Lectures 1 & 2: Introduction to weeds Seed ecology Weed management methods

Definition of Weeds

• Weeds are unwanted and undesirable plants.
• Plants out of place.
• According to the Weed Science Society of America (WSSA), weeds are any plant that is objectionable or interferes with human activities or welfare.
• The European Weed Research Society (EWRS) defines weeds as any plant or vegetation, excluding fungi, that interferes with the objectives or requirements of people.

Characteristics of Weeds

• Long seed life in soil.
• Quick emergence.
• Ability to survive and thrive in disturbed conditions, such as a cropped field.
• Rapid early growth.
• Lack of special environmental requirements for seed germination.

Losses Due to Weeds

• Weeds cause the highest losses among agricultural pests.
  • Weeds: 45%
  • Insects: 30%
  • Diseases: 20%
  • Other pests: 5%
• Economic losses:
  • Australia: AU$ 3.3 billion/year to grain growers.
  • USA: US$ 33 billion/year.
  • India: US$ 11 billion/year in top 10 crops.

Harmful Effects Caused by Weeds

• Direct $ losses
• Indirect $ losses
• Reduce crop yield.
• Increase production cost.
• Reduce quality.
• Harbour other pests.
• Reduce land value (perennial weeds).
• Limit the choice of crops.
• Reduce labour efficiency.

Reduce Crop Yield

• In Australia, weeds cause losses of 2.8 million tonnes of grain/year.
• Competition for sunlight, nutrients, water, and space.
• Intensity of weed competition depends on:
  • Weed species.
  • Weed density.
  • Time of weed emergence.
  • Duration of infestation.
  • Competitive ability of the crop.
  • Climatic conditions (whether they favor the crop or the weed).

Increase Crop Production Cost

• Herbicide use (equipment, labor, and application).
• Number of tillage/weeding operations.
• Farm equipment depreciation.
• Cost of harvesting.

Reduce Quality

• Animal products:
  • Wool (e.g., cocklebur, cobbler’s peg).
  • Milk (goat weed and parthenium weed).
• Leafy vegetables (wild onion in onion) and oil (Mexican poppy in mustard).
• Contaminate crop grain (weedy rice in rice; wild oats in oats).

Harbour Other Pests

• Alternate hosts of insects (rice stem borer on barnyard grass; tomato stalk borer on fat hen).
• Alternate hosts of diseases (black stem rust of wheat on wild oats; rice blast on nutgrass; cucumber mosaic virus on chickweed).
• Alternate hosts of nematodes (nematodes on barnyard grass).

Reduce Land Value

• Heavy infestation of some weeds can make land unsuitable (e.g., nutgrass and parthenium weed).
• Economic value of lakes for recreation purposes (e.g., water hyacinth).
• Block irrigation channels and drains (e.g., water hyacinth).
• Road edges and railway lines (grass species).

Limit the Choice of Crops

• Presence of a particular weed limits the choice of crops (e.g., canola in fields infested with Orobanche spp.).

Reduce Human Efficiency

• Through physical discomfort caused by allergy & poisoning (parthenium weed and ragweed causes itching and other diseases).
• Through spines (Mexican poppy in chickpeas).

Weed Classification

• Life cycle:
  • Annual (wild oats).
  • Biennial (puncture vine).
  • Perennial (nutgrass).
• Growth characters:
  • Broadleaf (pigweed).
  • Grass (FTR).
  • Sedge (nutgrass).
• Reproductive propagules:
  • Seed (sexual).
  • Vegetative (asexual).
• Photosynthetic pathways:
  • C3 and C4.
• Photoperiod:
  • Short day, long day, and day neutral.
• Plant architecture:
  • Herbaceous, semi-woody, and woody.
• Origin:
  • Native and introduced.

Weed Seed Dispersal

• Most weeds are good “travelers”.
• Use various forces or agents to transport and scatter themselves from place to place.
  • Wind
  • Water
  • Animals
  • Birds
  • Humans

Weed Seed Bank

• Seed rain + seed dispersal (water, wind, animal, machinery).
• Decay (pathogens).
• Predation (birds, mice, insects).
• Germination:
  • Germinate & die.
  • Germinate & establish.

Seed Germination & Dormancy

• Germination (G) is a critical factor for the establishment of a species.
• G occurs when the environmental conditions required for the mechanism are favorable.
• Dormancy is a state in which a viable seed fails to germinate even under conditions of moisture, light & temperature favorable for plant growth (a type of resting stage for the seeds).
• When the conditions for G are not favorable, seeds can become dormant to survive in the soil (efficient survival mechanism of weeds).

Factors Affecting Seed Germination

• Light
• Temperature (constant & fluctuating)
• Seed scarification and fire
• Burial depth (for successful seedling emergence)
• Crop residue

Light

• Weeds respond differently to light & darkness.
• Responses are species-specific
  • Germinate equally in light and dark
  • Light stimulates G
  • Absolute light requirement for G (+vely photoblastic)
  • Light inhibits G

Light (species-specific)

• CS – Chinese sprangletop
• RF – Rice flatsedge
• CST – Common sowthistle
• IHM – Indian hedge mustard
• MM – Marsh mallow
• WT – Wild turnip
• BK – Bladder ketmia
• WMG – Windmill grass
• TW – Turnip weed
• The light and dark germination percentages of different species vary.

