Interaction of Herbicides with other agro chemicals

• Simultaneous or sequential application of herbicides, insecticides, fungicides, fertilizers,PGR etc., is followed in a single cropping season.

• These chemicals may undergo a change in physical and chemical characters, which could lead to enhancement or reduction in the efficacy of one or more compounds.

• The interaction effects were seen much later in the growing season or in the next season due to build up of persistent chemicals or their residues in the soil.

• When two or more chemicals accumulate in the plant, they may interact and bring out responses

• i) Additive effect: It is the total effect of a combination, which is equal to the sum of the effects of the components taken independently.

• ii) Synergistic effect: The total effect of a combination is greater or more prolonged than the sum of the effects of the two taken independently.

• iii) Antagonistic effect: The total effect of a combination is smaller than the effect of the most active component applied alone.

• iv) Independent effect: The total effect of a combination is equal to the effect of the most active component applied alone.

• v) Enhancement effect: The effect of a herbicide and non-toxic adjuvant applied in combination on a plant is said to have an enhancement effect if the response is greater than that obtained when the herbicide is used at the same rates without the adjuvant. Eg. Mixing Ammonium sulphate with glyphosate.

Herbicide and herbicide interactionMixtures of selected herbicides offer several advantages over the use of a

single herbicide, including (a) a reduction in cost of cultivation by saving time and labour, (b) a reduction in soil compaction by eliminating multiple field operations, (c) an increase in the spectrum or range of weeds controlled or an

extension of weed control over a longer period of time, (d) an improvement in crop safety by using minimum doses of selected

herbicides applied in combination rather than a single high dose of one herbicide,

(e) a reduction in crop or soil residues of persistent herbicides by using minimum doses of such herbicides, and

(f) a delay in the appearance of resistant weed species to selected herbicides

Objective: The optimum herbicide combinations would be

those that exhibit enhanced activity on target weed species and decreased toxicity on crops (increased selectivity).

This is difficult to predict since the behaviour of each single herbicide in the mixture is often affected by the presence of the other(s) and the activity of the mixture may also vary considerably depending on plant species, growth stage, and environmental conditions.

Types of herbicide interactions

Antagonistic interactions in herbicide mixture often cause significant problems in weed control.

For example, the application of pyrithiobac in mixture with fluazifop-P hasbeen reported to reduce the efficacy of fluazifop-P on large crabgrass (Digitaria sanguinalis) .

Similarly, the application of tribenuron in mixture with diclofop has been reported to reduce the efficacy of diclofop on wild oat (Avena fatua) .It is obvious that such herbicide combinations should be avoided.

Antagonistic interactions, however, may be consideredbeneficial when they reduce herbicide activity on crops

For example, mixtures of fenoxaprop with MCPA showed reduced toxicity of fenoxaprop on wheat and barley compared with fenoxaprop applied alone.

Furthermore, mixtures of thifensulfuron with bentazon showed reduced toxicity of thifensulfuron on soybean compared with thifensulfuron applied alone.

(Hart& Roskamp, 1998; Lycan & Hart, 1999).

Synergistic interactions may be particularly beneficialwhen they result in more effective control of troublesome

weeds.

For example, Flint and Barrett (1989a) found that mixtures of glyphosate with 2,4-D were more effective on field bindweed (Convolvulus arvensis) compared with separate applications.

Similarly, Scott et al. (1998) found that mixtures of sethoxydim with dimethenamid were more effective on johnsongrass (Sorghum halepense) compared with separate applications.

Synergistic interactions, however, may causesignificant problems when they result in increased herbicide

activity on crops.

For example, mixtures of ethametsulfuron with haloxyfop, fluazifop, fluazifop-P, quizalofop, and quizalofop-P may cause phytotoxicity and yield losses in Brassica napus and Brassica rapa (Harker et al., 1995).

Furthermore, mixtures of thifensulfuron (sulfonylurea) with imazethapyr (imidazolinone) may cause phytotoxicity in soybean which is generally resistant to sulfonylureas (Simpson & Stoller, 1996). It is obvious that such herbicide combinations should be avoided

Mechanisms of herbicide interactions.

Interactions in herbicide mixtures can occur prior, during, or after application of the mixture.

This means that herbicides may interact physically or chemically in the spray solution or biologically in the plant.

Mechanisms of interactions in herbicide mixtures can be broadly grouped into four categories: biochemical, competitive, physiological, and chemical .

