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Webber, C.L. III. 1993. The effects of metolachlor and trifluralin on kenaf yield components. p. 413-416. In: J. Janick and J.E. Simon (eds.), New crops. Wiley, New York.

The Effects of Metolachlor and Trifluralin on Kenaf Yield Components

Charles L. Webber III*

    1. Plant Injury and Weed Control
    2. Plant Populations
    3. Plant Heights
    4. Stalk Yields
  5. Table 1
  6. Table 2

While kenaf (Hibiscus cannabinus L.) shows great promise as a new source of fiber, commercialization depends on successful development of production systems. Researchers have reported that kenaf is a good competitor with weeds once the plants are of sufficient size to shade the ground (Burnside and Williams 1968; Orsenigo 1964), yet weeds can significantly reduce kenaf yields. Weed control therefore becomes an important consideration in obtaining optimum kenaf yields.

Williams (1966) reported that weed competition, with moderate weed pressure, reduced stalk yields during one season by an average of 1.0 t/ha. Weed competition in a three year Nebraska study significantly reduced yields by an average of 9.0 t/ha (69%) and reduced plant height and stalk diameter (Burnside and Williams 1968).

Presently, no herbicides are registered for kenaf in the United States. Literature examining the effects of preemergence herbicides on kenaf development and stalk yields is limited. Many herbicides originally evaluated for use in kenaf production are either no longer available, phytotoxic to kenaf, or reduce kenaf populations (Orsenigo 1964; Williams 1966; Burnside and Williams 1968). Two efficacious herbicides that have registration potential are trifluralin and metolachlor; trifluralin has been the standard herbicide used by kenaf researchers (White et al. 1970).

Burnside and Williams (1968) tested seven herbicides and found that kenaf was most tolerant to trifluralin, which also provided excellent weed control. However trifluralin, at 2.2 kg/ha, significantly reduced kenaf yields by 3.9 t/ha (25%) during the first year; although stalk heights and diameters were unaffected by the application of trifluralin. Orsenigo (1964) reported a 50% phytotoxicity and a 50% stand reduction when trifluralin was applied to kenaf at 6.7 kg/ha, but 100% tolerance and no stand reductions when applied at 2.2, 3.4, and 4.5 kg/ha. In south Texas, trifluralin, at 0.9 and 1.7 kg/ha, and metolachlor at 3.4 kg/ha provided excellent (90%) grass control while acceptable (80%) total weed control was obtained with metolachlor at 3.4 kg/ha (Hickman and Scott 1989). Trifluralin did reduced stalk yields at the rates tested. In Mississippi, metolachlor (3.0 kg/ha) gave no visual injury to the kenaf, although stalk yields may have been reduced (Kurtz and Neill 1990).

As the commercial production of kenaf in the United States grows closer, weed control strategies must be developed and the best herbicides identified and registered. The objective of this research was to determine the effects of metolachlor and trifluralin on kenaf plant development and stalk yields.


A two-year field plot study was conducted at Lane, Oklahoma on a Bernow fine sandy loam, 0 to 3% slope, (fine-loamy, siliceous, thermic Glossic Paleudalf). Fertilizer was applied and incorporated prior to herbicide application at a rate of 168-72-139 kg/ha (N-P-K). Trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl) benzenamine] and metolachlor [2-choro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl) acetamide] were applied at 0.56, 1.12, and 2.24 kg ai/ha using fan nozzles at 187 liters/ha. The trifluralin treatments were incorporated twice to a depth of 5 to 7.5 cm, with the second incorporation at 90° to the first. A combination secondary tillage tool with cultivator shovels, cutting blades, spike tooth harrow, and rolling baskets was used for the trifluralin incorporation and to prepare the seedbed for all treatments prior to planting. Metolachlor was applied after seedbed preparation and prior to planting. The experiments also included a weed-free (kept as such via handweeding) and weedy check treatments.

Plots were 3 m wide (four 76-cm rows), 6 m long, and were oriented in a east-west direction. Kenaf cultivar 'Tainung #1' was planted on June 22, 1989 and June 12, 1990. All plots were planting the same day as herbicide application.

Crop injury ratings were collected at two and four weeks after planting using a 0 to 5 rating system; 0 represents no visual injury, 5 represents crop death. Grass and broadleaf weed control ratings were collected at four weeks after planting, using a 0 to 10 rating system; 0 represents no weed control, 10 represents 100% weed control. Kenaf plant populations, plant heights, and stalk data were collected at harvest. Kenaf plots were hand-harvested 18 weeks after planting on Oct. 23, 1989 and Oct. 17, 1990. A 2.25 m2 (1.5 by 1.5 m) quadrant was harvested from the center of the second and third row of each plot. Plant counts from the harvest quadrant were used to determine plant populations. The harvested plants were cut at ground level and fresh weights determined. Three plants were randomly selected from the harvested material for plant height. Leaves, flowers and flower buds were removed from the stalks and weighed separately before and after samples were oven dried at 66deg.C for 48 h. The fresh and oven dry weights of the three plants were used to determine the percent moisture of the plants and the percent stalks by weight. The percent plant moisture and percent stalks were used to convert the fresh weight of the 2.25 m2 quadrant sample to dry weight of stalks. Stalk yields are based on oven dry weights.

