Henbit (Lamium amplexicaule L.) control and forage legume tolerance to selected postemergence herbicides

0261-2194(95)00110-7

Henbit (Lamium amplexicaule L.) control and forage legume tolerance to selected postemergence herbicides W. J. Grichar*+, G. W. Ever&, B. A. Besler* and A. J. Jaks* *Texas

Agricultural

Experiment

Station,

Yoakum,

TX 77995,

USA,

and

*Texas

Agricultural

Experiment Station,

Over-ton, TX 75684, USA

Field studies were conducted one annual

from 1990 through

1992 to evaluate

medic species to selected postemergence

herbicides.

the tolerance

of four clover species and

Circle Valley medic, arrowleaf

clover,

clover, red clover and sweetclover were injured by 2,4-D and MCPA while bentazon at 0.8 and I.7 kg ha-’ caused consistent injury only to sweetclover. 2,4-DB at 2.2 kg ha-’ resulted in consistent injury to annual medic, arrowleaf clover, crimson clover, and sweetclover. Pronamide caused no legume injury while MCPA injured crimson clover, red clover and sweetclover. crimson

Keywords: herbicide;

crop suppression;

Emphasis on reducing nitrogen fertilizer requirements, increasing forage quality, and developing year around grazing systems has stimulated interest in intensive legume-grass pasture management systems in the

southeastern USA. Cool-season annual clovers are the primary forage legumes used with warm-season perennial grass systems in this area. Each autumn, annual clovers must be established from seed. Weed competition reduces clover stands, seedling growth, and nodulation which hinders early forage production and Nz-fixation. Weeds can also be toxic to livestock (Carlisle, Watson, and Cole, 1980) and are a major problem in clover seed production (Lee, 1985). Since weed stands are not predictable, herbicides applied postemergence are considered more cost effective than herbicides applied pre-emergence. Only propham, pronamide, and 2,4-DB were registered for early postemergence use on all clover species (Lee, 1985). However, propham is no longer available in the USA. Germination and seedling growth of arrowleaf clover (Trifolium vesiculosum Savi .) were affected less by 2,4DB than by several other postemergence herbicides (Smith, 1975). ‘Amclo’ arrowleaf clover growth was not visibly reduced by pronamide, bentazon, and 2,4-DB leaf stage applied at the two- to three-trifoliolate (Smith and Powell, 1979). Chlorpropham and 2,4-D reduced growth slightly (Smith and Powell, 1979). Arrowleaf clover, 5-15 cm tall, was tolerant to 2,4-DB at -0.56 kg ha-’ applied in early spring but suffered some yield reduction from MCPA, 2,4-D, bromoxynil, and dicamba (Conrad and Stritzke, 1980). Evers, Grichar, Pohler, and Schubert (1993) reported that field studies conducted in Texas to evaluate the tolerance of several clover species to selected posttAuthor to whom correspondence

should be addressed

winter cover crop

emergence herbicides indicated 2,4-D at 0.8 and 1.7 kg ha& injured rose clover (Trifolium hirtum All.) and berseem clovers (Trifolium alexandrinum L.), while a high rate of 2,4-D injured subterranean clover (Trifofium subterranean L.). Bentazon and pronamide did not injure rose or subterranean clover; however, pronamide at 3.4 kg ha-’ caused 18% injury to berseem clover when rated 72 days after treatment. In the greenhouse, 2,4-DB and bentazon applied at the four- to five-leaf stage reduced growth of arrowleaf, crimson (Trifolium incarnatum L.), subterranean, red (Trifolium pratense L.) and white (Trifolium repens L.) clovers by 30-40% (Nichols, Miller and Wells, 1982). In field studies, dicamba, 2,4-DB, and combinations of the two were applied in early April to the same species as above (Nichols et al., 1982) plus common vetch (Vi& sutiva L.) (Griffin, Watson, Knight and Cole, 1984). Vetch was killed by all treatments. White and crimson clovers were the most tolerant (5-15% injury) to dicamba at 0.15 and 0.28 kg ha-‘. Dicamba at 0.6 kg ha-’ reduced vigor of white clover by 50% and red and arrowleaf clovers by 90%. Bentazon or 2,4-DB at 1.1 kg ha-’ did not injure arrowleaf clover and subterranean clover (Smith and Powell, 1979). Subterranean clover has been rated tolerant to bentazon, susceptible to 2,4-D, and moderately susceptible to 2,4-DB (Hawton, Johnson, Loch, Harvey, Marley, Hazard, Bibo and Walker, 1990). We found little data on phytotoxicity of postemergence herbicides to some of the cool-season annual legume cultivars and species being evaluated and used in the southwestern USA. Selective postemergence herbicides to control weeds with minimal legume injury are needed. The objectives of this study were: (1) evaluate Circle Valley medic, arrowleaf clover, crimson clover, red clover, and sweetclover

