In this chapter covered with tropical region weeds and its problems and management through seed ecological research

1. Improving Weed Management Strategies
Through Seed Ecological Research In Tropics

2. Conclusion
Responses of Weed Seed Germination to Stresses
Responses of Weed Seed Germination to Seed Burial Depth, Tillage, and
Surface Mulches
Responses of Weed Seed Germination to Seed Scarification and Fire
Responses of Weed Seed Germination to Light
Role of Seed Ecology in Determining Weed Populations
Challenges for Weed Management in the Tropics
Introduction

3. ❖ Unwanted and undesirable plants - utilization - affect human welfare.
❖ Manage all the land and water resources,
but effective impact is greater on Agriculture.
❖ Losses caused by weeds
INTRODUCTION

4. ❖Arable crop systems - reducing weed density - early stages of crop growth
(Zimdahl,1988)
❖ weed seed bank, - reservoir of weed seeds
(Forcella,1993)
❖Weed seeds can be lost to the seed bank by
➢ germination,
➢ death or predation,
➢ physical distribution of seeds in the soil,
➢ physiological changes in the seeds, and
➢ flux of dormancies (Cousens et al., 1995)
Reduce the impact on crop yield

5. ❖ Weed seed germination is commonly influenced by
➢ Soil moisture,
➢ Seed burial depth due to tillage operations,
➢ Use of mulches,
➢ Fire for land clearance, and
➢ Flooding of the soil.
❖ Harnessing these factors to influenced germination
can provide
major entry point for improved weed management.

6. HARMFUL EFFECTS CAUSED BY WEEDS
➢ Reduction in crop yield
➢ Reduction in land value
➢ Limited choice of crops
➢ Loss of quality
➢ Reduced human efficiency
➢ Increased costs of insect and disease control

7. ❖ Weeds - biotic constraint to crop production in the tropics.
❖ In Asia, actual losses in rice and wheat production due to weeds have been
estimated as
Rice – 10 % Wheat – 13%
(Rao et al., 2007)
CHALLENGES FOR WEED MANAGEMENT IN THE TROPICS

8. ❖ Weed control in the tropics is commonly achieved by manual weeding, But
now a days more scarcity of labour
➢ increasing costs of labour
➢ migration of rural labour to the cities
❖ Alternative method of manual weeding is use of chemical herbicides.
❖ Impact-
➢ Herbicides resistance in weeds,
➢ Weed species population shifts,
➢ Increased the cost of chemical control measures
➢ Rapid degradation of environment
(Johnson et al., 2008)
Because of

9. Eg. In India, reliance on the isoproturon (substituted urea herbicide) led to a
rapid increase in resistance on
(Malik et al.,1995)
❖ Weed reduce wheat yield up to 80%
(Singh et al., 1999)
(Singh, 2007)
Phalaris minor wheat
Haryana, India
Weed surveys conducted during
2005–2006 revealed Phalaris minor
infestations in 82% of the wheat
fields

10. ❖ Changes in crop establishment methods have occurred in many countries.
Example,
❖ Indo-Gangetic Plains of India,
➢ Conventional tillage (CT) in wheat in the rice wheat cropping system
- replaced by no-till (NT)
(Singh, 2007)
❖ Similarly, different tillage systems for the establishment of corn (Zea mays)
-replaced by reduced and minimum tillage.
(Nakamoto et al., 2006).
❖ Transplanting of rice in some countries has been replaced by direct seeding as
farmers respond to increased costs or decreased availability of labor or water.
(Pandey et al.,2005)

11. ❖ Risk of crop yield loss
compare,
direct-seeded rice is higher than in transplanted rice
Why ?
➢ Absence - size differential between the crop and weeds
➢ Suppressive effect of standing water on weed growth at crop
establishment. (Rao et al., 2007)
❖ Changes in crop management and establishment methods
associated with
➢ Size of weed infestations
➢ As well as a shift in the weed flora (Johnson et al., 2005)
competition from weeds

12. ROLE OF SEED ECOLOGY IN DETERMINING WEED POPULATIONS
Ecology
❖ Interrelationship between organisms and their environment.
Weed ecology
❖ Mainly concerned with growth characteristics and
❖ Adaptations that enable weed to survive changes in the environment.
❖ Environment includes climatic, edaphic and biotic factors - determines the
➢ Distribution
➢ Prevalence
➢ Competing ability
➢ Behaviour & survival of weeds

13. Survival mechanism of weed
Annual weeds seed
perennial weeds
vegetative plant propagules
such as buds, rhizomes, tubers,
bulbs and stolons
Major adaptations for survival of weeds - include
➢ Prolific seed production
➢ Survival of vegetative propagules under adverse conditions
➢ Seed dissemination
➢ Dormancy
➢ Allelopathy
* Dormancy is the single most important characteristic of weeds that enable them
to survive when crops cannot.