Interactions between herbicides in mixtures may be attributed to

a) changes in the amount of an herbicide that reaches its site of action through absorption, translocation or metabolism caused by the presence of the other herbicide,

b) Interaction at the site of action between the combined herbicides where one herbicide of the mixture affects the binding of the other at its site of action

c) interaction between combined herbicides that produces opposite effects on the same physiological process of the plant or synergizes the overall effect

d) chemical reaction between the combined herbicides that leads to formation of inactive complex or an increase in the rate of metabolism

Factors affecting herbicide interactions

The type and the extent of interactions depend primarily on properties of the combined herbicides (chemical group, absorption, translocation, mechanism of action, pathway of metabolism).

In general, antagonism has been found to occur three times more often than synergism regardless of the species or the herbicides in which is recorded (Zhang et al., 1995).

Same Chemical group: Synergism :has been found to occur more

frequently in mixtures where the companion herbicides belong to the same chemical group .

These herbicides normally have similar chemical structure, the same mechanism of action, and similar pathway of metabolism.

The high frequency of antagonism in such same chemical herbicide mixtures could be attributed in plant inability to metabolize simultaneously two or more herbicides.

Different chemical groups : Antagonism,: unlike synergism, has been found to

occur more frequently in mixtures where the companion herbicides belong to different chemical groups .

These herbicides normally have different chemical structure, different mechanism of action, and different pathway of metabolism.

This is because these herbicides probably have a greater chance to interact at the site of action (enzyme or physiological process) or to react chemically and form an inactive complex.

• The point of entrance and the mobility of the combined herbicides into the plant may affect significantly the behaviour of the herbicide mixture In particular,

• when the combined herbicides enter into the plant through the same point (root or foliage) then the presence of one herbicide in the mixture may reduce the absorbed amount of the other and consequently can reduce its efficacy.

• The translocated amount of an herbicide to its site of action can be reduced by the presence or the concomitant translocation of another herbicide into the plant

Other Factors affecting herbicide interactioon

It was observed that when aryloxy phenoxy propionate and cyclohexanedione herbicides was used. members of both herbicide families were found to be affected more when mixed with systemic rather than contact broadleaf herbicides.

The type of interactions between companion herbicides may depend on target plant species.

For example, the combination of acifluorfen and bentazon showed an increased efficacy against common lambsquarters (Chenopodium album) and velvetleaf (Abutilon theophrasti)…. but reduced efficacy against jimsonweed (Datura stramonium) and red root pigweed (Amaranthus retroflexus)

. The combination of herbicides that inhibit acetolactate synthase (ALS) (e.g. imazaquin, chlorimuron) with herbicides of the diphenylether group (e.g. acifluorfen, fomesafen) showed increased efficacy on prickly sida (Sida spinosa)………….. but reduced efficacy on common cocklebur (Xanthium strumarium).

The growth stage of weeds may often affect the extentof interactions between combined herbicides

Liebl and Worsham (1987) observed that the postemergence application of chlorsulfuron and diclofop decreased efficacy of diclofop on italian ryegrass (Lolium multiflorum) and the effect was more severe at the two-leaf growth stage than the application was performed at the three-leaf growth stage.

This may be attributed to reduced detoxification ability from the younger plants (Two leaf stage) and also to their thinner cuticle that probably allowed retention, absorption, and translocation of greater amounts of the applied herbicides.

postemergence application graminicides in mixture with one or more broadleaf herbicides

Antagonistic interactions between graminicides and broadleaf herbicides are probably due to morphological and physiological differences between grasses and broadleaf weeds.

Broadleaf weeds have meristems at the top of the plant; whereas, grasses have them at the base.

This difference probably affects absorption and mainly translocation of the foliar applied herbicides particularly the systemic ones that are translocated and accumulated at the meristematic tissues of the plant where they act.

The simultaneous application of various graminicides with certain broadleaf herbicides limits considerably graminicide absorption by foliage and translocation to the meristematic tissues. This has been confirmed by the results of Zhang et al. (1995) who found that the frequency of antagonistic interactions was four times greater than synergistic interactions in grasses .

whereas, the corresponding frequencies were almost equal in broadleaf weeds . It is worth mentioning that almost 80% of the interactions that has been observed in species of the family Poaceae (grasses) refer to cases of antagonism.

Herbicide - Insecticide Interactions

Herbicide-insecticide interactions : synergistic action and injury to crop plants Crop injury results because some insecticides

temporarily render crop plants unable to metabolize and detoxify herbicides.