Crop Protection

1996 Volume

15 Number 1

55

Henbit control

and tolerance

to herbicides:

W.J. Grichar et al.

on 6 January. Herbicides were not applied in the 1991 test until mid-February, because of extremely heavy rains from December to early February (446.7 mm) which prevented entry into the field. Clover growth during the period of heavy rain and overcast conditions was extremely slow. Henbit (Lamium amplexicaule L.) was the primary weed and the population was considered uniform enough to rate only in crimson clover, red clover and sweetclover plots. Clover injury (% stunting or kill) and weed control were rated visually 12-60 days after application. Ratings were based on a scale of 0 = no cover injury or weed control; to 100 = complete clover or weed kill. Untreated plots were not hand-weeded.

tolerance to selected postemergence herbicides; and (2) to determine the control of henbit with these herbicides.

Materials and methods General procedure

All plots were located on prepared seedbeds at the Texas Agricultural Experiment Station near Yoakum, Texas. Agronomic practices are reported in Table 1. Soil type was a Denhawken-Elmendorf clay loam (fine, montmorillonitic, hyperthermic Vertic Ustochrepts) with a pH of 7.8. ‘Yuchi’ arrowleaf clover and Circle Valley medic were planted in 1991 and 1992 while ‘Kenland’ red clover, ‘Dixie’ crimson clover, and ‘Hubam’ sweetclover were planted in separate studies from 1990 to 1992. All forage legumes were seeded at 16.8 kg ha-‘. Seeds were drilled 1.2 cm deep in 18 cm rows on 1.8 by 7.6 m plots with four replications. Two randomly selected 0.4 m* quadrants were handcut from each plot. Cuttings were taken on 6 March and 15 April 1990; 24 March and 22 April 1991; and 24 February and 19 April 1992. Weeds and clover were hand-separated to determine forage composition, and dried for 72 h at 65°C. After samples were dried, clover yields on a dry matter basis were determined. Herbicides tested included bentazon (Basagran 4E, BASF Corporation, Germantown, TN); pronamide (Kerb 5OW, Rohm and Haas, Philadelphia, PA); 2,4DB dimethylamine salt (Butyrac 200, Rhone-Poulenc, Research Triangle Park, NC); 2,4-D (2,4-D Amine4, Riverside/Terra Corporation, Sioux City, IA); and MCPA ester (Rhonox, Rhone-Poulenc, Research Triangle Park, NC). Herbicide rates included 2,4-D at 0.8 and 1.7 kg ha-‘, 2,4-DB at 1.1 and 2.2 kg ha-‘, bentazon at 0.8 and 1.7 kg ha-‘, pronamide at 1.7 and 3.4 kg ha-’ and MCPA at 0.6 and 1.1 kg ha-‘. MCPA was not applied to Circle Valley medic or arrowleaf clover. Herbicides were applied with a compressed-air bicycle sprayer in a spray volume of 190 I ha-’ at 180 kPa. Agridex (Helena Chemical Company, 5100 Popular Ave, Memphis, TN 38137) was included with bentazon treatments. Herbicides in 1990 were applied on 10 December, while in 1992 herbicides were applied

Statistical analysis Each species was evaluated separately using a randomized complete block design with four replications. All data were subjected to analysis of variance with year, herbicide treatment and year X herbicide treatment interaction tested for significance at the 0.05 level (SAS Institute, 1985). Means were separated by Fisher’s

Table 2. Henbit control at 30 days of herbicide application in crimson, red and sweetclover when treated with various postemergence herbicides Control %* Rate (kg ha-‘)

Treatment

-

Untreated 2,4-D 2,4-D 2,4-DB 2,4-DB Bentazon Bentazon Pronamide Pronamide MCPA MCPA LSD (0.05)

