Lecture 2: Biology of weeds Flashcards
the world population is predictive to reach __ by 2050
9 billion
more than ____ people of not have adequate food
800 million
___ people live on less than $1 a day
1.3 billion
At least ___% of global food production is lost pre-harvest to weeds, pests & diseases in developed countries
30%
- this rises to 40-50% for some crops in the developed world
weeds, pests, and diseases are a __ to food security
THREAT
Weed control examples:
- cultivation practices
- biological control
- chemical control
- biotech solutions
- plant breeding strategies
- integrated control strategies
cultivation practises e.g.:
good drainage, manuring + liming, choice of crop and varieties, rotation of crops, weeding & good farm hygiene
biotech solutions to weed control e.g.
genetic modification of crops
plant breeding strategies to weed control e.g.
resistant cultivars
2 definitions of a weed:
“a plant growing in any situation where it is considered to be undesirable”
” A plant growing in the wrong place at the wrong time”
can weeds be both monocotyledonous and dicotyledonous plants??
YES,
can be broad leaved dicotyledonous plants or monocotyledonous e.g. grasses
is a weed always a weed?
no, a plant may be a weed in some circumstances but not in others
of the 250,000 plant species in the world only ____ are troublesome weeds
only a few hundred
plants classified as agricultural weeds possess
certain characteristics which contribute to their success
biological characteristics commonly possessed by weedy species:
- Competitiveness
- Persisitence
competitiveness in weeds:
is the ability of weeds to draw resources (light, water, nutrients and CO2) away from crop plants
persistence in weeds:
the ability to survive from year to year on a given area of land despite farmers attempts to control them
traits contributing to competitiveness and persistence in weeds include:
- rapid seedling establishment
- high growth rates
- prolific root systems
- large leaf area
- high reproductive output
- plasticity (the ability to adapt to changes in the environment)
- type of life cycle to the crop
- germination characteristic of seeds
life cycle of weeds: Annuals:
complete their life cycle in one season e.g. poppies, nettles, thistles
life cycle of weeds: biennials:
take two years to complete their life cycles e.g. ragwort
life cycle of weeds:
- Annual
- biennials
- ephermerals
- perennials
life cycle of weeds: Ephermerals:
produce several generations of seed per year e.g. hairy bittercress & chickweed
life cycle of weeds: perennials:
- persists from year to year
- the aerial shoots die back & plants persist vegetatively as stolons, rhizomes, bulbs or tap roots
Perennials: Coltsfoot
persists by means of underground rhizomes
perennials: creeping buttercup
persists by means of stolons, underground stems
perennials: wild garlic and wild onion
persist by means of a bulb
understanding there germination characteristics of weed species is important for
weed control
all agricultural soils have ___ seed banks
LARGE
things to understand of the large seed banks in agricultural soils
- depth of germination
- -> most seed have amax depth from which they can germinate depending on seed size and food reserves
- -> the majority of annual seeds are to be found in the top 5cm of soil
- periodicity of germination
- dormancy
periodicity of germination:
- most seeds of a given weed species germinate & emerge at a particular time of the year
- means there is a close association between certain weed sp. and certain crop plants
- maximum period of germination is known then correct herbicide can be chosen
periodicity of germination: Ive leaved speedwell
germinates in the autumn and winter and is a problem in winter cereals
periodicity of germination: Chickweed
emerges in the spring and is a problem in spring and summer crops
dormancy is a mechanism that
has evolved to ensure survival of the species. Most seed have primary or innate dormancy when they are direct produced
primary or innate dormancy may be due to:
- impermeability of the seed coat to water
- impermeability of the seed coat to gases
- mechanical resistance to embryo growth
- immaturity of the embryo
- the embryo may require a period of drying or cold
how do weeds damage crops?
- competition for resources
- loss in the quality of produce
- poisoning of livestock
- harbouring of pests & diseases
- pollination
how do weeds damage crops? competition or resources &
are most weeds parasitic?
- competition for resources (light, water, CO2, nutrients)
- -most weeds are not parasitic on their hosts & compete, indirectly for resources. Few are parasitic on crop species & Compete for host resources directly
the effect of weed density on rice yields:
- as weed density increases, the yield of cultivated rice decreases
- the extent of the decrease in yield differs depending upon the weed
- the field decrease occurs as a result of competition for, water & sometimes light
competition for water:
those plants outcompeted for water are a lot smaller and lack growth
where are parasitic weeds important?
