Transcript 18 Agrochemicals

AGROCHEMICALS
Population:
Crop Yields:
1B in 1830; 5B in 1986; > 6.76B today (Jan 09)
~ 25 bushels/acre (1800) to >110 b/a today
~ 1.68 tonnes/hectare (1800); 7.39 tonnes/hecate today
FERTILIZERS
NITROGEN: plants require N as NO3-, either from natural sources or
by symbiotic N2 fixing bacteria
Legume crops have root nodules that contain bacteria capable of
converting N≡N to nitrate (via Mo, V based nitrogenase enzymes)
egs. alfalfa, lover, most beans and peas
ALL others need nitrate – add as fertilizer
BUT water soluble so much washes into lakes
and streams causing algal blooms
PHOSPHORUS
Phosphate rock (and bone meal) is mostly Ca3(PO4)2 very insoluble
Treatment with sulfuric acid gives more soluble SUPERPHOSPHATE
Ca(H2PO4)2 + 2CaSO4
more easily absorbed by plants BUT more easily washed in to lakes
TRIPLE SUPERPHOSPHATE uses phosphoric acid to give Ca(H2PO4)2
POTASSIUM
Potash:
technically this is K2CO3 from wood ashes but the term
now applied to any potassium deposit such as KCl
Global
~ 36M tonnes/y (as K2O equivalents)
Canada 32% (Saskatchewan and Alberta)
NB: Saskatchewan led Canada in jobs, economic growth in
2008 due largely to potash, uranium and oil industries
MIXED BAG FERTILIZERS
N-P-K
13-10- 6
13% N, as elemental N
10% P, as P2O5 (‘real’ %P = P number x 0.44)
6% K, K2O (‘real’ %K = K number x 0.83)
so 13-10-6 is really 13-4.4-5.0 !
ASIDE:
THE ENERGY PROBLEM
Corn uses ~ 950L fuel per hectare so to get 4 kg of corn uses about
1 kg of fuel
ENERGY IN: 45 MJ for fuel
ENERGY OUT: 10-60 MJ (depending on
efficiency of processes)
Diverting corn to fuel use may not make energy sense!
What is alternative to more fertilizers?
Plant Growth Hormones (>100 M$)
Auxins cause enlargement of cells
Gibberellins stimulate division and enlargement
Cytokinins stimulate division
Gibberellic acid
O
OH
CO
Treatment of sugar cane with 100 g
gibberellin per hectare, increases yield
~ 12 tonnes/hectare
HO
COOH
Good on grapes - larger
No good on cabbage - makes it flower
Monsanto’s Polaris HOOCCH2N(CH2PO3H2)2 = Glyphosphine is an
aminoacid derivative which increases sugar content 10-20% at
harvest (introduced in sugar shortage of 1974, enormous profits) –
very similar to structure to their herbicide Roundup:
LD50 = 4 g / kg body mass!
Very low toxicity to mammals (inhibits
plant protein synthesis, specifically
of aromatic amino acids)
Cyanamid’s Cycocel (Me3NCH2CH2Cl)+Clalso ripens sugarcane, but reduces the
stem length of cereal crops like wheat
- reduces fall over in heavy wind/rain
Ethephon ClCH2CH2PO3H2 stimulates latex flow in rubber trees,
stimulates flowering in pineapples...
HERBICIDES (> 15B $ World, > 7B US alone) ~ ½ B kg /y
Weeds compete for fertilizer and water; reduce crop yields
90% corn, soya-beans, cotton, pea-nuts, rice treated;
75% of herbicides go on farm crops
2,4-D (1946) and 2,4,5-T (late 50’s) (mixture = Agent Orange):
OCH2COOH
OH
Cl
Cl
Cl
Cl
Cl
HO-
Cl
Cl
2,4-D
Cl
Cl
OH
Cl
OCH2COOH
Cl
Cl
Cl
Cl
2,4,5-T
‘Dioxin’
Cl
O
Cl
Cl
O
Cl
2,4-D (1946) kills broadleaf plants at 200 g – 2.5 kg per hectare
(much less than the inorganic defoliants used to that point)
Cheap (use >100M kg/y); Low mammalian toxicity ~0.5 g / kg orally,
1.5 g / kg dermally; degrades quickly in soil, does not concentrate
Can use on wheat, barley, corn, sugar, rice, GRASS (home use)
then the relative 2,4,5-T was found to be more effective on brush,
blackberries: used around power lines AND THE VIETNAM JUNGLE
US used ~70M L of 50:50 2,4-D/2,4,5-T (AGENT ORANGE) Vietnam
sprayed neat, not diluted as used commercially in NA
Causes puss oozing acne and birth defects due to the Dioxin
impurity in 2,4,5-T - stopped use in 1970
EPA permitted use on rice, rights of way, home use till 1985
(later in Canada)
Viktor Yushchenko, Ukrainian
Presidential Candidate 2004,
Poisoned by ‘dioxin’ (?)
