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AG CHEMICAL AND CROP NUTRIENT INTERACTIONS – CURRENT UPDATE
Don M. Huber, Emeritus Professor, Purdue University
ABSTRACT: Micronutrients are regulators, inhibitors and
activators of physiological processes, and plants provide a primary
dietary source of these elements for animals and people. Micronutrient
deficiency symptoms are often indistinct (“hidden hunger”) and commonly
ascribed to other causes such as drought, extreme temperatures, soil pH,
etc. The sporadic nature of distinct visual symptoms, except under
severe deficiency conditions, has resulted in a reluctance of many
producers to remediate micronutrient deficiency. Lost yield, reduced
quality, and increased disease are the unfortunate consequences of
untreated micronutrient deficiency. The shift to less tillage, herbicide
resistant crops and extensive application of glyphosate has
significantly changed nutrient availability and plant efficiency for a
number of essential plant nutrients. Some of these changes are through
direct toxicity of glyphosate while others are more indirect through
changes in soil organisms important for nutrient access, availability,
or plant uptake. Compensation for these effects on nutrition can
maintain optimum crop production efficiency, maximize yield, improve
disease resistance, increase nutritional value, and insure food and feed
safety.
INTRODUCTION
Thirty+ years ago, U.S. agriculture started a conversion to a
monochemical herbicide program focused around glyphosate (Roundup®). The
near simultaneous shift from conventional tillage to no-till or minimum
tillage stimulated this conversion and the introduction of genetically
modified crops tolerant to glyphosate. The introduction of genetically
modified (Roundup Ready®) crops has greatly increased the volume and
scope of glyphosate usage, and conversion of major segments of crop
production to a monochemical herbicide strategy. Interactions of
glyphosate with plant nutrition and increased disease have been
previously over looked, but become more obvious each year as glyphosate
residual effects become more apparent.
The extensive use of glyphosate, and the rapid adoption of
genetically modified glyphosate-tolerant crops such as soybean, corn,
cotton, canola, sugar beets, and alfalfa; with their greatly increased
application of glyphosate for simplified weed control, have intensified
deficiencies of numerous essential micronutrients and some
macronutrients. Additive nutrient inefficiency of the Roundup Ready®
(RR) gene and glyphosate herbicide increase the need for micronutrient
remediation, and established soil and tissue levels for nutrients
considered sufficient for specific crop production may be inadequate
indicators in a less nutrient efficient glyphosate weed management
program.
Understanding glyphosate’s mode of action and impact of the RR gene,
indicate strategies to offset negative impacts of this monochemical
system on plant nutrition and its predisposition to disease. A basic
consideration in this regard should be a much more judicious use of
glyphosate. Glyphosate damage is often attributed to other causes such
as drought, cool soils, deep seeding, high temperatures, crop residues,
water fluctuations, etc. Table X provides some of the common symptoms of
drift and residual glyphosate damage to crops. This paper is an update
of information on nutrient and disease interactions affected by
glyphosate and the RR gene(s), and includes recently published research
in the European Journal of Agronomy and other international scientific
publications.
UNDERSTANDING GLYPHOSATE
Glyphosate (N-(phosphomonomethyl)glycine) is a strong metal chelator
and was first patented as such by Stauffer Chemical Co. in 1964 (U.S.
Patent No. 3,160,632). Metal chelates are used extensively in
agriculture to increase solubility or uptake of essential micronutrients
that are essential for plant physiological processes. They are also
used as herbicides and other biocides (nitrification inhibitors,
fungicides, plant growth regulators, etc.) where they immobilize
specific metal co-factors (Cu, Fe, Mn, Ni, Zn) essential for enzyme
activity. In contrast to some compounds that chelate with a single or
few metal species, glyphosate is a broadspectrum chelator with both
macro and micronutrients (Ca, Mg, Cu, Fe, Mn, Ni, Zn). It is this
strong, broadspectrum chelating ability that also makes glyphosate a
broad-spectrum herbicide and a potent antimicrobial agent since the
function of numerous essential enzymes is affected (Ganson and Jensen,
1988).