- sub Saharan Africa e.g. Strega species
- some Mediterranean agro-ecosystems e.g. Orobanche species
how do parasitic weeds lower yield:
-Directly compete with the host for host resources (e.g. carbon, nitrogen and inorganic solutes) as they parasitise the host
Striga species not only parasitise their host but cause
a molecular reprogramming of host development
example of 2 root parasites:
- Orobanche cumana (broomrape) (obligate holoparasite)
- Striga hermonthica (witchweed) (obligate hemiparasite)
Striga hermonthica & S. asiatica infect:
Maize, sorghum, millet & upland rice directly affecting the lives of 100M people in sub Saharan Africa
___ ha land infested with Striga seed & yield losses of ___% are common
- 70M ha
- 40-100%
Orobanche species infect
many legumes & vegetables & Crops such as sunflower, tomato & tobacco
where are Orobanche species a problem & how much loss do they cause:
problem in Russia, Ukraine, Mediterranean regions, North Africa & the middle east again causing losses form 10-100%
control of parasitic weed is ___
very difficult
Effects of Striga on the host:
1) lowers biomass and height
2) Reduction in biomass & grain production due to:
- -utilisation of host nutrients by the parasite (competition)
- -Molecular reprogramming of host growth & development
the genus Orobanche includes more than
100 species
distribution of orobanche:
mainly in the mediterranean Basin, but also in Eastern Europe, Former Soviet Union
which genus are holoparasitic and what does this mean
OROBANCHE, lacks chlorophyll
does Orobanche form direct connections with the host phloem & xylem
YES
how does orobanche lower the yield of host
by acting as a very efficient sink for host carbon, nitrogen & inorganic nutrients
five species of orobanche for economic important
O. aegyptiaca O. ramosa O. cumana O. cernua O. Crenata
Orobanche parasitised cultivated species:
sunflower, cabbage, cucumber, potato, tomato, tobacco, carrot
__ ha infected with O. cumana
73 million hectares infested with O. cumana
Orabanche can lead up to yield losses of up to
50%
existing management methods have ___ against Orobanche
limited success & applicability
how do weeds damage crops? loss in the quality of produce
- Contamination of produce for processing e.g.
• Wild oats in malting barley
• Black nightshade berries in peas - Weed seeds are an impurity in crop seed which can lead to failure to reach EEC standards
- Harvesting problems
• The presence of weeds in a crop
slows down combine harvesters
• If the weed ripens after the cereal it may raise the moisture content of the grain h increasing drying costs.
how do weeds damage crops? Poisoning of livestock
- Many common weeds are poisonous to livestock e.g. ragwort, bracken and hemlock.
- Most cattle will avoid these species in the field but the real problem come if they get into silage.
- Horses and cows are more susceptible to ragwort poisoning than other livestock.
- Young animals are even more susceptible than older animals.
- Ragwort destroys the liver of the animal by poisoning.
- Little can be done after clinical symptoms appear.
how do weeds damage crops? harbouring of pests & diseases
• Weeds may be a problem if they harbour pests and disease of crops.
• Several weed species act as a host for a range of fungal, viral and nematode borne diseases e.g.
– Couch grass/Take-All of wheat – Chickweed/nematodes.
how do weeds damage crops? Pollination
- Weeds can compete with trees in orchards for pollination by bees.
- The flowers of some weeds attract bees more than apple and plum blossom.
- Weeds in orchards also reduce yield by competing for water and nutrients
3 approaches to weed management:
- prevention
- Eradication
- control
prevention aim:
is to keep weed seeds & vegetative materials from being introduced into a field e.g.
–• Make sure that seed that is sown is pure
• Clean implements when moving from one field to another
• Mow weeds in field borders to prevent seed se
Eradication: because weeds are persistent once they become established it is _____ to eradicate them
is almost impossible and impractical
Eradication is ___ attempted
RARELY
and is only considered when a noxious weed has been inadvertently introduced from one area to another.
a successful eradication programme requires:
a long term connected effort consisting of several preventative & Control measures e.g. the attempt to eliminate Striga from the USA