Shows the classic disfigurement
Caused by severe chloracne
ATRAZINE
(Ciba-Geigy, Shell, Syngenta)
Used at 2-5 kg / hectare
LD50 ~5 g / kg
Corn is able to remove -Cl and deactivate so atrazine is widely used
Disrupts photosynthesis
Controversy: It was one of the more widely used, but Banned in EU
in 2004 because detected in ground waters. Some associate the
feminization of frogs to atrazine; US disagrees:
http://www.syngentacropprotection-us.com/prod/herbicide/Atrazine/
http://www.epa.gov/safewater/contaminants/dw_contamfs/atrazine.html
http://www.pan-uk.org/pestnews/Actives/atrazine.htm
Major herbicides in use today
•Imazapyr, is a non-selective herbicide used for the control of a broad range of weeds including terrestrial
annual and perennial grasses and broadleaved herbs, woody species, and riparian and emergent aquatic
species.
•Imazapic, is a selective herbicide for both the pre- and post-emergent control of some annual and perennial
grasses and some broadleaf weeds. Imazapic kills plants by inhibiting the production of branched chain
amino acids, which are necessary for protein synthesis and cell growth.
•Glyphosate, a systemic nonselective (it kills any type of plant) herbicide used in no-till burndown and for
weed control in crops that are genetically modified to resist its effects. It is an example of an EPSPs inhibitor.
•Paraquat, a nonselective contact herbicide used for no-till burndown and in aerial destruction of marijuana
and coca plantings. More acutely toxic to people than any other herbicide in widespread commercial use.
•2,4-D, a broadleaf herbicide in the phenoxy group used in turf and in no-till field crop production. Now mainly
used in a blend with other herbicides that act as synergists, it is the most widely used herbicide in the world,
third most commonly used in the United States. It is an example of synthetic auxin.
•clopyralid, is a broadleaf herbicide in the pyridine group, used mainly in turf, rangeland, and for control of
noxious thistles. Notorious for its ability to persist in compost. It is another example of synthetic auxin.
•metoalachlor, a pre-emergent herbicide widely used for control of annual grasses in corn and sorghum; it
has largely replaced atrazine for these uses.
•dicamba, a persistent broadleaf herbicide active in the soil, used on turf and field corn. It is another example
of synthetic auxin.
•picloram, a pyridine herbicide mainly used to control unwanted trees in pastures and edges of fields. It is
another synthetic auxin.
•atrazine, a triazine herbicide used in corn and sorghum for control of broadleaf weeds and grasses. Still
used because of its low cost and because it works as a synergist when used with other herbicides, it is a
photosystem II inhibitor.
Pesticides and Insecticides
Pesticide residues on fruit and
vegetables are very small
Natural carcinogens, eg. aflatoxin in
the mould on peanuts and corn is
much more dangerous
Risk from carcinogens:
1/3000 motor cycle
1/300 smoking
1/25 car driving
25x strike by lightening!
Daminozide (ALAR)
Uniroyal (1963)
Before 1989, BC and Washington apples were sprayed with ALAR
to stops premature falling and bruising
Feb 1989: ‘60 Minutes’ aired a report that ALAR was found in apple
juice and caused tumors in rats – at the equivalent to 20,000 L of
apple juice per day per rat!
Uniroyal took ALAR off market voluntarily (Apple growers filed a
libel suit against CBS, dismissed in ‘94)
Now apples can develop a natural fungus in
bruises that produces the mycotoxin patulin:
An antibiotic but also a carcinogen that may be
more potent than ALAR!
