades) roots.

Sweetpotato yield loss was calculated relative to the weed-free check plot in each

replication. All data were subjected to regression analysis and fit to the

appropriate models. Sweetpotato yield loss and Palmer amaranth light

interception were fit to the rectangular hyperbola model (Cousens 1985):

YL = (ID)/[1 + (ID/A)]

where YL is predicted percent sweetpotato yield loss or light interception due to

weed competition, I is percent yield loss or light interception per unit weed

density as density approaches zero, D is weed density, and A is percent yield loss

or light interception as density approaches infinity.

Note: The I parameter is often used as a measure of competitive ability where

lower I values indicate a more competitive species.

Palmer amaranth height and canopy width are measured (A); the plants are then chopped,

placed in a paper bag (D), and dried to obtain shoot dry biomass. The effect of Palmer

amaranth density on plant height and canopy width (B) and shoot dry biomass plant-1 and m-1

of row (C).

Figure 2. Sweetpotato Yield Loss

Sweetpotatoes are dug, graded,

and weighed (A). The effect of

Palmer amaranth density on

sweetpotato yield and quality loss

(B). One, jumbo, and canner

yields (front to back) decrease as

Palmer amaranth density

increases (left to right) (C).

Palmer amaranth shades

sweetpotato vines three weeks after

transplanting (A). The effect of

Palmer amaranth density on light

interception (B). The effect of

Palmer amaranth light interception

on sweetpotato yield loss (C).

Figure 3. Palmer Amaranth Light Interception

Palmer amaranth density (plants m-1 crop row)

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7

● Jumbo__ __ predicted jumbo R2=0.97

I=259(85) A=100(8)

▪ No.1_____ predicted No.1 R2=0.95

I=86(21) A=100(12)

Marketable

----- predicted marketable R2=0.95

I=121(34) A=90(9)

Sw

eetp

ota

to y

ield

lo

ss (

%) B

A

Palmer Amaranth (Amaranthus palmeri) Interference in SweetpotatoStephen L. Meyers, Katie M. Jennings, David W. Monks, and Jonathan R. Schultheis

- Department of Horticultural Science -

Literature Cited: Bensch, C. N., M. J. Horak, and D. Peterson. 2003. Interference of redroot pigweed (Amaranthus retroflexus), Palmer amaranth (A. palmeri), and common waterhemp (A. rudis) in soybean. Weed Sci. 51:37-43. Burke, I. C., M. Schroeder, W. E. Thomas, and J. W. Wilcut. 2007. Palmer amaranth interference and seed production in peanut. Weed Technol. 21:367-371. Cousens, R. 1985. A simple model relating yield loss to weed density. Ann. Appl. Biol. 107:239-252. Keeley, P. E., C. H. Carter, and R. J. Thullen. 1987. Influence of planting date on growth

of Palmer amaranth (Amaranthus palmeri). Weed Sci. 35:199-204. Massinga, R. A., R. S. Currie, M. J. Horak, and J. Boyer Jr. 2001. Interference of Palmer amaranth in corn. Weed Sci. 49:202-208. Massinga, R. A., R. S. Currie, and T. P. Trooien. 2003. Water use and light interception under Palmer amaranth (Amaranthus palmeri) and corn competition. Weed Sci. 51:523-531. [NCDA & CS] N. C. Dept. of Agr. and Consumer. Services. 2009. North Carolina Agricultural Statistics. Raleigh, NC: NC Dept of Agr, p. 82. Sellers, B. A., R. J. Smeda, W. G. Johnson, J. A.

Kending, and M. R. Ellersieck. 2003. Comparative growth of six Amaranthus species in Missouri. Weed Sci. 51:239-333. [USDA-NASS] U.S. Department of Agriculture- National Agricultural Statistics Service. 2009. 2007 Census of Agriculture. Washington D.C.: U.S. Department of Agriculture. Webster, T. M. 2006. Weed survey-southern states. Proc. South Weed Sci. Soc. 59:260-277.

Figure 1. Palmer Amaranth Height, Width, and

Shoot Dry Biomass

A

D

A

Jumbo

One

Canner

0 0.5 1.1 1.6 3.3 6.5

Palmer amaranth density (plants m-1 crop row)

C

0

0.5

1

1.5

2

2.5

0 1 2 3 4 5 6 7

Palmer amaranth density (plants m-1 of row)

Sho

ot

dry

bio

mas

s (k

g)

C

Palmer amaranth biomass plant-1

---- predicted biomass plant-1 Y = -0.15x + 1.18 R2 = 0.98

Palmer amaranth biomass m-1

____ predicted biomass m-1 Y = -0.12x2 + 0.95x + 0.19 R2 = 0.99

Palmer amaranth density (plants m-1 of row)

Pal

mer

am

aran

th h

eight

and

wid

th

(cm

)

0

50

100

150

200

250

0 1 2 3 4 5 6 7

B

Palmer amaranth height

____ predicted height Y = -1.56x2 + 12.85x + 171.02 R2 = 0.65

Palmer amaranth width

---- predicted width Y = -12.67x + 151.21 R2 = 0.94

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7

● 6-7 WAP__ __ predicted light interception 6-7 WAP R2=0.59 I=278 (90) A=71 (4)

■ 10 WAP____ predicted light interception 10 WAP R2=0.80 I=197 (69) A=88 (8)

▲ 13-14 WAP

----- predicted light interception 13-14 WAP R2=0.81 I=184 (38) A=76 (4)

Palm

er

am

ara

nth

lig

ht

inte

rcepti

on (

%)

Palmer amaranth density (plants m-1 row)

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7

● 6-7 WAP__ __ predicted light interception 6-7 WAP R2=0.59 I=278 (90) A=71 (4)

■ 10 WAP____ predicted light interception 10 WAP R2=0.80 I=197 (69) A=88 (8)

▲ 13-14 WAP

----- predicted light interception 13-14 WAP R2=0.81 I=184 (38) A=76 (4)

Palm

er

am

ara

nth

lig

ht

inte

rcepti

on (

%)

Palmer amaranth density (plants m-1 row) B

6 WAP

13 WAP

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80

● Jumbo__ __ predicted jumbo yield loss Y=1.35x -1.93 R2=0.99

▪ No.1____ predicted No.1 yield loss Y=1.12x - 6.53 R2=0.86

▲ Marketable

---- predicted marketable yield loss Y=1.11x - 4.5 R2=0.93

Palmer amaranth light interception 6 to 7 wk after planting (%)

Sw

eetp

ota

to y

ield

lo

ss (

%)

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80

● Jumbo__ __ predicted jumbo yield loss Y=1.35x -1.93 R2=0.99

▪ No.1____ predicted No.1 yield loss Y=1.12x - 6.53 R2=0.86

▲ Marketable

---- predicted marketable yield loss Y=1.11x - 4.5 R2=0.93

Palmer amaranth light interception 6 to 7 wk after planting (%)

Sw

eetp

ota

to y

ield

lo

ss (

%)

CPalmer amaranth light interception 6 WAP (%)