Primary emphasis in understanding glyphosate’s herbicidal activity
has been on inhibition of the enzyme 5-enolpyruvylshikimate-3-phosphate
synthase (EPSPS) at the start of the Shikimate physiological pathway for
secondary metabolism. This enzyme requires reduced FMN as a co-factor
(catalyst) whose reduction requires manganese (Mn). Thus, by
immobilizing Mn by chelation, glyphosate denies the availability of
reduced FMN for the EPSPS enzyme. It also can affect up to 25 other
plant enzymes that require Mn as a co-factor and numerous other enzymes
in both primary and secondary metabolism that require other metal
co-factors (Co, Cu, Fe, Mg, Ni, Zn). Several of these enzymes also
function with Mn in the Shikimate pathway that is responsible for plant
responses to stress and defense against pathogens (amino acids,
hormones, lignin, phytoalexins, flavenoids, phenols, etc.). By
inhibiting enzymes in the Shikimate pathway, a plant becomes highly
susceptible to various ubiquitous soilborne pathogens (Fusarium, Pythium, Phytophthora, Rhizoctonia,
etc.). It is this pathogenic activity that actually kills the plant as
“the herbicidal mode of action” (Johal and Rahe, 1984; Levesque and
Rahe, 1992, Johal and Huber, 2009). If glyphosate is not translocated
to the roots because of stem boring insects or other disruption of the
vascular system, aerial parts of the plant may be stunted, but the plant
is not killed.
Recognizing that glyphosate is a strong chelator to immobilize
essential plant micronutrients provides an understanding for the various
non-herbicidal and herbicidal effects of glyphosate. Glyphosate is a
phloem-mobile, systemic chemical in plants that accumulates in
meristematic tissues (root, shoot tip, reproductive, legume nodules) and
is released into the rhizosphere through root exudation (from RR as
well as non-RR plants) or mineralization of treated plant residues.
Degradation of glyphosate in most soils is slow or non-existent since it
is not ‘biodegradable’ and is primarily by microbial co-metabolism when
it does occur. Although glyphosate can be rapidly immobilized in soil
(also spray tank mixtures, and plants) through chelation with various
cat-ions (Ca, Mg, Cu, Fe, Mn, Ni, Zn), it is not readily degraded and
can accumulate for years (in both soils and perennial plants). Very
limited degradation may be a “safety” feature with glyphosate since most
degradation products are toxic to normal as well as RR plants.
Phosphorus fertilizers can desorb accumulated glyphosate that is
immobilized in soil to damage and reduce the physiological efficiency of
subsequent crops. Some of the observed affects of glyphosate are
presented in table 1.
TABLE 1. Some things we know about glyphosate that influence plant nutrition and disease.
1. Glyphosate is a strong metal chelator (for Ca, Co, Cu, Fe, Mn, Mg, Ni, Zn) – in the spray tank, in soil and in plants.
2. It is rapidly absorbed by roots, stems, and leaves, and moves systemically throughout the plant (normal and RR).
3. Accumulates in meristematic tissues (root, shoot, legume nodules, and reproductive sites) of normal and RR plants.
4. Inhibits EPSPS in the Shikimate metabolic pathway and many other plant essential enzymes.
5. Increases susceptibility to drought and disease.
6. Non-specific herbicidal activity (broad-spectrum weed control).
7. Some of the applied glyphosate is exuded from roots into soil.
8. Immobilized in soil by chelating with soil cat-ions (Ca, Co, Cu, Fe, Mg, Mn, Ni, Zn).
9. Persists and accumulates in soil and plants for extended periods
(years) – it is not ‘biodegradable,’ but is rapidly immobilized by
chelation generally.
