Herbicides weed control and sesame tolerance

Several herbicides provide excellent control of weeds with minimal to no damage to sesame. However, in evaluating herbicides, there have been conflicting results, and it is difficult to sort out why some herbicides work in one area and do not work in another. Also, in some cases, at the same location, the herbicides effectively control weeds and little sesame injury is noted in one year; however, the opposite may be true the following year. With most herbicides, herbicide dose, formulation, soil texture, pH, moisture, method of incorporation, and temperature before and after application are all a factors affecting herbicide persistence (Smith, 1989). Since soil organic matter, temperature, and aeration are more favorable for microbial activity in the topsoil than in the subsoil, degradation rates may decrease if a herbicide is leached into the subsoil (Smith, 1989). Soil pH can affect degradation directly if the stability of the herbicide is dependent upon acidity or alkalinity, and indirectly via its effects on the absorption of the herbicide to the soil (Smith, 1989). Increased rates of non-biological reactions and biological processes are favored by increasing temperature, herbicide degradation rates should increase also. Adequate moisture is also essential for microbiological activity (Smith, 1989). Martin (1995) reported that rainfall amounts during germination and establishment can markedly affect herbicide phytotoxicity to sesame, a possible factor in the reported erratic behavior of many herbicides. Many herbicides will delay sesame maturity while a few herbicides will completely kill the sesame. In many of the studies mentioned below, it will be seen with some herbicides that even with severe stand reduction, sesame yields are good because the plants can compensate for open space by putting out branches with capsules. In some herbicide studies in the U.S. where multiple varieties were used, there have been differences in varietal susceptibility. Some of the clues have not been followed up because the moving baselines of new varieties has been fast, and the emphasis has always been placed on the use of the most recent released variety to use in herbicide evaluations. More work needs to be done in this area; particularly to determine whether a specific genotype may have more tolerance to a particular herbicide.

A review of sesame herbicide information from 21 countries has shown that there are approximately 16 herbicides that are used or have the potential to be used in commercial sesame production somewhere in the world (Langham et al., 2007). Some of these products are not available in the U.S. or have been discontinued. Table 1 shows the active ingredients of these 16 current herbicides that show the greatest potential for weed control in sesame production. The table does not contain herbicides such as flumioxazin that is used commercially in other parts of the world, but have resulted in considerable sesame injury in the U.S. (Grichar et al., 2001a; Grichar & Dotray, 2007).

Just as important as knowing the potential use of herbicides, it is important to note herbicides that have resulted in severe sesame injury or have had mixed results. In some cases, another application method of a herbicide in Table 2 can be toxic, e. g., glyphosate postemergence over-the-top.

Use

Preemergence

Postemergence

Postemergence-directed

Diuron

Fluchloralin

Fluometuron

Glyphosate

Linuron

Metobromuron

S-metolachlor

Pendimethalin

Trifluralin

Clethodim Diuron

Fluazifop-P-butyl

Sethoxydim

Glyphosate (only between rows or wiper application)

Potential

Acetochlor Diuron + linuron S-metolachlor + diuron S-metolachlor + linuron

Pendimethalin S-metolachlor Alachlor Acetochlor

Diuron + linuron Linuron Diuron Prometryn

Table 1. Current and potential herbicides for use in sesame.

Table 1. Current and potential herbicides for use in sesame.

There are many preemergence herbicides that have been successfully used in sesame growing regions worldwide. These would include: alachlor, diuron, fluchloralin, fluometuron, linuron, metobromuron plus metolachlor, metolachlor, pendimethalin, and trifluralin. In the U.S., the main herbicides are S-metolachlor, diuron, linuron, and alachlor. Fluchloralin and metobromuron are not available in the U.S. Glyphosate is often applied with the preemergence herbicide to control emerged weeds. Herbicides act differently under certain environmental conditions which include variability in soil texture, organic matter, temperature, pH, humidity, rainfall timing and intensity, and under different methods and timing of application (Grichar et al., 2001a; 2001b). Pendimethalin and trifluralin are particularly difficult to use with results ranging from exceptional weed control with no damage to the sesame to little or no sesame stand (Grichar & Dotray, 2007). Poor sesame stands with the use of pendimethalin or trifluralin have resulted from incorporating either of the herbicides too deep. Since sesame is planted shallow, it is difficult to properly incorporate the dinitroaniline herbicides effectively and not have the herbicides come in contact with the sesame seed or roots (Grichar & Dotray, 2007).