Herbicide - Insecticide Interactions in corn

Application of some organophosphate corn rootworm insecticides (Counter, Thimet, Lorsban, etc) in combination with with ALS inhibitor herbicides (Accent, Beacon, Exceed, Lightning, etc.) can injure corn significantly.

Symptoms of this injury : Stunting, yellowing, chlorosis in leaves,

bleached bands on leaves

Organophosphate insecticides generally used in corn

Aminoacid synthesis (ALS) and pigment (HPPD) inhibitor herbicides in corn

The severity of injury is dependent: environmental conditions,(rainfall)

the insecticide used, the method of insecticide application, .Method of insecticide application.: Furrow application : Injury is more rather

than banded. The insecticides that tend to cause the most problems are Counter and Thimet, especially when applied in-furrow. application:

Environmental conditions: insecticide interaction is likely to be most severe

when rain is adequate to ensure effective insecticide and herbicide uptake and activity. i.e more absorption by plat roots.

Some studies have shown that significant rain during the week prior to an Accent (Nicosulphuron) (H)or Beacon application increases the severity of injury.

stress from weather : Injury may be more likely to occur

How to avoid this problem in corn

Pyrethroid-type insecticides (Force) are substituted for organophosphate: which do not increase the risk of injury from a herbicide,.

Apply organophosphate insecticides as a band rather than in-furrow to minimize the risk of injury

Herbicide - Insecticide Interactions in Peanut

Acephate and aldicarb applied in the seed furrow at planting did not affect injury potential of peanut following postemergence application of acifluorfen plus bentazon or bentazon;

However, the insecticide phorate applied in the seed furrow enhanced visible injury associated with bentazon.

chlorpyrifos applied at planting: did not affect peanut response to preemergence application of diclosulam, S-metolachlor, or flumioxazin applied or post emergence application of acifluorfen, acifluorfen plus bentazon, imazapic, or paraquat plus bentazon.

Efficacy of graminicides can be affected by insecticides applied to peanut.

1.Carbaryl and dimethoate applied postemergence in combination with sethoxydim reduced annual grass control;

2.But, when acephate was mixed with sethoxydim no adverse effect was noted

3.Pyrethroid insecticides did not affect efficacy of postemergence herbicides

Herbicide - Insecticide Interactions in other crops:The Phyto-toxicity of monuron and diuron is

increased on cotton when applied with phorate. Similar effects were also observed on oats.

Combination of Organo-phosphate insecticide and Atrazine on phyto-toxicity appeared to involve an effect of the insecticides on herbicides absorption and translocation in corn.

Interactions of nicosulfuron and pyrithiobac-sodium increased injury in corn (Zea mays L.) and cotton (Gossypium hirsutum L.),

Herbicide – Fungicide interactions

• Herbicides interact with fungicides as the disease causing organisms. Dinoseb was known to reduce the severity of stem rot ( White mould) in groundnut.

• Diuron and Atrazine which inhibit photosynthesis may make crops susceptible to tobacco mosaic virus.

• But, diuron may decrease the incidence of root rot in wheat.

• Atrazine was found to have antagonistic interaction with the fungicide Dexon on many crops because reduced uptake of atrazine in the presence of Dexon in corn, cucumber and soyabean .

Herbicides reduced disease severity: Sharma and Sohi (1983) showed that bromacil, diuron, nitrofen, and alachlor all reduced disease severity in Phaseolus vulgaris by Rhizoctonia.

Herbicides increased disease severity: In a survey of the effects of twelve herbicides (bentazon,

acifluorfen, chlorimuron, fluazifop, diclofop, sethoxydim, imazaquin, metribuzin, oryzalin, thidiazuron, diaminozide, and mefluidide) on disease severity of four plant pathogens (Alternaria cassiae, Colletotrichum coccodes, C. truncatum, and Fusarium lateritium), all of the herbicides enhanced disease severity of at least one of the pathogens to a host plant (Caulder et al., 1987).

Reasons to reduce disease severity by herbicides

Some Herbicides have: fungitoxic, antimicrobial activity, Production of Phytoalexins

Effects of glufosinate on plant disease are due to direct fungitoxic effects. It is a non-selective herbicide, so its effects are best seen in glufosinate-resistant crops. Glufosinate has antimicrobial activity in glufosinate-resistant soybeans , rice and protecting these crops from bacterial and fungal diseases.

