The simultaneous or sequential application of herbicides with other agrochemicals like insecticides, fungicides, and fertilizers can result in interactions that influence the efficacy and toxicity of the applied chemicals. These interactions may be additive, synergistic, antagonistic, or have no effect. Factors like chemical properties, mode of action, application method, and environmental conditions determine the type and extent of interactions. While some interactions like synergism can enhance weed control, others like antagonism can reduce efficacy or increase toxicity. Understanding these interactions is important for optimizing agrochemical combinations and avoiding adverse effects.

1. Interaction of Herbicides with
other agro chemicals

2. • 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.

3. • 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.

4. Herbicide and herbicide interaction
Mixtures 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

5. 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.

6. Types of herbicide interactions

7. 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.

8. Antagonistic interactions, however, may be considered
beneficial 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).

9. Synergistic interactions may be particularly beneficial
when 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.

10. Synergistic interactions, however, may cause
significant 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

11. 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 .

12. 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

13. 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).

14. 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.

15. 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.

17. • 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

18. 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.

19. 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).

20. The growth stage of weeds may often affect the extent
of 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.

21. 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.

22. 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.

23. 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.

24. 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

25. Organophosphate insecticides
generally used in corn

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

29. 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:

30. 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

31. 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

32. 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.

33. 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

34. 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.),

35. 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 .

36. 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).

38. 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

39. 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

41. 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

42. 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

43. • 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.

44. 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

45. 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).

46. 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.

47. 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.

48. 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.