10. Desorbed from soil particles by phosphorus and is available for root uptake by all plants.
11. Toxic to soil organisms that facilitate nutrient access, availability, or absorption of nutrients.
12. Inhibits the uptake and translocation of Fe, Mn, and Zn at very low, non-herbicidal rates.
13. Stimulates soilborne pathogenic and other soil microbes to reduce nutrient availability.
14. Reduces secondary cell wall formation and lignin in RR and non-RR plants.
15. Inhibits nitrogen fixation by chelating Ni for ureide synthesis and is toxic to Rhizoiaceae.
16. Reduces physiological availability and concentration of Ca, Cu, Fe, K, Mg, Mn, and Zn in plant tissues and seed.
17. Residual soil activity can damage plants through root uptake.
18. Increases mycotoxins in stems, straw, grain, and fruit.
19. Reduces photosynthesis (CO2 fixation).
20. Causes fruit (bud) drop and other hormonal effects.
21. Accumulates in food and feed products to enter the food chain as an item of food safety.
UNDERSTANDING THE ROUNDUP READY® GENE
Plants genetically engineered for glyphosate-tolerance contain the
Roundup Ready® gene(s) that provide an alternate EPSPS pathway
(EPSPS-II) that is not blocked by glyphosate. The purpose of these gene
inserts is to provide herbicidal selectivity so glyphosate can be
applied directly to these plants rather than only for preplant
applications. As an additional physiological mechanism, activity of this
duplicate pathway requires energy from the plant that could be used for
yield. The RR genes are ‘silent’ in meristematic tissues where
glyphosate accumulates so that these rapidly metabolizing tissues are
not provided an active alternative EPSPS pathway to counter the
physiological effects of glyphosate’s inhibition of EPSPS. Meristematic
tissues also are areas of high physiologic activity requiring a higher
availability of the essential micronutrients needed for cell division
and growth that glyphosate immobilizes by chelation.
Residual glyphosate in RR plant tissues can immobilize Fe, Mn, Zn or
other nutrients applied as foliar amendments for 8-35 days after it has
been applied. This reduces the availability of micronutrients required
for photosynthesis, disease resistance, and other critical physiological
functions.The presence of the RR gene(s) reduces nutrient uptake and
physiological efficiency and may account for some of the ‘yield drag’
reported for RR crops when compared with the ‘normal’ isolines from
which they were derived. Reduced physiological efficiency from the RR
gene is also reflected in reduced water use efficiency (WUE) and
increased drought stress (table 2).
It should be recognized that:
1. There is nothing in the glyphosate-tolerant plant that operates on the glyphosate applied to the plant.
2. All the technology does is insert an alternative enzyme (EPSPS-II) that is not blocked by glyphosate in mature tissue.
3. When glyphosate enters the plant, it is not selective; it
chelates with a host of elements influencing nutrient availability,
disease resistance, and the plant’s other physiological functions.
4. Glyphosate is present for the life of the plant or until it is
exuded into soil or groundwater through the roots. Degradation products
are toxic to RR and non-RR plants.
TABLE 2. Some things we know about the glyphosate-tolerance (RR) gene(s).
1. Provides selective herbicidal activity for glyphosate.
2. Inserts an alternative EPSPS pathway that is not sensitive to glyphosate action in mature tissue.
3. Reduces the plant’s physiological efficiency of Fe, Mn, Ni, Zn, etc.
4. Inactive (silent) in meristematic tissues (root and shoot tips, legume root nodules, and reproductive tissues).
5. Reduces nutrient uptake and efficiency.
6. Increases drought stress.
7. Reduces N-fixation.
8. Lowers seed nutrient content.
9. Transferred in pollen to plants, and from degrading plant tissues to microbes.
10. Generally causes a yield ‘drag’ compared with near-isogenic normal plants from which it was derived.
11. Has greatly increased the application of glyphosate.
12. Permanent in plants once it is introduced.
INTERACTIONS OF GLYPHOSATE WITH PLANT NUTRITION
Glyphosate can affect nutrient efficiency in the plant by chelating
essential nutrient co-factors after application since there is many
times more ‘free’ glyphosate in the plant than all of the unbound
cat-ions. Chelation of Mn and other micronutrients after application of
glyphosate is frequently observed as a ‘flashing’ or yellowing that
persists until the plant can ‘resupply’ the immobilized nutrients. The
duration of ‘flashing’ is correlated with the availability of
micronutrients in soil. Symptom remission indicates a resumption of
physiological processes, but is not an indicator of plant nutrient
sufficiency since micronutrient deficiencies are commonly referred to as
‘hidden hunger.’ As a strong nutrient chelator, glyphosate can reduce
physiological efficiency by immobilizing elements required as
components, co-factors or regulators of physiological functions at very
low rates. Thus, plant uptake and or translocation of Fe, Mn and Zn are
drastically reduced (up to 80 %) by commonly observed ‘drift’ rates of
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Glyphosate is not readily degraded in soil and can probably
accumulate for many years chelated with soil cat-ions. Degradation
products of glyphosate are as damaging to RR crops as to non-RR crops.