14. ❖ Annual weeds are characterized by the production of very large number of
seeds.
Ex.
➢ Trianthema species – plant may produce about 10,000 seeds / plant and
➢ Parthenium species - produce 10,000 to 20,000 seeds / plant, depending
upon its size.
(Stevens, 1982)
Type of weeds Seeds/plant
Perennials 16,629
Biennials 26,600
Annuals 20,832
Prolific seed production

15. SEED DISSEMINATION
❖ Most weeds are good travellers.
❖ Use - various forces or agents to transport and scatter themselves
from
place to another place
❖ Agents by which weeds are disseminated through
➢ wind,
➢ water,
➢ animals and
➢ human’s.

16. WIND
❖ Seeds or small fruits - modify with tufts of hair or wing-like appendages - easily
carried by wind over long distances, the lighter seeds may drift for miles.
❖ Various modifications of seed structure – travel one place to another are termed as,
➢ saccate,
➢ parachute Ex. Wind dispersal
➢ winged comate (hair-covered),
➢ plumed.
Taraxacum officinale
Cirsium spp.
Imperata cylindrical
Saccharum spontaneum
Asclepias syriaca
calotropis gigantea

17. Taraxacum officinale Cirsium spp Imperata cylindrical
Saccharum spontaneum Asclepias syriaca calotropis gigantea

18. Water
❖ Many species are light or covered with an oily film, - enabling to float on the
water surface.
❖ Some weeds have air-filled membranous seed envelopes which can float on
the surface of water.
❖ Floodwater, running streams and irrigation water are important in spread of
weed seeds.
Ex. Water dispersal
Ambrosia spp.
Amaranthus spp.
Xanthium strumarium

19. Ambrosia spp. Amaranthus spp.
Xanthium strumarium

20. Animals
❖ Many weed seeds pass through - digestive tracts of animals - without loss of
viability.
❖ Birds also consume large quantities of weed seeds and scatter them in
droppings.
❖ The dispersal of seeds – in the form of incompletely digested material -
passing through the animals is termed as endozoochory.
Ex. Animal dispersal
Endozoochory is Prosopis spp.

21. Man
❖ Seeds of many seeds have specialized structures such as
➢ Hooks
➢ Spines
➢ Barbs
➢ Awns
➢ Tend to cling - man’s clothing and footwear or agricultural implements used
by man.
Ex.
✓ Fruits of Tribulus terrestris (Puncturevine) - have sharp spines - easily cling
to animals and man and get dispersed far and wide .
✓ Chrysopogam aciculatus (Silkgrass) have awns and get disseminated by
sticking to human’s clothing.
✓ Phalaris minor (littleseed canarygrass) seeds spread in India through
human’s indifference or carelessness.

22. Tribulus terrestris Chrysopogam aciculatus
Phalaris minor

23. Seed dormancy
❖ Viable seed fails to germinate - moisture, temperature and oxygen
favourable for plant growth.
❖ Resting stage for the seed.
❖ Many annual weeds - produce dormant seeds to germinate under a narrow
range of environmental conditions.
Ex.
Cucurbitaceae
Leguminaceae
Graminaceae
Boraginaceae
Convolvulaceae
* This species having long dormancy period often running into several years.