0.8 1.7 1.1 2.2 0.8 1.7 1.7 3.4 0.6 1.1

Crimson clover 0 74 83 69 79 82 78 78 91 58 81 16

Red clover

Sweet clover

0 30 44 32 40 5.5 56 55 44 38 55 23

0 79 78 66 88 79 81 85 83 64 70 16

*% Control based on scale of 0 = no control to 100 = complete control. Data

were combined

over years since there was no treatment x year interaction

Table 1. Management practices for herbicide evaluations at Yoakum, TX in 1990, 1991, and 1992 Variable Planting date Fertilizer rate

1990

(kg ha-‘)

Spray date Air temperature at herbicide application Soil temperature at herbicide application (mm) Rainfall 1 week prior to applicaton (mm) Rainfall 1 week after application Crop leaf stage when sprayed: Arrowleaf clover Crimson clover Red clover Sweetclover Annual medic Weed height (cm) Harvest dates: First Second

56

Crop Protection

1996 Volume

15 Number 1

1991

16 October 1990 0-69-O 10 December 1990 13°C 3.3 0

12-20 7-14 4-6 5-6 6 March 15 April

1991 1991

27 November 0-79-O 19 February 16°C 13°C 33.0 68.3

1992 1991 1992

29 October 1992 0-69-O 6 January 1993 11°C 12°C 8.1 24.9

16-20 14-18 4-8 4-6 26-32 6-8

12-16 12-16 6-10 6-8 18-24 6-8

24 March 1992 22 April 1992

24 February 1993 19 April 1993

Henbit control and tolerance to herbicides: W.J. Grichar et al. Protected Least Significant Difference level of significance.

Results

(LSD) at the 5%

interaction for all data except henbit control in crimson clover, red clover and sweetclover. Consequently, data from each year were presented separately unless stated otherwise. Henbit populations in the annual medic and arrowleaf clover studies were low and not uniform enough to warrant a rating. After the first harvest, henbit never was a problem in any of the clover studies.

and discussion

Analysis

of variance

revealed

a treatment

by year

Table 3. Herbicide injury to five forage legumes with various postemergence herbicides Injury (%)* Circle Valley medic Arrowleaf clover

Red clover

Crimson

clover

Sweetclover

Treatment

Rate (kg ha-‘)

1991 12 D

1992 20 D

1991 12 D

1992 20 D

1990 64 D

1991 12 D

1992 20 D

1990 64 D

1991 12 D

1992 20 D

I990 64 D

1991 I2 D

1992 20 D

Untreated 2,4-D 2,4-D 2,4-DB 2,4-DB Bentazon Bentazon Pronamide Pronamide MCPA MCPA

0.8 1.7 1.1 2.2 0.8 1.7 1.7 3.4 0.6 1.1

0 66 93 99 100 6 20 0 (I _

0 45 56 6 20 5 5 0 I -

0 79 80 _ 95 1 33 3 0 _ -

0 36 79 18 0 5 6 0 _

0 89 95 45 53 8 5 3 8 48 40

0 36 78 3 9 0 0 11 34

0 60 73 4 8 6 3 3 4 -

0 100 100 23 35 8 I8 0 13 40 52

0 51 81 26 53 0 0 33 55

0 24 48 11 30 1 4 3 0 _

0 100 100 86 98 23 80 0 5 87 100

0 85 95 96 99 68 97 1 I9 85 94

0 84 90 43 74 43 38 49 I4 _

19

16

20

I8

20

16

12

22

13

I1

I6

15

30

LSD (0.05)

*% Injury based on a scale of 0 = no injury IO 100 = complete kill D, daysafter herbicide treatment

Untreated 2. 4 -D 0.8 2. 4 - D 1.I LSD = 0.05 Cut 1 ~__~___~_

2, 4 - DB 1.1 2, 4 - DB 2.2

cut 2

I-------I

Tot*,

I---------I

1

Bentazon0.8 Bentazon 1.7 Pronamide

1.7

Pronamide

3.4

Untreated 2, 4 - D 0.8 2, 4 - D

1.7

2. 4 - DB 1.1

E

LSD

= 0.05

cut

1 I-----I

cut 2

I------I

Total l--------I

2, 4 - DB 2.2

Beatazon 0.8 Bentazon 1.7 Pronamide I .I Pronamide3.4 0

1

2

3

4

5

6

Thousands (kg ha-‘) fqqfj 1991 (cut 1)

rJ

1991 (cut 2)

m

1992 (cut 1)

W

1992 (cut 2) I

Figure 1. Influence of postemergence

herbicides on Circle Valley medic yields for 1991 and 1992

Crop Protection 1996 Volume 15 Number 1

57

Henbit control and tolerance to herbicides: W.J. Grichar et al.