It is important to realize that the many preemergence herbicides reduce sesame populations, but in mechanized sesame growing, this reduction is not noticed because of the cultural practices. One of the most difficult aspects of growing sesame is getting an uniform stand. The seeds are very small as compared to other field crops such as corn, soybean, cotton, wheat, and peanuts. One of the trends in mechanized agriculture is to singulate the larger seeded crops to attain the optimum plant population. Singulation has not worked in sesame because the seeds need adjacent seeds to help emerge out of the soil. Even with seed that has over 95% germination, rarely do more than 60% of the seeds emerge (Langham et al., 2010). In addition, there are many variations in soil type and row configurations within the sesame growing areas. In order to compensate for poor land preparation, the seeding rate is increased. Sesame varieties have been selected to compensate in high populations by self-thinning and in low populations by branching (Langham 2007). Various studies have shown that the yields are comparable between the untreated check and herbicide treatments that have some stand reduction (unpublished data).

Active

Preemergence

Postemergence

Postemergence

Harvest

ingredient

over-the-top

directed

Aid

2,4-DB

Toxic

Toxic

Acetochlor

Potential

Potential

Acifluorfen

Toxic

Potential

Alachlor

Commercial

Potential

Allidochlor (CDAA)

Mixed results

Ametryn

Toxic

Amiprophosmethyl

Toxic

Asulam

Semi-selective

Atrazine

Toxic

Toxic

Benefin

Toxic

Benfuresate

Toxic

Bensulide

Selective

Bentazon

Toxic

Bifenox

Toxic

Bromoxynil

Toxic

Carbuthioate

Semi-toxic

Carfentrazone

Semi-toxic

Not effective

Chloramben

Mixed results

Chlorimuron

Toxic

Chloroxuron

Toxic

Chlorpropham (CIPC)

Mixed results

Chlorsulfuron

Mixed results

Chorthal-dimethyl

Semi-toxic

Clethodim

Commercial

Clomazone

Toxic

Clopyralid

Semi-selective

Toxic

Cloransulam

Toxic

Toxic

Dicamba

Toxic

Dichlobenil

Toxic

Dichlormate

Semi-selective

Diclosulam

Toxic

Toxic

Diethatyl

Semi-selective

Diethylacetanilide

Semi-selective

Diflufenican

Semi-toxic

Toxic

Diflufenzopyr

Toxic

Dimethenamid

Mixed results

Dinitramine

Toxic

Dinoseb

Toxic

Diphenamid

Selective

Selective

Diquat

Effective

Diuron

Commercial

Commercial

Potential

DSMA

Semi-toxic

Active

Preemergence

Postemergence

Postemergence

Harvest

ingredient

over-the-top

directed

Aid

Endothall

Toxic

Toxic

EPTC

Mixed results

Ethalfluralin

Mixed results

Fenoxaprop

Inconclusive

Fluazifop-P-butyl

Commercial

Fluchloralin

Commercial

Flufenacet

Toxic

Flumetsulam

Toxic

Toxic

Flumioxazin

Toxic

Semi-toxic

Mixed results

Fluometuron

Commercial

Semi-selective

Fluorodifen

Toxic

Fomesafen

Mixed results

Glufosinate-ammonium

Mixed results

Effective

Glyphosate

Commercial

Toxic

Mixed results

Effective

Haloxyfop

Selective

Imazapic

Toxic

Toxic

Imazethapyr

Semi-selective

Toxic

Isopropalin

Toxic

Lact of en

Toxic

Semi-toxic

Linuron

Commercial

Toxic

Potential

Mesotrione

Toxic

Methabenthiazuron

Semi-selective

Methazole

Mixed results

Metobromuron

Commercial

Metolachlor

Commercial

Metribuzin

Toxic

Metsulfuron

Mixed results

Monolinuron

Mixed results

Monuron

Selective

MSMA

Semi-toxic

Napropamide

Mixed results

Naptalam (NPA)