Persistence and accumulation of glyphosate in perennial plants, soil,
and root meristems, can significantly reduce root growth and the
development of nutrient absorptive tissue of RR as well as non-RR plants
to further impair nutrient uptake and efficiency. Impaired root uptake
not only reduces the availability of specific nutrients, but also
affects the natural ability of plants to compensate for low levels of
many other nutrients. Glyphosate also reduces nutrient uptake from soil
indirectly through its toxicity to many soil microorganisms responsible
for increasing the availability and access to nutrients through
mineralization, reduction, symbiosis, etc.
Degradation of plant tissues through growth, necrosis, or
mineralization of residues can release accumulated glyphosate from
meristematic tissues in toxic concentrations to plants. The most
damaging time to plant wheat in ryegrass ‘burned down’ by glyphosate is
two weeks after glyphosate application to correspond with the release of
accumulated glyphosate from decomposing meristematic tissues. This is
contrasted with the need to delay seeding of winter wheat for 2-3 weeks
after a regular weed burn-down’ to permit time for immobilization of
glyphosate from root exudates and direct application through chelation
with soil cat-ions. The Roundup® label for Israel lists recommended
waiting times before planting a susceptible crop on that soil.
One of the benefits of crop rotation is an increased availability of
nutrients for a subsequent crop in the rotation. The high level of
available Mn (130 ppm) after a normal corn crop is not observed after
glyphosate-treated RR corn. The lower nutrient availability after
specific RR crop sequences may need to be compensated for through
micronutrient application in order to optimize yield and reduce disease
in a subsequent crop.
THE INFLUENCE OF GLYPHOSATE ON SOIL ORGANISMS
IMPORTANT FOR ACCESS, MINERALIZATION, SOLUBILIZATION, AND FIXATION OF
ESSENTIAL PLANT NUTRIENTS
Glyphosate is a potent microbiocide and is toxic to earthworms,
mycorrhizae (P & Zn uptake), reducing microbes that convert
insoluble soil oxides to plant available forms (Mn and Fe, Pseudomonads, Bacillus, etc.), nitrogen-fixing organisms (Bradyrhizobium, Rhizobium),
and organisms involved in the ‘natural,’ biological control of
soilborne diseases that reduce root uptake of nutrients. Although
glyphosate contact with these organisms is limited by rapid
chelation-immobilization when applied on fallow soil; glyphosate in root
exudates, or from decaying weed tissues or RR plants, contacts these
organisms in their most active ecological habitat throughout the
rhizosphere. It is not uncommon to see Cu, Fe, Mg, Mn, Ni, and Zn
deficiencies intensify and show in soils that were once considered fully
sufficient for these nutrients. Increasing the supply and availability
of Co, Cu, Fe, Mg, Mn, Ni, and Zn have reduced some of the deleterious
effects of glyphosate on these organisms and increased crop yields.
In contrast to microbial toxicity, glyphosate in soil and root
exudates stimulates oxidative soil microbes that reduce nutrient
availability by decreasing their solubility for plant uptake, immobilize
nutrients such as K in microbial sinks to deny availability for plants,
and deny access to soil nutrients through pathogenic activity. Plant
pathogens stimulated by glyphosate (table 3) include ubiquitous
bacterial and fungal root, crown, and stalk rotting fungi; vascular
colonizing organisms that disrupt nutrient transport to cause wilt and
die-back; and root nibblers that impair access or uptake of soil
nutrients.
TABLE 3. Some plant pathogens stimulated by glyphosate.
Botryospheara dothidea Gaeumannomyces graminis
Corynespora cassicola Magnaporthe grisea
Fusarium species Marasmius spp.