MCPA provided less than 70% control. Evers et al. (1983) reported greater than 90% henbit control in rose clover with 2,4-D; however, in berseem clover (Trifolium alexandrinum L.), henbit control was 80-90% with 2,4-D. The early season henbit growth rate, like many annual weed species, exceeds the growth rate of many clover species (personal observation). Therefore, early season weed control is essential to maximize clover yields.

For ease of reporting, test years were designated as the year in which the clovers were planted, although the herbicide treatments were applied and clover plots were harvested during the following year. Henbit control

With crimson clover, henbit control was 17-33% better with the high rate of pronamide than the low rates of 2,4-D, 2,4-DB, or MCPA (Table 2). Rate effects were apparent only with MCPA. The high rate of MCPA improved control by 23% over the lower rate of MCPA. Henbit control in the red clover was less than 60% with all herbicides tested. Rate effects were not apparent with any of the herbicide treatments. The stands of red clover were thinner and slower growing than the other clovers at the time of herbicide spray application. This was due in part to wet, overcast weather soon after clover emergence up until herbicide application. Henbit population was heavy and henbit growth was fast during this period. This may explain some of the reduced henbit control observed in the red clover plots. With sweetclover, the high rates of 2,4-DB, bentazon, and both rates of pronamide provided 81-88% henbit control. The low rates of 2,4-DB and

Legume injury

Circle Valley medic injury from 2,4-D varied from 45 to 93% when rated 12 and 20 days from treatment (Table 3). In 1991, a rate response to 2,4-D was apparent; however, in 1992, this rate response was not apparent. 2,4-DB resulted in complete medic kill in 1991, while in 1992 only the high rate resulted in significant injury. Previous work indicated 2,4-D and 2,4-DB were highly toxic to annual medics (Grichar, Evers, Pohler and Schubert, 1993). Bentazon at the high rate caused 20% injury in 1991, but only 5% injury in 1992. The low rate of bentazon and pronamide caused no annual medic injury. Earlier work on Circle Valley medic in Texas reported that the high rate of bentazon caused considerable injury (Grichar et al. 1993).

Untreated 2, 4 - D 0.8 2.4-Dl.7

Z 01 ti

2, 4 - DB 2.2

LSD = 0.05

Bentazon 0.8

cut 1 Icut 2 I-

,. I + I Total I- _i..--_---,

Bentazon 1.7

.--_s ._w_

Pronamide 1.7 Pronamide 3.4

Untreated 2, 4 - D 0.8 2,4-D

LSD = 0.05

1.7

cut 1 I-+ cut 2 I-i---I

2, 4 - DB 2.2

Total

Bentazon 0.8

+--I

Bentazon 1.7 Pronamide 1.7 Prom&de 3.4

i 2

4

3

5

6

Thousands (kg ha-‘) gig 1991 (cut 1)

a

1991 (cut 2)

m

1992 (cut 1)

m

1992 (cut 2) t

Figure 2. Influence of postemergence

58

herbicides on arrowleaf clover yields for 1991 and 1992

Crop Protection 1996 Volume 15 Number 1

Henbit control and tolerance to herbicides: W.J. Grichar et al.

Arrowleaf clover was severely injured (>75%) with 2,4-D at 1.7 kg/ha-’ in both years while the low rate resulted in severe injury (79%) in 1991 and moderate injury (36%) in 1992 (Table 3). 2,4-DB at 2.2 kg ha-’ caused 95% injury to arrowleaf clover in 1991 and 18% in 1992. Conrad and Stritzke (1980) reported that arrowleaf clover was tolerant to 2,4-DB at 0.56 kg ha-‘. Bentazon, at the high rate, injured clover in 1991 but no injury was noted in 1992. Pronamide did not cause any arrowleaf clover injury. Red clover was injured by 2,4-D in each year while 2,4-DB injured clover in one of the two years (Table 3). 2,4-D at 0.8 kg ha-’ caused 36-89% injury to red clover while the high rate of 2,4-D caused 73-95% clover injury. 2,4-DB and MCPA resulted in 240% injury in 1990 but ~35% injury in other years. Bentazon or pronamide did not injure red clover. Nichols et al. (1982) reduced the growth of red clover by 30-40% with 2,4-DB and bentazon when applied to the four- to five-trifoliolate in greenhouse studies. Crimson clover was injured by 2,4-D, 2,4-DB, and MCPA in all years of the study (Table 3). In field studies, Griffin et al. (1984) reported 2,4-D at 1.12 kg ha-’ caused 40% injury to crimson clover. Bentazon at 0.8 kg ha-’ injured crimson clover in 1991 but caused no injury in any of the other years. Pronamide produced no crimson clover injury. Pronamide caused the least amount of injury to sweetclover (Table 3). In 1991, the high rate of