24. ❖ Seeds of some species never experience dormancy.
Forcella et al., (2000)
❖ Nondormant seeds, the primary factors governing seed germination are
temperature and moisture.
❖ Longevity of weed seeds in the seed bank - depend upon
➢ Inherent dormancy characteristics of the seed populations,
➢ Environmental conditions present in the soil - that influence to release
dormancy.
Eg.
✓ Environmental conditions Light, temperature, water, and gas environment
and
✓ Biological (predation and allelopathy)
✓ Interactions
(Radosevich et al., 1996)

25. ❖ Usually, seeds lose their viability through germination and mortality
more rapidly
when present near the soil surface than buried deeper in the soil.
(Mohler, 1993)
❖ In temperate areas, for example,
➢ Seed populations of Bromus sterilis declined by 85% during July and
August in an uncultivated field and no viable seeds - remained the following
April.
Froud-Williams (1983)
❖ The rapid decline in viable seeds - at the surface may be due to unsuitable
germination conditions and greater pathogen and predator incidence at the
surface. (Taylorson, 1970)

26. ❖ Germination patterns - differences in weed to weed species – on
different seasons.
(Johnson et al., 2004).
❖ Important environmental factors affecting weed germination and emergence,
and these can be manipulated through management practices are
➢ Light,
➢ soil moisture,
➢ soil salinity,
➢ fire,
➢ tillage, and
➢ surface residue (mulch)

27. Responses of Weed Seed Germination to Light
❖ Germination of various weed species has different responses to light and
darkness.
❖ Exposure to light - breaks dormancy and - increases germination in many species,
especially small-seeded species.
(Cousens et al.,1993)
❖ Exposure of seeds to light flashes from several μ s to 1 s is sufficient to stimulate
germination.
(Scopel et al., 1994).
❖ Soil tillage operation - brings seeds to the surface, - provides sufficient light to
meet the requirements for most seeds.

28. ❖ Seed germination responses to light - species specific.
❖ Some species germinate equally in light and dark
e.g.,
➢ Avena fatua
➢ Mimosa invisa
➢ Eleusine indica
➢ Melochia concatenata
❖ Some species require light to stimulate germination are
e.g.,
➢ Digitaria ciliaris
➢ Echinochloa colona
➢ Portulaca oleracea

29. ❖ Some species have an absolute light requirement for germination
E.g.,
Cyperus difformis
Digitaria longiflora
Eclipta prostrata
➢ Absolute light requirement species are positively photoblastic, - response to
be controlled by phytochrome, “light-absorbing pigment” within plants.
❖ In photoblastic seeds, light exposure may convert inactive-phytochrome ‘‘red’’
to activephytochrome ‘‘far-red’’.
(Rollin, 1972)

30. ❖ No tillage systems - large proportion of weed seeds remains on the surface and
exposed to light.
(Chauhan et al., 2008)
❖ Even shallow burial can induce dormancy in light-requiring seeds
(Wesson et al.,1969)
❖ Less than 1% of incident light penetrates beyond 2.2mm into the soil is
sufficient for germination weed seeds
(Egley, 1986)

31. Weed species
Germination (%)
Light Dark
Avena fatua 74 74
Borreria ocymoides 91 93
Celosia argentea 31 2
Chromolaena odorata 80 37
Cyperus difformis 81 0
Cyperus iria 94 0
Digitaria ciliaris 93 7
Digitaria longiflora 89 0
Echinochloa colona 76 12
Echinochloa crus-galli 69 24
Eclipta prostrata 93 0
Eleusine indica 97 96
Effect of light conditions (light and dark) on seed germination of different weed species
Chauhan et al., (2008)

32. RESPONSES OF WEED SEED GERMINATION TO SEED SCARIFICATION AND FIRE
Effect of seed scarification on germination
❖ The seed coat itself act as a physical barrier can impose dormancy.
(Turner et al., 2005)
❖ Hard seeds present - absolute form of imposed dormancy due to the
impermeability of the seed coat to water or gasses.
(Foley, 2001)
❖Hard seeds generally require
➢ physical or chemical scarification, or to enhance germination
➢ Normal weathering in the soil.
(Foley, 2001)
❖ Seeds with an impermeable seed coat may have a long life in the soil.
(Egley et al.,1983)
,

33. ❖ Microbial and fungi attack
and may break dormancy in hard-seeded
abrasion by soil particles -species.
(Baskin et al.,2000)
❖ Small proportion of seeds – Scarified during the time of tillage operation.
❖ Scarification occur - trampling by animals or by passage through an
animal’s digestive tract.
❖ Other possible natural factors – to break dormancy in hard-seeded species
are extreme changes in temperature and moisture regimes, and fire.
(Baskin et al., 1998)