INSECTICIDES
500 BC
1500
1800
1900’s
Burning Sulfur;
Arsenic
Nicotine (Sulfate)
Lead Arsenate
1900 Extract of dried chrysanthemum = PYRETHRUM
Have to hit insect
Decomposes in sunlight
(no good for crops)
Low mammalian toxicity
By 1949 had developed more stable synthetic ones:
O
H
O
H
O
Cl
O
Allethrin
Cl
O
O
Permethrin
O
O
O
O
O
Piperonyl butoxide
Allethrin could be used in sunlight but needs a synergist, piperonyl
butoxide, to prevent insect recovery - used in RAID and expensive
Since 1970, for agricultural use, Permethrin (Ambush, Pounce) is
better, need only 100g/hectare – more stable, cheaper, BUT still
relatively expensive LD50 = 2g/kg - low – known as PYRETHROIDS
THE CHLORINATED PESTICIDES
DDT first made in 1873 in Germany (Baeyer) but insecticidal
properties not recognized until 1939 (Geigy labs in Switzerland)
acid
Cl
CHO
CCl3
ALL CHEAP
Cl
Cl
Cl
CCl3
CHEAP < 50c/kg in 1940
dichlorodiphenyltrichlorethane
Wartime: pyrethrum supply cut off by
Japan (troops were lice/tick infested)
Tried DDT as 10% powder mixed with talc
directly on millions of troops: 3M people
treated in Naples in 1943 alone!
DDT THE GOOD
Cheap
Effective against all insects
Extremely persistent
Low toxicity to non-insects
DDT THE BAD
Cheap - used indiscriminately
Kills ‘good’insects
Stays around for >20 years
Estrogenic effect on birds
[LD50 300-500 mg/kg, 2-3 g/kg dermally, but about 10 mg/kg to insects]
Insects do not need to eat - absorbed through cuticles: allows Na+ to
leak into nerve channels, continuous transmission of nerve impulses,
overload, death
Crop yields went up
Stopped Gypsy Moth in East, Spruce Budworm in West
Reduced malaria in tropics (1g/m2 on house walls), also kills louse
(Typhoid), Flea (Plague), Tetse Fly (Sleeping sickness)
Peak production in 1961 ~ 400 Mkg/y; World total production >3B kg,
i.e. > 3M tons or about ½ kg per person alive today!
Insect Resistance – overrated! Some insects have developed DDTase which dehydrochlorinates DDT to DDE; however that is easy to
overcome by blocking the enzyme, eg. with chlorofenthol
OH
Cl
Cl
Cl
DDE
Cl
Cl
Cl
Chlorofenthol
Non-selectivity is bad – most insects not pests!
SOLUBLE IN FAT: passes up food chain
ALSO VERY PERSISTENT: half life ~ 20 years!
UVIC students 1970: ~12 ppm in tissues
NOW: ~2-3 ppm even though use in NA was
stopped in 1972!
BIRDS: DDE and DDT metabolites mimic
diethylstilbestrol and have weakly estrogenic
activity: affect Ca transport – thin egg shells
See: ‘Silent Spring’
by Rachel Carson (1962)
for one view of DDT
In Canada, Methoxychlor is widely used – degrades more quickly
Cl
Cl
OMe
MeO
CCl3
Methoxychlor
Cl
Cl
Cl
CCl2
Cl
Cl
Cl
Lindane
O
Cl
Cl
Dieldrin
But many much more toxic ones were also made:
Lindane
Dieldrin
LD50
LD50
100 mg/kg
50 mg/kg highly toxic to fish and mammals
Most have been phased out now but residues remain buried in
soils, ocean muds, lake bottoms, etc.
ORGANOPHOSPHATE NERVE AGENTS
Less environmentally persistent insecticides (hydrolyze)
Acetylcholine esterase inhibitors
Relative
Toxicity
1
125
Malathion LD50 = 1 g/kg
Parathion LD50 = 8 mg/kg
12.5
Diazinon LD50 = 80 mg/kg
100,000
Sarin LD50 = 10 mg/kg
All species use acetylcholine as a nerve transmitter:
Botulinus toxin prevents synthesis of acetylcholine - no impulses paralysis of muscles, etc.
Organophosphates block acetylcholine esterase: prevents break
down of acetylcholine - nerve impulses continuously sent, overload,
convulsions, death
~300,000 farm workers per year suffer from pesticide poisoning!!
Other important strategies not covered here:
Insect biology strategies:
pheromones
interfere with sexual development
interfere with maturation
Genetic engineering of crops:
disease resistance
insect resistance (kill insects when feed on plant)
herbicide resistance (to kill weeds but not plant)
better climate tolerance (drought, heat, cold)