pronamide resulted in 19% injury while in 1992 the low rate caused 49% injury. In all years, 2,4-D, 2,4-DB, bentazon, and MCPA resulted in 23-100% clover injury. Legume yield

Both rates of 2,4-DB and 2,4-D at 1.7 kg ha-’ ultimately killed the Circle Valley medic in 1991 (Figure I). Dry matter yields were reduced with 2,4-D at 0.8 kg ha over the untreated check by at least 63% at the second harvest and 82% for total annual medic production in 1991. In 1992, 2,4-D at 0.8 kg ha-’ reduced first harvest yields by 80% when compared with the untreated check (Figure I). Second harvest yields of 2,4-D treated plols at 1.7 kg ha-’ resulted in a 43% reduction of dry matter over the untreated check. Cicle Valley medic total dry matter yields were reduced by 43 and 75% when treated with 2,4-D at 0.8 and 1.7 kg ha-‘, respectively. While 2,4-DB killed annual medic in the 1991 study, first harvest yields and total dry matter production were reduced with both rates of 2,4-DB in the 1992 study. In earlier work, Grichar et al. (1993) reported that annual medics were killed by both 2,4-D and 2,4-DB. Bentazon at 1.7 kg ha-’ reduced Circle Valley medic first harvest yields in both years, but by the second harvest no differences in yield were noted. However, overall forage production from two harvests was

Untreated 2. 4 - D 0.8 t 2,4-D

1.7

2. 4 - DB 1.1 2. 4 - DB 2.2

3

Bentazon

0.8

LSD

Bentazon

1.7

= 0.05 I-_-3-_ cut 1 Cut2 I---T-----I

Pronamide

t .7

Total

Pronamide

3.4

s-1

I_-__-_----

1

MCPA 0.6 MCPA 1.1 Untreated 2. 4 - D 0.8 2. 4 -D

1.7

Bentazon

0.8

Cut 1 I----l

LSD

= 0.05

_

Bentazon

1.7

cut 2 I---I

g

Pronamide

1.7

Total

-

Pronamide

3.4

I

______ I

MCPA 0.6 MCPA

1.1

Untreated LSD

2. 4 - D 0.8 294-D 2,4-DB

-__c--“c”:f : I--’

1.1

c1

2, 4 - DB 2.2

$

Bentazon

0.8

-

Bentazon

1.7

Pronamide

1.7

Pronamide

3.4

Total

1

0

2

3

4

5

Thousands

(kg ha-‘)

(cut 1)

t 990 (cut 2)

1991 (cut 1)

1992 (cut 1)

1992 (cut 2)

of postemergence

herbicides

I

I ________ I

b

1991 (cut 2)

1990

Figure 3. influence

= 0.05

1.7

on crimson clover yields for 1990, 1991 and 1992

Crop Protection 1996 Volume 15 Number 1

59

Henbit control and tolerance to herbicides: W.J. Grichar et al.

reduced. In 1991, pronamide at 1.7 kg ha-’ reduced first harvest yields and overall dry matter production (Figure 2). However, in 1992, only pronamide at the highest rate (3.4 kg ha-‘) reduced first harvest dry matter production as well as overall production. In 1991, no arrowleaf clover was produced in plots treated with the high rate of 2,4-D or 2,4-DB (Figure 2). The low rate of 2,4-D reduced first and second harvest yields by 88 and 91% and total dry clover production by 90%, respectively, when compared to the untreated check. None of the other herbicide treatments had any effect on clover production. In 1992, all 2,4-D and 2,4-DB treatments reduced first harvest arrowleaf clover production from 41 to 97% (Figure 2). However, at the second harvest, only 2,4-D at 1.7 kg ha-’ reduced dry matter production. Total clover yields from the untreated check and plots sprayed with 2,4-D at 1.7 kg ha-’ were 22 and 73% less, respectively, than plots sprayed with the low rate of pronamide. Total clover production from plots treated with the high rate of 2,4-D was 72% less than the untreated weedy check. 2,4-DB significantly reduced total clover yield when compared to plots treated with the low rate of pronamide. The high rate of 2,4-D killed crimson clover in 1990 and 1991 (Figure 3). In 1992, the high rate of 2,4-D reduced total crimson clover production more than 50% when compared to the untreated check. The low