F. avenaceum Monosporascus cannonbalus
F. graminearum Myrothecium verucaria
F. oxysporum f.sp. cubense Phaeomoniella chlamydospora
F. oxysporum f.sp. (canola) Phytophthora spp.
F. oxysporum f.sp. glycines Pythium spp.
F. oxysporum f.sp. vasinfectum Rhizoctonia solani
F. solani f.sp. glycines Septoria nodorum
F. solani f.sp. phaseoli Thielaviopsis bassicola
F. solani f.sp. pisi Xylella fastidiosa
Clavibacter michiganensis subsp. nebraskensis (Goss’ wilt)
HERBICIDAL MODE OF ACTION OF GLYPHOSATE
As a strong metal micronutrient chelator, glyphosate inhibits
activity of EPSPS and other enzymes in the Shikimate metabolic pathway
responsible for plant resistance to various pathogens. Plant death is
through greatly increased plant susceptibility of non-RR plants to
common soilborne fungi such as Fusarium, Rhizoctonia, Pythium, Phytophthora,
etc. that are also stimulated by glyphosate (Johal and Rahe, 1984;
Levesque and Rahe, 1992; Johal and Huber, 2009). It is very difficult to
kill a plant in sterile soil by merely shutting down the Shikimate
pathway (secondary metabolism) unless soilborne pathogens are also
present. It is the increased susceptibility to soilborne pathogens, and
increased virulence of the pathogens, that actually kills the plants
after applying glyphosate. Disease resistance in plants is manifest
through various active and passive physiological mechanisms requiring
micronutrients. Those metabolic pathways producing secondary
anti-microbial compounds (phytoalexins, flavenoids, etc.), pathogen
inhibiting amino acids and peptides, hormones involved in cicatrisation
(walling off pathogens), callusing, and disease escape mechanisms can
all be compromised by glyphosate chelation of micronutrient co-factors
critical for enzyme function. Genetic modification of plants for
glyphosate tolerance partially restores Shikimate pathway function to
provide a selective herbicidal effect.
INTERACTIONS OF GLYPHOSATE WITH PLANT DISEASE
Micronutrients are the regulators, activators, and inhibitors of
plant defense mechanisms that provide resistance to stress and disease.
Chelation of these nutrients by glyphosate compromises plant defenses
and increases pathogenesis to increase the severity of many abiotic
(bark cracking, nutrient deficiencies) as well as infectious diseases of
both RR and non-RR plants in the crop production system (table 4). Many
of these diseases are referred to as ‘emerging’ or reemerging’ diseases
because they rarely caused economic losses in the past, or were
effectively controlled through management practices.
Non-infectious (Abiotic) Diseases: Research at Ohio State
University has shown that bark cracking, sunscald, and winter-kill of
trees and perennial ornamentals is caused by glyphosate used for
under-story weed control, and that glyphosate can accumulate for 8-10
years in perennial plants. This accumulation of glyphosate can be from
the inadvertent uptake of glyphosate from contact with bark (drift) or
by root uptake from glyphosate in weed root exudates in soil. Severe
glyphosate damage to trees adjacent to stumps of cut trees treated with
glyphosate (to prevent sprouting in an effort to eradicate citrus
greening or CVC) can occur through root translocation and exudation
several years after tree removal.
Infectious Diseases: Increased severity of the take-all root and crown rot of cereals (Gaeumannomyces graminis)
after prior glyphosate usage has been observed for over 20 years and
take-all is now a ‘reemerging’ disease in many wheat producing areas of
the world where glyphosate is used for weed control prior to cereal
planting. A related disease of cereals, and the cause of rice blast (Magnaporthe grisea),
is becoming very severe in Brazil and is especially severe when wheat
follows a RR crop in the rotation. Like take-all and Fusarium root rot,
this soilborne pathogen also infects wheat and barley roots, and is a
concern for U.S. cereal production.
Fusarium species causing head scab are common root and crown
rot pathogens of cereals everywhere; however, Fusarium head scab (FHB)
has generally been a serious disease of wheat and barley only in warm
temperate regions of the U.S. With the extensive use of glyphosate, it
is now of epidemic proportions and prevalent throughout most of the
cereal producing areas of North America. Canadian research has shown
that the application of glyphosate one or more times in the three years previous to planting wheat
was the most important agronomic factor associated with high FHB in
wheat, with a 75 % increase in FHB for all crops and a 122 % increase
for crops under minimum-till where more glyphosate is used. The most
severe FHB occurs where a RR crop precedes wheat in the rotation for the
same reason. Glyphosate altered plant physiology (carbon and nitrogen
metabolism) increasing susceptibility of wheat and barley to FHB and
increased toxin production, is also associated with a transient
tolerance of wheat and soybeans to rust diseases.