34. Effect of seed scarification on germination of different weed species
*Seeds were scarified either by physical cut with a scalpel or chemically with sulfuric acid.
Sources: Chauhan et al., (2006)
Weed species
Germination (%)
Control Scarified
Corchorus olitorius 3 93
Malva parviflora 10 88
Melochia concatenata 23 97
Urena lobata 2 78
Mimosa invisa 2 100
Mimosa pudica 6 90
Sida rhombifolia 5 65

35. Effect of Fire on Germination
❖ In many countries, small farmers use fire to clear their land straw or debris
before crop planting.
(Roder et al., 1997)
❖ Heat generated by burning affects populations of many species through - effects
of physical and biological properties of the soil.
(Uhl, 1982)
❖ Fire is the important factor to reduce the weed seeds,
➢ Affect plant communities with persistent seed banks,
➢ kills unburied seeds of susceptible species.

36. ❖ In India, wheat straw was removed or burned after combine harvesting,
- soil samples were taken and washed to separate the seeds of Phalaris minor ,
these seeds were tested for germination.
❖ The data showed - germination was 60% less in the field - where wheat straw
was burned after combine harvesting compared with its removal
(87% germination).
(Hari et al., 2003)
❖ Similarly, 97% of ungerminated seeds of Bromus sterilis on the soil surface –
were destroyed and seedling numbers reduced by upto 94% with straw
burning in Europe.
(Froud-Williams, 1983)
❖Germination was completely inhibited - when depending on weed species - were
exposed at 160–200 C.
Research Evidence

37. Temperature (C) required for 50% inhibiting (T50) and completely inhibiting
(T100) seed germination of different weed species
Weed species
Temperature
T50 T100
Digitaria ciliaris 148 180
Digitaria longiflora
116 160
Echinochloa colonam 139 180
Eclipta prostrata 167 200
Eleusine indica 133 180
*A known number of seeds were placed in an oven at different high temperatures for 5 min. The treated
seeds were then tested for germination at optimum temperatures for 14 days.
Chauhan et al., (2008)

38. ❖ Fire – influence in releasing physical dormancy - hard-seeded species.
(Tieu et al., 2001)
❖ Fire also creates - mass release of dormant seeds, were fire coincides with
- hot and wet conditions.
(van klinken et al., 2006)
❖ seeds of Parkinsonia aculeate that act high temperature
produce cracks on the seed coat
increase imbibition and germination
Scott (2006)

39. Responses of Weed Seed Germination to Tillage, Seed Burial Depth and Surface Mulches
❖ Tillage - determines the vertical distribution of weed seeds - soil profile.
(Chauhan et al., 2006)
❖ Different tillage systems – have different distribution of weed seeds in soil profile.
1. Effect of tillage systems on vertical seed distribution

40. ➢ No-till system - retain 56% of the weed seeds in the top 1 cm soil layer, but
➢ Conventional tillage system - buried 65% of the seeds to a depth of 1–5 cm and
only 5% of the seeds remained in the top 1 cm soil layer.
(Chauhan et al., 2006)
❖ 85% weed seeds in the upper 5 cm of soil in a no - tillage system,
❖ but only 28% of seeds found in surface layer in a mold board plow for CT.
Pareja et al., (1985)
Research Evidence

41. ❖ Likewise, 60% weed seeds - present in the top 1 cm of soil in - NT system
but only 30% of seeds found in surface layer in a chisel plow system.
(Yenish et al., 1992)
❖ Low soil disturbance systems - leave most of the weed seeds in the top soil
layer - compared with high soil disturbance systems.

42. 2.Effect of seed burial depths on weed germination
❖ Due to tillage operation – weed seeds are distributed to various soil burial
depth.
❖ Some of the evidence shows - seedling emergence decreases with increasing
seed burial depth.
❖ Several factors, depending on the weed species, could be responsible for this
response are
➢ Light and
➢ seed size
-generally limited seedling emergence from deep layer of soil.