rate of 2,4-D reduced clover yield at all harvest dates except for the second harvest in the 1992 study. The high rate of 2,4-DB reduced first harvest yields in 1990 and 1992, while the low rate reduced first harvest yields in 1992 (Figure 3). By the second harvest in 1990 and 1992, crimson clover had recovered completely. Bentazon reduced crimson clover yield at first harvest in 1991 (Figure 3). Crimson clover recovered by the second harvest from the low rate of bentazon, however, the high rate reduced growth throughout the growing season. Bentazon did not reduce crimson clover production in 1990 or 1992. Pronamide reduced crimson clover production only at the second harvest in 1991. MCPA at the high rate reduced clover production at the first harvest in 1990 and 1991 (Figure 3). Clover treated with the high rate of MCPA in 1990 did not recover by the second harvest, therefore, total prodcution was also reduced. 2,4-D at both rates reduced total red clover yields in 1990 and 1991, while only the high rate of 2,4-D reduced yields in 1992 (Figure 4). No forage was harvested from plots treated with 2,4-D at the 1.7 kg ha-’ rate in 1991. In 1992, 2,4-D at the high rate reduced red clover production at the second harvest by 59% and total clover production by 61% when compared to the untreated check. Bentazon at 1.7 kg ha-’ affected red clover in 1990

Untreated 2. 4 - D 0.8 2.4-D 1.7 2. 4 - DB 1.1 2, 4 - DB 2.2 h

LSD = 0.05

Bentazon 0.8 Bentazon 1.7 Pronamide 1.7

cut 1 I---l Cat 2 ~---_-_--I Total

Pronamide 3.4 MCPA 0.6 MCPA 1.1 Untreated 2, 4 -D 0.8 2.4-D 1.7 s s

+-------

LSD = 0.05

Bentazon 0.8 Bentazon 1.7 Pronamide 1.7

cat 1 I----I cut2

l---l

Total I-----I

Pronamide 3.4 MCPA 0.6 MCPA 1.1 Untreated 2. 4 - D 0.8 2,4-D1.7

LSD = 0.05 cut 1 1-I cut2 I---I

2, 4 - DB 1.1 2, 4 - DB 2.2 3

Total t---J

Bentazon 0.8 Bentazoo 1.7 Pronamide 1.7

i

Pronamide 3.4 0

2

Thousadds

I

i

3

4

(kg ha-‘)

EEEI1990 (cut 1)

El

1990 (cut 2)

a

1991 (cat 1)

w

m

1992 (cut 1)

a

1992 (cut 2)

1991 (Cut 2)

Figure 4. Influence of postemergence herbicides on red clover yields for 1990, 1991 and 1992

60

5

Crop Protection 1996 Volume 15 Number 1

Henbit control and tolerance to herbicides: W.J. Grichar et al. Untreated 2, 4 - D 0.8 2. 4 - D 1.7 1.1 2.4-DB 2, 4 - DB 2.2

-

Bentazon Bentazon Pronamide Pronamide

LSD = 0.05

0.8 1.7 1.7 3.4

Cut 1 j-_-_-I Cut 2 ,______) Total

MCPA 0.6 MCPA 1.1

;;: 2

Untreated 2, 4 - D 0.8 1.7 2,4-D 1.1 2.4-DB 2, 4 - DB 2.2 Bentazon 0.8 Bentazon 1.7 Pronamide 1.7 Pronamide 3.4

I-----_-_I

LSD = 0.05 Cut 1 I__1 Cut2

I-_-I

Total

,____I

MCPA 0.6 MCPA 1.1

z *

Untreated 2. 4 - D 0.8 2, 4 - D 1.7 1.1 2, 4 - DB 2.2 Bcntazon Bentazon Pronamide Pronamide

LSD = 0.05 Cut 1 I--I Cat 2 I-_-I Total I-_-_-I

0.8 1.7 1.7 3.4 0

5

4

3

2

1

Thousands (kg ha-‘) EEEll 1990 a

Figure 5.