Phytoalexins• pretilachlor and butachlor trigger accumulation of the phytoalexins

momilactone A and sakurantetin in rice leaves.• Pendimethalin, induces the synthesis of the phytoalexin tomatine in

tomato

THE SPECIAL CASE OF GLYPHOSATE

Glyphosate is the most widely used herbicide worldwide. Its mode of action is inhibition of the shikimic acid pathway which produces aromatic amino acids, as well as secondary plant products involved in resistance of plants to plant pathogens.

glyphosate caused lowered phytoalexin levels and increased susceptibility to plant pathogens

Glyphosate reduced lignification and alterations in root exudates which contributed to susceptibility to Pythium spp

Glyphosate-resistant crops

There should be no effect on phytoalexin of glyphosate on disease resistance in glyphosate-resistant crops, as the shikimic pathway is not blocked by the herbicide in these transgenic crops.

However, reports of both enhanced and reduced disease severity have been reported in glyphosate-resistant crops (Duke & Cerdeira, 2005).

Recently, glyphosate was reported to have both preventative and curative properties on rust diseases in both glyphosate-resistant wheat and -soybean

Herbicide- Fertilizer Interactions

• Application of fertilizer with herbicides is becoming increasingly popular in developed countries. No detrimental properties were observed when herbicide were combined with suspension of fertilizers.

• Application of complete fertilizer ( Containing N.P and K) reduce the atrazine absorption by plants, thus reducing phytoxicity. Atrazine was more toxic in the presence of PK than in the presence of NP and NK due to increased absorption of herbicides by plants.

• The addition of Urea or ammonium sulphate in 2,4-D and glyphosate increased the efficiency of herbicides.

Effect of herbicides on nutrient uptake:• 2,4 D and Dicamba decreased the Nitrogen uptake in pea plants

• Effect of Nitrogen Fertilizer:• Herbicides that appear to benefit from the addition of

ammonium are the relatively polar, weak acid herbicides such as Basagran, the sulfonylureas (Accent, Beacon, Classic, and Pinnacle, etc.), and the imidazolinones (Pursuit and Raptor).

• Nitrogen fertilizers may replace surfactant or crop oil concentrate with some of the contact-type herbicides.

• Ammonium-based fertilizers and, in particular, ammonium sulfate (AMS) are also being promoted to reduce potential antagonism with hard water or antagonism with other pesticides.

• Roundup (glyphosate) is one product that specifically recommends on its label the addition of ammonium sulfate (or a higher rate of Roundup) for hard water, or drought conditions.

Herbicide – Micronutrient interactions

Boron and manganese are the primary micronutrients applied to peanut. Occasionally, these can affect herbicide performance. For example, efficacy of clethodim and imazethapyr was reduced by micronutrients for some weeds evaluated

Herbicide – Plant growth regulator interactions in Groundnut

Prohexadione calcium is the primary plant growth regulator available for use in peanut.

Efficacy of the herbicides acifluorfen, acifluorfen plus bentazon, bentazon, imazethapyr, imazapic, lactofen, and 2,4-DB was not affected by prohexadione calcium (Beam et al., 2002).

Growth regulator herbicides are also known as synthetic auxins, because their effects on plants mimic those of naturally occurring plant hormones called “auxins.” In extremely small doses, synthetic auxin herbicides stimulate plant growth, but in proper concentrations, synthetic auxins disrupt numerous biochemical pathways and will kill susceptible plants.

Most growth regulator herbicides can be categorized as one of three types: phenoxy (2,4-D; 2,4-DP; MCPA; MCPP),

benzoic acid (dicamba), or pyridine (triclopyr). All of these are selective, broadleaf herbicides, meaning they

will kill broadleaf weeds without harming grasses.

No single, selective herbicide controls all broadleaf weeds commonly found in turf. The combination of two or more selective herbicides can dramatically increase the spectrum of weed control. As such, most growth regulator herbicides are available as mixtures. For example, Trimec is a combination of MCPP, 2,4-D and dicamba.

Conclusion

The effect of herbicides on performance of fungicides and insecticides is limited but no less important than defining impacts on herbicide efficacy. As new active ingredients and new formulations of active ingredients become available, additional research will be needed to define interactions among these agrochemicals. Although interactions of herbicide-herbicide combinations have been defined broadly and in some cases in detail, research elucidating the mechanism of reduced control associated with co-application of fungicides, insecticides, or plant growth regulator and micronutrients is limited. Finally, determining the impact of interactions in the overall production system would be beneficial.