Toxic

Toxic

Nicosulfuron

Mixed results

Toxic

Nitralin

Mixed results

Nitrofen

Toxic

Norea

Mixed results

Norflurazon

Toxic

Oxadiazon

Semi-selective

Semi-toxic

Oxasulfuron

Toxic

Oxyfluorfen

Semi-selective

Toxic

Paraquat

Toxic

Semi-toxic

Effective

Pebulate

Semi-selective

Active ingredient

Preemergence

Postemergence over-the-top

Postemergence directed

Harvest Aid

Pendimethalin

Commercial

Potential

Perfluidone

Selective

Phenmediphan

Toxic

Piraflufen ethyl

Semi-selective

Proatryne

Selective

Profluralin

Selective

Prometryn

Toxic

Toxic

Potential

Pronamide

Toxic

Propachlor

Selective

Propanil

Semi-selective

Propazine

Mixed results

Toxic

Selective

Prosulfuron

Mixed results

Toxic

Pyraflufen ethyl

Semi-toxic

Selective

Not effective

Pyridate

Mixed results

Pyrithiobac

Toxic

Toxic

Toxic

Rimsulfuron

Selective

Toxic

Sesone

Mixed results

Sethoxydim

Commercial

Simazine

Toxic

S-metolachlor

Commercial

Potential

Sufentrazone

Toxic

Sulfonamide

Mixed results

Thiobencarb

Toxic

Triasulfuron

Mixed results

Trifloxysulfuron

Mixed results

Toxic

Toxic

Trifluralin

Commercial

Mixed results

Vernolate

Toxic

aIn the evaluation the following categories of effectiveness are used:

Commercial: used commercially in at least one country

Potential: potential to use commercially Selective to sesame: does not damage sesame

Semi- selective to sesame: some damage to sesame, but helps

Mixed results with some showing some selectivity and others showing toxicity Toxic: substantial reduction of production

Semi- toxic: enough reduction that probably cannot be used

Effective as a harvest aid

Not effective as a harvest aid

Table 2. Summary of herbicides that have been evaluated for weed control and sesame tolerancea.

Table 2. Summary of herbicides that have been evaluated for weed control and sesame tolerancea.

Until 2000, little or no research has been done on the use of postemergence herbicides in sesame (Grichar et al., 2001b). Most of the herbicide work has been at crop establishment. From initial work done in the U.S. in Arizona, several postemergence herbicides have done a very good job controlling grasses and not damaging the sesame. Grass herbicides, fluazifop-P-butyl, haloxyfop, and sethoxydim have been used successfully in many parts of the world. More recently, clethodim has proven equally good controlling both annual and perennial grasses (particularly johnsongrass) and not damaging sesame (Grichar et al., 2001b). There is a label in the U.S. for clethodim (Select Max®) use in sesame which allows spraying in all phases except flowering (Langham et al., 2010). Concerns have been raised on the use of clethodim after extensive glyphosate applications and improper clean-out of spray tanks. Sesame capsule inhibition has been noted when glyphosate carryover has been noted in spray tanks that have been used to apply clethodim. The cleaning and removal of any glyphosate residues in spray tanks after each herbicide use is essential to prevent herbicide carryover.

To date there is no postemergence over-the-top broadleaf herbicide that will control the weeds without damaging the sesame (Grichar et al., 2001b). There are products such as alachlor and metolachlor that cause minimum injury to sesame when applied postemergence, will not control emerged weeds, but will provide some soil residual activity (Grichar et al., 2001a; Grichar et al., 2009). In the case of herbicides such as diuron, sesame will recover, but the farmer will notice stunting and leaf necrosis on sesame leaves for about 10 days after herbicide application (Grichar et al., 2009). In some sesame herbicide research, severe sesame plant stunting and leaf necrosis has resulted in good weed control and produced higher yields than the untreated check because of the loss of production to weeds in the untreated check (Grichar et al., 2009). A controversial use of herbicides is what is known as a "rescue treatment", which is using a herbicide that will injure the sesame, but will bring weeds under some control and allow the sesame to be harvested at an economic return. As an example, a farmer used clopyralid on a portion of a field that was being overwhelmed by common cocklebur (Xanthium strumarium L.). Where he did not spray, he lost the crop; however, where he sprayed there was damage to the sesame with control of the cocklebur and he harvested about 660 kg/ha. However, many sesame growers in the U.S. are not tolerant of any type of sesame herbicide injury even knowing that the sesame will recover.