Temperature

• Info helps us to understand the spread in different regions/seasons.
• Seeds of WMG germinated at all the tested temperature regimes, indicating its ability to germinate throughout spring, summer & autumn in Qld.
• Ability to germinate in spring would make weed control difficult as winter cereal crops are still growing in the fields at that time.

Seed Coat

• Seed coat can impose dormancy (impede water absorption).
• Hard seeds require scarification (physical/chemical) to enhance G.
• Scarification significantly increases germination percentage in weeds like Bladder ketmia, Marsh mallow, Sensitive plant, and Wild jute.

Seed Burial Depth

• Soil disturbance by any means affects vertical seed distribution.
• Optimum burial depth from which a max no. of sown seeds can produce established plants.
• Generally, greatest seedling emergence (E) from the soil surface but species differ in their response to depth.

Seed Burial Depth

• Weed seedling E decreases with increasing burial depths (tillage/inter-cultivation or other soil disturbance operations).
• Light & seed size generally limit E from depths.

Seed Burial Depth

• Greater E from seeds on or near the surface suggests that NT practices would enhance E of these small-seeded spp as a large proportion of the SB remains on the surface after planting.
• With few exceptions, E from the weed SB occurs only from the upper few cm soil layer.
• Therefore, weeds could be managed by burying their seeds below the maximum depth of E with deep tillage & subsequently to using shallow tillage to avoid bringing the seeds back onto the soil surface.
• Greater knowledge on the impact of seed burial depth on target species could improve weed control & reduce reliance on other options.

Crop Residue

• Provides a physical barrier to emerging weeds (reduce & delay E).
• Impact depends on the quantity, position relative to weed seeds & allelopathic potential of the residue.
• Cover crops & their residues are also used to suppress weeds.

Management

Preventive Measures

• Plant crop in weed-free conditions.
• Weeds are just the symptom of the problem.
• The real problem is weed seed banks.
• Farmers would benefit from management practices that reduce weed seed input, increase seed losses, and reduce the probability that residual seeds establish.
• Stop seed set.

Fallow-Focused Weed Management

• Soil moisture is an important issue in dry land agriculture and weeds may deplete most of this moisture.
• In addition, weeds may harbor pests, including nematodes.
• Therefore, weed control in the fallow phase is very important.
• WeedSeeker® sensor technology is gaining increased acceptance in Australian agriculture (GRDC 2014).

WeedSeeker® sensor technology

• The sensor captures infrared reflection from green tissue and delivers herbicide as the boom passes over the plant.
• This technology has advantages because less area in a paddock needs to be covered and therefore offers economic and environmental benefits.
• Growers are able to try different modes of action herbicides due to less operation cost compared to whole farm spraying.
• Up to 90% savings in chemical.

Cover Cropping

• Cover cropping is the process of growing a crop during a fallow period, creating ground cover that helps increase water infiltration, slow evaporation, and manage weeds.
• The concept is very new in Australia.
• Need to evaluate the impact of cover crops in different situations, provided that soil moisture is not limited (a good tool to manage HR weeds, especially in HT crops).

Strategic Tillage

• We are NT country but tillage (one-off operation) can be used to bury the seeds below their maximum depth of emergence (effective for surface germinating seeds; FTR and sowthistle).
• Subsequent tillage operations - shallow Tillage is back, but not as we knew it.

Crop Competition: Narrow Rows

• Narrow row spacing gives advantages to the crop in terms of weed competitiveness by developing faster canopy closure and less light penetration through the leaves.

Crop Competition: High Seeding Rates

• High seeding rates are used in many crops to suppress weeds, where weed control is expected to be poor.
• Growers in some countries (e.g., Uruguay) use high seeding rates (up to 165 kg/ha) and narrow row spacing (15 cm), mainly to suppress weeds.

Crop Competition: Weed-Competitive Cultivars

• Taller genotypes & early crop vigor were both closely associated with suppressed weed seed production and tolerance to weed competition
• Grow a more competitive crop:
  • Wheat could suppress weeds better than chickpea.
  • Barley could suppress weeds better than wheat.

Harvest Weed Seed Control

• Weed seed control at harvest (e.g., Harrington Seed Destructor).
• Found very effective on annual ryegrass in WA.
• Works well on early harvested crops before WO drop their seeds.
• Summer weeds mature early, so seeds shatter before sorghum harvest (50-60 d vs >100 days); may work in short duration crops, e.g., mungbean.

Herbicides

• Herbicide rotation and mixtures; double knocks (see labels)

Rotations

• Rotations are the best way to manage weeds.
• Rotations of crops (including HT crops), herbicides, and tillage systems allow to use different mgt practices and thus help managing weeds more effectively than monoculture systems.
• Need to revisit this area again to manage HR weeds.
• Depend on market price, farm & farmers’ situation.