In both years, the experiments were randomized complete block designs with four replications. Crop injury and weed control data were converted to percentages and transformed using an arcsin transformation before analysis (Snedecor and Cochran 1967).

Rainfall during the growing season, from planting to harvest, was 6.1 cm below and 9.2 cm above the 20-year average rainfall for 1989 and 1990 respectively.


Plant Injury and Weed Control

No visual crop injury was observed during 1989 or 1990 as a result of the herbicides applied at the given rates (data not shown). The only year by treatment interaction detected was reflected in the degree of grass weed control, which was less in 1990 by metolachlor (Table 1). Metolachlor at 0.56 kg/ha, in 1990, was the only herbicide treatment in either year that had less grass weed control than any other herbicide treatment (Table 1). Broadleaf weed control data showed no differences in weed control (Table 1).

The primary weeds present during both years were large crabgrass [Digitaria sanguinalis (L.) Scop.] and tumble pigweed (Amaranthus albus L.). Both these weed species were present at moderate populations.

Plant Populations

Except for metolachlor at 0.56 kg/ha and trifluralin at 1.12 kg/ha all herbicide treatments reduced plant populations compared to the weed-free plots (Table 2). No kenaf population differences were detected between the herbicide treatments or between the herbicide treatments and the weedy check plots (Table 2). No weed control by year interactions were detected for plant populations, plant heights, or stalk yields as a result of the combined analysis of the eight weed control treatments and the two years. Populations, when combined over weed control treatments, were greater in 1990 than those in 1989 (Table 2). The differences in populations were attributed to less than ideal planting conditions and less rainfall in 1989 compared to 1990.

Plant Heights

Plant heights were greater in 1990 (311 cm) than in 1989 (176 cm) when combined over all weed control treatments (Table 2). Increased height in 1990 may in part be from an earlier planting date (10 days) and 15.3 cm greater seasonal rainfall. No differences in plant height were detected between herbicide rates within either trifluralin or metolachlor (Table 2).

Stalk Yields

Stalk yields were greater in 1990 (21.3 t/ha) than 1989 (5.2 t/ha) (Table 2). An earlier planting date in 1990 (10 days) and greater seasonal rainfall contributed to greater stalk yields. No differences in stalk yields were detected between any of the weed control treatments indicating that rates of trifluralin and metolachlor which decrease plant populations and plant height did not significantly reduce stalk yields (Table 2).


Trifluralin and metolachlor provided excellent (>90%) weed control of moderate weed populations. The herbicides did not induce visual injury or reductions in stalk yields, though plant populations were adversely affected. The moderate weed populations did not reduce stalk yields, but weed interference did reduce kenaf heights and populations. Trifluralin and metolachlor are promising for use in kenaf production. The probability of expanding the registration label of these herbicides to include use in kenaf remains a serious problem to overcome.


*I am indebted to David A. Iverson, Research Technician, Agricultural Research Service, South Central Agricultural Research Laboratory, Lane, Oklahoma for field plot work, data entry and data analysis.
Table 1. Influence of herbicide application on the percentage of grass and broadleaf weed control in 1989 and 1990.

Weed control (%)
Grass Broadleaf
Herbicide treatment Rate (kg/ha) 1989 1990 1989 1990
Trifluralin 0.56 98az 93b 99a 99a
1.12 99a 95b 98a 99a
2.24 100a 96b 100a 100a
Metolachlor 0.56 99a 70c 99a 98a
1.12 99a 91b 100a 97a
2.24 100a 96b 100a 100a
Weedy Check --- 0b 0d 0b 0b
Weed-Free --- 100a 100a 100a 100a
zMeans within each column followed by the same letters are not significantly different at the 0.05 level using LSD.

Table 2. Influence of trifluralin and metolachlor on kenaf stand establishment, heights, and stalk yields for 1989 and 1990.

Plant pop.
Plant heights
Stalk yields
Trifluralin 0.56 116 245 13.6
1.12 137 240 13.7
2.24 115 235 13.2
Metolachlor 0.56 134 251 13.6
1.12 117 248 13.1
2.24 117 253 13.5
Weedy Check --- 124 232 12.2
Weed-Free --- 154 247 13.3
LSD (0.05) 26 12 NS
Across all herbicide treatments
1989 107 176 5.2
1990 147 311 21.3
LSD (0.05) 13 6 1.0

Last update April 23, 1997 aw