Starting in 2003, research has been conducted using postemergence-directed herbicides with and without the use of hooded sprayers. This work is very encouraging; however, there are many cropping patterns that preclude the use of hooded sprayers. There is a label for glyphosate (Roundup Max®) that allows wiper applicators or hooded sprayers to be used between sesame rows (Langham et al., 2010). While this does not provide effective weed control in the sesame seed row, it helps with vining weeds such as morningglory species (Ipomoea spp.) and smellmelon (Cucumis melo L.) that spread across the rows. While morningglory is becoming increasingly tolerant of glyphosate, glyphosate will slow the growth of morningglory and reduce the damage to the sesame from this weed. In the case of Amaranthus, which quickly can become taller than the sesame, wiper applicators using glyphosate have been very successful, particularly in areas with high relative humidity, as long as the glyphosate does not drip on the sesame. Initial work with spraying glyphosate on the sesame stem showed little injury; however, in subsequent studies, there have been instances of severe damage. In further observations, when the sesame was under moisture stress, there was little damage, but when the plants were in a rapid growth phase following rainfall or irrigation.

One of the major problems in using postemergence-directed herbicides has been the timing of the application and the height of the spray application on the sesame stem as related to the height of the plant. Recent work has shown that there are differences in applying herbicides at

5 cm above the surface versus 15 cm; differences in applying 4 weeks after planting versus 6 weeks; and differences is the heights of the plants in different locations in a field. In waiting for the sesame to get tall enough to spray a postemergence-directed herbicide, weeds also become tall and herbicides may not control taller weeds (Langham et al., 2010). In reviewing research using postemergence herbicides, it is sometimes difficult to understand exactly at what stage of growth the herbicide was applied (Langham et al. 2007). Many of the documents will cite the number of days after planting or the height of the plants. However, there are many differences in the cultivars of the world in terms of number of days in each stage and in the heights of the plants in each stage as shown in Table 3. In order to standardize terminology, a phenology chart has been developed to specify the beginning and end points of the stages (Langham, 2007). Table 4 summarizes sesame phenology.

Days from planting

Phase length

Phase

Range

Mean

Range

Mean

Vegetative

29-59

42

29-59

42

Reproductive

56-116

89

16-70

47

Ripening

77-140

108

(14)b-54

11

Drying

102-181

150

11-57

38

a Based on sesame germplasm from Sesaco Corporation (Langham 2007) b In some cultivars, there are dry capsules above green leaves while the upper portion of the plant is still flowering creating a negative range.

Table 3. Range and mean of number of days in phases for sesame germplasm.a

Stage/Phase

End point of stage DAPa

No. weeks

Vegetative

Germination

Emergence

0-5

1-

Seedling

3rd pair true leaf length=2nd

6-25

3-

Juvenile

First buds

26-37

1+

Pre-reproductive

50% open flowers

38-44

1-

Reproductive

Early bloom

5 node pair of capsules

45-52

1

Mid bloom

Branches/minor plants stop flowering

53-81

4

Late bloom

90% of plants with no open flowers

82-90

1+

Ripening

Physiological maturity

91-106

2+

Drying

Full maturity

All seed mature

107-112

1-

Initial drydown

1st dry capsules

113-126

2

Late drydown

Full drydown

127-146

3

a DAP, days after planting. These numbers are based on S26 (Sesaco Corp.) in 2004 near Uvalde, TX under irrigation.

Table 4. Phases and/or stages of sesame.

Table 4. Phases and/or stages of sesame.

Future work on sesame herbicides should specify the stage of the sesame. Recent application timing work has shown that some herbicides are phytotoxic in the seedling stage, are neutral in the juvenile stage, and reduce yield in the pre-reproductive through mid bloom stages. Additional work is needed to verify these initial findings as to the exact neutral stages, but there is enough data to know that plant stage at application is critical. A second problem in reviewing the literature is that some of the work has not been carried through to completion of the sesame crop (Langham et al. 2007). Sesame has a remarkable ability to compensate. Recent work has compared the stunting/damage ratings of some contact-based herbicides and showed that the amount of damage to sesame was reduced over time and the yields of stunted/damaged materials was comparable to the weed-free checks. Sesame injury ratings should only be done by researchers familiar with sesame. Sesame yields are related to the number of capsules and the seed weight per capsule per square meter. There have been herbicide treatments that apparently damage the sesame, i.e., the yellow splotching of leaves by such herbicides as diuron, but the number and weight of the capsules were not affected. In some cases, the herbicide delayed flowering, but the plants flowered longer. A reduction in plant height may not affect yield. Below is a discussion of the most promising and effective herbicides for use across the sesame growing areas of the world. A discussion of research in various sesame growing areas is also included.

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Responses

  • Nebay
    What causes the difference in sesame tolerance levels to pre metolachlor and alachlor?
    4 months ago
  • atte katajisto
    Do sesame seeds contain Glyphosate?
    17 hours ago

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