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Publication #SS-AGR-288

Spiny Amaranth (Spiny Pigweed) Control in Pastures1

Jason Ferrell and Brent Sellers2

Figure 1. 

Spiny amaranth (Amaranthus spinosus), also known as spiny pigweed, is very common throughout Florida (Figure 1). This summer annual species is often observed in pastures, particularly in bareground areas (near feeding pens and water troughs). This weed seems to thrive in well-worn, highly compacted areas where stockings rates are high and desirable grasses are few. If left unchecked, spiny amaranth can eventually take over entire pastures (Figure 2).

Figure 2. 

Spiny amaranth is particularly troublesome because sharp spines proliferate on the stem (Figure 3). This greatly deters grazing around the plant because animals avoid the sharp spines. Also, this weed is an abundant seed producer with well over 100,000 seeds per plant produced each year. The seeds germinate throughout the warm summer months and each rainfall event results in another flush of spiny amaranth plants.

Figure 3. 

Since spiny amaranth seed germinates so frequently, any control measure will generally only last a few weeks before a new flush of seedlings overtakes the area once again. Therefore, it is important to determine if herbicides that provide soil residual activity can be used to provide long-term control of spiny amaranth. Conversely, if residual control cannot be obtained, then low-cost options must be found so that multiple applications can be made each season.

Control

The herbicides Telar (chlorsulfuron) and Milestone and GrazonNext HL (both possessing aminopyralid as the active ingredient) have been shown to provide extensive residual control of some weeds. Therefore, these herbicides were chosen to determine if they could adequately control spiny amaranth for an extended period of time. Additionally, these herbicides do not possess any grazing restrictions for beef or dairy animals.

It was observed that Telar and Milestone provided excellent spiny amaranth control at 1 month after treatment (Table 1). However, by 3 months, multiple seedlings had germinated and had resumed growth in the treated area. Therefore, neither of these herbicides provided sufficient residual control.

Since long-term control cannot be obtained with these herbicides, low (less expensive) use rates were explored. It was observed that low rates of GrazonNext HL and Telar were effective on spiny amaranth (Table 2). Therefore, for only a few dollars per acre, Telar can be used to manage this weed. Since long-term control will not be obtained, 2 or 3 applications per season should effectively manage spiny amaranth for the entire season.

Although Telar is very effective on spiny amaranth, there are few other weeds that it can control. Ragweed, coffeeweed, mexican tea (Jerusalem oak), tropical soda apple, and thistle will not be controlled with Telar. Conversely, GrazonNext HL is excellent on each of these weeds (depending on the application rate). Therefore, Telar is ideal for areas where spiny amaranth is the dominant species, but GrazonNext HL would be a better choice for areas that contain a mixture of different weeds.

Tables

Table 1. 

Control of spiny amaranth with Telar and Milestone.

Herbicide

Rate

Spiny amaranth control (%)

$/A

   

1 MAT1

3 MAT

Telar

0.5 oz/A

93

50

$10

Telar

0.75 oz/A

95

60

$15

Milestone

7 fl. oz/A

90

50

$20

1Data collected at 1 and 3 "months after treatment" (MAT).

Table 2. 

Control of spiny amaranth with Telar and GrazonNext HL.

Herbicide

Rate

Spiny amaranth control (%)

$/A

   

1 MAT1

 

Telar

0.5 oz/A

95

$10

Telar

0.3 oz/A

95

$7

Telar

0.1 oz/A

94

$3

GrazonNext HL

24 oz/A

91

$8

GrazonNext HL

18 oz/A

89

$6

1Data collected at 1 "month after treatment" (MAT).

Footnotes

1.

This document is SS-AGR-288, one of a series of the Agronomy Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date October 2007. Revised December 2010 and September 2012. Visit the EDIS website at http://edis.ifas.ufl.edu.

2.

Jason Ferrell, associate professor, Agronomy Department; Brent Sellers, associate professor, Agronomy Department, Range Cattle Research and Education Center, Ona, FL; Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611.

The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS does not guarantee or warranty the products named, and references to them in this publication do not signify our approval to the exclusion of other products of suitable composition. All chemicals should be used in accordance with directions on the manufacturer's label.


The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other UF/IFAS Extension publications, contact your county's UF/IFAS Extension office.

U.S. Department of Agriculture, UF/IFAS Extension Service, University of Florida, IFAS, Florida A & M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Nick T. Place, dean for UF/IFAS Extension.