Influence

(cut 1)

lzi

1990 (cut 2)

EQ

1991 (cut 1)

1991 (cut 2)

m

1992 (cut 1)

[=1

1992 (cut 2)

of postemergence

herbicides

on sweetclover yields for 1990, 1991 and 1992

and 1991 while MCPA at 1.1 kg ha& affected red clover only in 1991 (Figure 4). The high rate of bentazon reduced total yields by more than 30% when compared with the untreated check in both years. In 1991, MCPA at the high rate reduced first harvest clover yield by 40% while total dry clover yield was reduced with both rates. No yield differences were noted with bentazon in 1992. Pronamide at 3.4 kg ha-’ reduced total red clover dry weight in 1990 and 1992. In each year, 2,4-D, 2,4-DB, and MCPA either killed sweetclover or significantly reduced production (Figure 5). Bentazon at 1.7 kg ha-’ reduced total clover production in 1990 and 1992. In 1991, bentazon at 0.8 kg ha-’ reduced clover production 66,57, and 60% for the first, second, and total dry clover production, respectively. Bentazon at 1.7 kg ha-’ killed sweetclover in 1991. The injury to crimson, red and sweetclover in 1991 from bentazon may have been caused in part by extremely wet conditions which prevailed from December through to early February. The cold, wet conditions resulted in little clover growth which reduced the plants’ ability to metabolize bentazon. The legume species used in this study demonstrated good safety tolerance to pronamide which is registered for general use on clovers. Bentazon caused reduced yield of crimson, red, and sweetclover one year partly due to environmental conditions.

Circle Valley medic, arrowleaf clover, crimson clover, red clover, and sweetclover were completely killed by 2,4-DB in at least 1 year of the study and suffered reduced yields in other years. 2,4-D which is registered on clovers, reduced all clover yields with complete kill at the high rate in several instances. MCPA, which had shown promise for use in earlier studies on some clover species (Conrad and Stritzke, 1980; Evers et al., 1993), reduced crimson and red clover yield when used at the high rate. Hubam sweetclover was completely killed with both rates of MCPA. References Carlisle,

R. J., Watson,

chemistry

V. H. and Cole, A. W. (1980) Canopy Weed Sci. 28, 139-142

and

of pasture weeds.

Conrad, J. D. and Stritzke, J. F. (1980) Response of arrowleaf herbicides. Agron. J. 72. 67G672

clover

to postemergence

Evers, G. W., Grichar, W. J., Pohler, C. I,. and Schubert, A. M. (1993) Tolerance of three annual forage legumes to selected postemergence herbicides. Weed Tech&. 7. 735-739 Grichar, W. J., Evers, G. W., Pohler, C. I,. and Schubert, A. M. (1993) Tolerance of four annual forage legumes to selected postemergence herbicides. Texus Agric. Exp. Srn MP-1747. 7 pp Griffin, J. L., Watson, Forage

V. H., Knight, W. E. and Cole, A. M. (1984) and 2.4-D applications. Agron.

legume response to dicamba

.I. 76, 487490

Crop

Protection

1996 Volume

15 Number

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Henbit control and tolerance to herbicides: W.J. Grichar et al. (1985) SAS Users

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Hawton, D., Johnson, I. D. G., Loch, D. S., Harvey, G. L., Marley, J. M. T., Hazard, W. H. L., Biho, J. and Walker, S. R. (1990) A

SAS Institute NC

guide to the susceptibility of some tropical crop and pasture weeds and the tolerance of some crop legumes to several herbicides. Trap. Pest M,gmr. 36, 147-150

Smith, A. E. (1975) Herbicide influence on arrowleaf clover seeding establishment. Crop Sci. 15, 539-541 Smith,

J. D. (1979) Herbicides for weed control of arrowleaf clover. Univ. Georgia, Res. Rep.

A. E. and Powell,

Lee, W. 0. (1985) Weed control. In: Clover Science and Technology (Ed. by N.L. Taylor). Agronomy 25, 295-308

during establishment

Nichols, R. L., Miller, J. D. and Wells, H. D. (1982) Tolerance of Trifolium sp. to bentazon and 2,4-DB. Proc. Seventh Trifolium Conf.

Received 3 April 1995 Revised 20 July 1995 Accepted 2 August 1995

13-14 April, MI, p. 54

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