43. Effect of seed burial depth (cm) on seedling emergence (%) of different weed species
Weed species
Seedling emergence (%) at different
burial depths
0 (cm) 0.5 (cm) 1 (cm) 2 (cm)
Cyperus difformis 58 0.3 0.0 0.0
Cyperus iria 86 0.7 0.0 0.0
Eclipta prostrata 83 0.0 0.0 0.0
Fimbristylis miliacea 55 0.0 0.0 0.0
Heliotropium indicum 70
2.0
0.3 0.0
Ludwigia hyssopifolia 84 2.8 0.0
0.0
Portulaca oleracea 72 0.9 0.7 0.0
*Known quantities of seeds were placed at different soil depths in pots and their emergence (%) was counted.
Chauhan et al., (2008)

44. ❖ Rice–wheat cropping system the greatest numbers of Phalaris minor seedlings
(83%) emerged at 1 cm soil depth but,
increasing depth declined the seedlings (15%) emerged at depth of 10 cm.
(Hari et al., 2003)
❖ seedlings of Avena fatua, another important weed of wheat,
emerge from depths ranging from near the surface to 20 cm depth.
(Sharma,1998)

45. 3.Role of tillage in weed management in dryland and wetland conditions
❖ Effect of tillage on weed emergence depends on factors include,
➢ intensity and timing of tillage,
➢ type and depth of the tillage equipment, and
➢ extent that the soil environment is modified by the tillage.
(Chauhan et al., 2006)
❖ Tillage - affects seedling establishment through
➢ seed predation,
➢ seed dormancy,
➢ seed longevity, and
➢ potential of a seedling to emerge from a given depth.
(Chauhan et al., 2006)

46. ❖ Tillage - stimulate germination of some weed seeds by
➢ Exposing buried seeds to light,
➢ aerating soil,
➢ increasing soil temperature,
➢ removing plant canopy,
➢ releasing soil-bound volatile inhibitors, and
➢ increasing seed– soil moisture contact. (Franke et al., 2007)
❖ Tillage - increases
➢ diffusion of O2 into soil and CO2 out from soil,
➢ increases the temperature fluctuations, and
➢ favours nitrogen mineralization. (Mohler,1993)

47. Effect of tillage systems on Conventional tillage and no-till on seedling emergence
(%) of different weed species
Weed species
Seedling emergence (%)
Conventional tillage No-tillage
Ageratum conyzoides 2.1 20.9
Eclipta prostrata 0.8 15.4
Portulaca oleracea 3.1 17.8
Digitaria ciliaris 3.1 22.4
Echinochloa colona 6.4 21.5
(Chauhan et al., 2008)

48. ❖ Soil tillage act as a pre emergence weed management tool to - reduce the seed
germination in various soil depths.
(Forcella et al., 2000)
❖ Lowland rice, puddling (cultivation in wet soil) is performed - to achieve
➢ pre sowing weed control,
➢ good crop establishment, and
➢ reduction of water permeability of the soil.
(Sharma and Bhagat, 1993).

49. Role of surface mulches in rainfed and irrigated direct-seeded crops
❖ Crop residues on the soil surface –
influence to reduce
germination and emergence of weeds by altering the environment
surrounding the seeds.
❖ Impact of crop residues on weed emergence,
➢ depends on the quantity,
➢ position relative to weed seeds (below or above), and
➢ allelopathic potential of the residue and the weed species
(Chauhan et al., 2006)
❖ Seedling emergence of many weeds - suppressed – if crop mulches are left on
the soil surface.
(Buhler et al., 1996)

50. ❖ Weed suppression by mulch is attributed to various factors,
➢ physical and chemical factors, and
➢ lower soil temperatures,
➢ shading, and
➢ physical obstruction provided by mulch.
(Facelli et al.,1991)
❖ Crop residues – reduce the soil temperature – inhibited the reduction of
weed emergence.