The increased FHB with glyphosate results in a dramatic increase in
tricothecene (deoxynivalenol, nivalenol, ‘vomitoxins’) and estrogenic
(zaeralenone) mycotoxins in grain; however, the high concentrations of
mycotoxin in grain are not always associated with Fusarium infection of kernels. Quite often overlooked is the increase in root and crown rot by FHB Fusaria
with glyphosate and the production of mycotoxins in root and crown
tissues with subsequent translocation to stems, chaff and grain. Caution
has been expressed in using straw and chaff as bedding for pigs or
roughage for cattle because of mycotoxin levels that far exceeded
clinically significant levels for infertility and toxicity. This also
poses a health and safety concern for grain entering the food chain for
humans. The list of diseases affected by glyphosate (see reference No.
18) is increasing as growers and pathologists recognize the cause-effect
relationship.
SPECIAL NUTRIENT CONSIDERATIONS IN A GLYPHOSATE-DOMINANT WEED MANAGEMENT ECOLOGICAL SYSTEM
There are two things that should be understood in order to remediate
nutrient deficiencies in a glyphosate usage program: 1) the effects of
glyphosate on nutrient availability and function and 2) the effect of
the RR gene on nutrient efficiency. With this understanding, there are
four objectives for fertilization in a glyphosate environment – all of which indicate a more judicious use of glyphosate as part of the remediation process. These four objectives are to:
1. Provide adequate nutrient availability for full functional
sufficiency to compensate for glyphosate and RR reduced availability or
physiological efficiency of micronutrients (esp. Mn and Zn but also Cu,
Fe, Ni).
2. Detoxify residual glyphosate in meristematic and other tissues, in
root exudates, and in soil by adding appropriate elements for chelation
with the residual glyphosate.
3. Restore soil microbial activity to enhance nutrient availability,
supply, and balance that are inhibited by residual glyphosate in soil
and glyphosate in root exudates.
4. Increase plant resistance to root infecting and reemerging
diseases through physiological plant defense mechanisms dependent on the
Shikimate, amino acid, and other pathways that are compromised by
micronutrient inefficiency in a glyphosate environment.
Meeting Nutrient Sufficiency: Extensive research has shown
that increased levels and availability of micronutrients such as Mn, Zn,
Cu, Fe, Ni, etc can compensate for reduced nutrient efficiency and the
inefficiency of RR crops. This need may not be manifest in high
fertility or nutrient toxic soils for a few years after moving to a
predominantly monochemical strategy. The timing for correcting
micronutrient deficiencies is generally more critical for cereal plants
(barley, corn, wheat) than for legumes in order to prevent irreversible
yield and/or quality loss. Nutrient sufficiency levels from soil and
tissue analysis that are considered adequate for non-GM crops may need
to be increased for RR crops to be at full physiological sufficiency.
Since residual ‘free’ glyphosate in RR plant tissues can immobilize most
regular sources of foliar-applied micronutrients for 8-15 days, and
thereby reduce the future availability of these materials, it may be
best to apply some micronutrients 1-2 weeks after glyphosate is applied
to RR crops.
The expense of an additional trip across the field for foliar
application frequently deters micronutrient fertilization for optimum
crop yield and quality. There are newly available micronutrient
formulations (nutrient phosphites) that maintain plant availability
without impacting herbicidal activity of the glyphosate in a tank-mix,
and plants have responded well from these micronutrient-glyphosate
mixes. Simultaneous application of some micronutrients with glyphosate
might provide an efficient means to overcome deficiencies in low
fertility soils, as well as mitigate the reduced physiological
efficiency inherent with the glyphosate-tolerant gene and glyphosate
immobilization of essential nutrients in the plant.
Under severe micronutrient deficiency conditions, selecting seed high
in nutrient content or a micronutrient seed treatment to provide early
nutrient sufficiency, establish a well-developed root system, and insure
a vigorous seedling plant with increased tolerance to glyphosate
applied later, has been beneficial even though excess nutrient applied
at this time may be immobilized by glyphosate from root exudates and not
available for subsequent plant uptake. Micronutrients such as Mn are
not efficiently broadcast applied to soil for plant uptake because of
microbial immobilization to non-available oxidized Mn, but could be
applied in a band or to seed or foliage.