51. (Chauhan, 2008)
Effect of residue amount (Mg ha-1) on percent seedling emergence (±SEm) of different weed species

52. ❖ Increasing the mulches on soil surface – it will decrease the weed emergence.
(Teasdale et al., 1991)
❖ cover crops – create unfavourable environment - for weed germination and
establishment.
(Teasdale, 1996)
❖ Cover crops and their residues - suppress weeds in several crops, including
corn ( Johnson et al., 1993) and soybean (Ateh and Doll, 1996).
❖ Decomposition of crop residues – results - the production or release many
phytotoxic organic substances

53. Responses of Weed Seed Germination to Stresses
❖ Weed seed germination is influenced by soil salinity and water stress.
❖ Salinity is a worldwide problem in agriculture.
❖ In Asia, a significant area (21.5 million ha) is reported as salt affected.
(Lafitte et al., 2006)
1.Effect of salt and moisture stress on weed germination

54. Weed species NaCl (mM) OP (MPa)
Chromolaena odorata 172 -0.58
Corchorus olitorius 230 -0.92
Cyperus difformis 23 -0.12
Cyperus iria 74 -0.46
Digitaria ciliaris 112 -0.41
Digitaria longiflora 66 -0.39
Echinochloa colona 106 -0.46
Eclipta prostrata 194 -0.58
Eleusine indica 78 -0.40
Mimosa invisa 255 -0.87
Leptochloa chinensis 50 -0.11
Melochia concatenata 198 -0.60
Concentrations of sodium chloride (NaCl) and osmotic potential (OP) required for 50% inhibition of
the maximum germination of different weed species
*Germination was evaluated by placing a known number of weed seeds in different concentrations of NaCl or OP at
optimum temperatures for 14 days. The concentrations were estimated from the fitted model.
Chauhan (2008)

55. Ex.
❖ Germination of Cyperus difformis , declined by 50% germination at NaCl
concentration of 23 mM, whereas more than 10 times of this concentration
(255 mM NaCl) was required to inhibit 50% of maximum germination of M.
invisa.
❖ Soils with more than 100 mM NaCl are considered to have high salt content.
(Tanji and Kielen, 2002)
❖ Fifty percent of maximum germination of most weeds occurred only at greater
than 100 mM NaCl

56. ❖ Germination of Corchorus olitorius and Mimosa invisa decreased by 50% of the
maximum germination at an osmotic potential of -0.9 MPa,
❖ But only -0.1 Mpa osmotic potential reduced the emergence of Cyperus difformis
and Leptochloa chinensis to this level.
❖ These experimental results suggest that seeds of some species remain
ungerminated until moisture is adequate,
❖ Because the emergence of a large number of seedlings at the beginning of the
rainy season or monsoon that commonly coincides with the beginning of the
cropping season.
❖ This is likely to be a valuable tactic in these species to delay germination until
favorable conditions occur, which ultimately increases the longevity of these
seeds in the seed bank.

57. 2.Role of flooding in lowland conditions
 Water
weed management in lowland rice
 Flooding influencing
➢ Severity of weed competition,
➢ Effectiveness of herbicides
(Kim et al., 2001)
❖ Rice is more tolerant than many weed species – flooding,
❖ Differential responses between rice and associated weeds –
farmers to use flooding
against weeds in lowland conditions.

58. ❖ Weed suppression by water is attributed to various factors,
➢ Timing,
➢ duration, and
➢ depth of flooding
(Hill et al., 2001)
❖ Flooding - inhibit germination and growth of weed species is varies - species
to species.
Ex.
➢ Cyperus difformis, Cyperus iria, Fimbristylis miliaceae, Ludwigia hyssopifolia
- controlled by flooding.
➢ While, Monochoria vaginalis and Sphenoclea zeylanica are well adapted -
Flooding conditions. (Kent et al., 2001)

59. Seedlingemergence(%of0cmfloodingdepth)
Depth of flooding (cm)
Meandrymatter(%of0cmfloodingdepth)
Emergence EmergenceDrymatter Drymatter
Effect of flooding depth (0, 2, 4, and 10 cm) and duration (intermittent: 2 days out of 7 days for 28 days, and
continuous: for 28 days) on seedling emergence (A) and mean dry matter (B) of different weed species.
(Chauhan et al.,2008)

60. This study was examining the effects of flooding duration & flooding depth of
emergence and growth of different species.
Here,
Continous flooding
Intermittent flooding
Both flooding in 28 days
Continous flooding
➢ Flooding to a depth of 2 cm compared with 0 cm greatly reduced emergence
of Cyperus difformis, Cyperus iria, Fimbristylis miliaceae, Ludwigia
hyssopifolia.
Intermittent flooding
➢ Intermittent (2 days out of 7 days for 28 days) flooding,
however, was effective only when flooding depth increased to 10 cm.

61. Thank y u…