Detoxifying Residual Glyphosate: Some nutrients are relatively
immobile in plant tissues (Ca, Mn) so that a combination of
micronutrients may be more beneficial than any individual one to chelate
with residual glyphosate and ‘detoxify’ it in meristematic and mature
tissues. Thus, foliar application of Mn could remediate for glyphosate
immobilization of the nutrient; however, it may be more effective when
applied in combination with the more mobile Zn to detoxify sequestered
glyphosate in meristematic tissues even though Zn levels may appear
sufficient. Gypsum applied in the seed row has shown some promise for
detoxifying glyphosate from root exudates since Ca is a good chelator
with glyphosate (one of the reasons that ammonium sulfate is recommended
in spray solutions with hard water is to prevent chelation with Ca and
Mg which would inhibit herbicidal activity).
Although bioremediation of accumulating glyphosate in soil may be
possible in the future, initial degradation products of glyphosate are
toxic to both RR and non-RR plants. This is an area that needs greater
effort since the application of phosphorus fertilizers can desorb
immobilized glyphosate to be toxic to plants through root uptake.
Micronutrient seed treatment can provide some detoxification during seed
germination, and stimulate vigor and root growth to enhance recovery
from later glyphosate applications.
Biological Remediation: The selection and use of plants for
glyphosate-tolerance that have greater nutrient efficiency for uptake or
physiological function has improved the performance of some RR crops,
and further improvements are possible in this area. Enhancing soil
microbial activity to increase nutrient availability and plant uptake
has been possible through seed inoculation, environmental modification
to favor certain groups of organisms, and implementation of various
management practices. There are many organisms that have been used to
promote plant growth, with the most recognized being legume inoculants (Rhizobia, Bradyrhizobia
species); however, glyphosate is toxic to these beneficial
microorganisms. Continued use of glyphosate in a cereal-legume rotation
has greatly reduced the population of these organisms in soil so that
annual inoculation of legume seed is frequently recommended.
Biological remediation to compensate for glyphosate’s impact on soil
organisms important in nutrient cycles may be possible if the
remediating organism is also glyphosate-tolerant and capable of over
coming the soils natural biological buffering capacity. This would be
especially important for nitrogen-fixing, mycorrhizae, and mineral
reducing organisms, but will be of limited benefit unless the introduced
organisms are also tolerant of glyphosate. Modification of the soil
biological environment through tillage, crop sequence, or other cultural
management practices might also be a viable way to stimulate the
desired soil biological activity.
Increasing Plant Resistance to Stress and Root-Infecting Pathogens: Maintaining
plant health is a basic requirement for crop yield and quality. Plant
tolerance to stress and many pathogens is dependent on a full
sufficiency of micronutrients to maintain physiological processes
mediated through the Shikimate or other pathways that are compromised in
a glyphosate environment. Sequential application(s) of specific
micronutrients (esp. Ca, Cu, Fe, Mn, Zn) may be required to compensate
for those nutrients physiologically lost through glyphosate chelation.
Breeding for increased nutrient efficiency and disease resistance will
be an important contributor to this objective.
SUMMARY
Glyphosate is a strong, broad-spectrum nutrient chelator that
inhibits plant enzymes responsible for disease resistance so that plants
succumb from pathogenic attack. This also predisposes RR and non-RR
plants to other pathogens. The introduction of such an intense mineral
chelator as glyphosate into the food chain through accumulation in feed,
forage, and food, and root exudation into ground water, could pose
significant health concerns for animals and humans and needs further
evaluation. Chelation immobilization of such essential elements as Ca
(bone), Fe (blood), Mn, Zn (liver, kidney), Cu, Mg (brain) could
directly inhibit vital functions and predispose to disease. The lower
mineral nutrient content of feeds and forage from a glyphosate-intense
weed management program can generally be compensated for through mineral
supplementation. The various interactions of glyphosate with nutrition
are represented in the following schematic:
Table X. Some symptoms of glyphosate damage to non-target plants.
1. Micronutrient (and often some macronutrient) deficiency
2. Low vigor, slow growth, stunting
3. Leaf chlorosis (yellowing) – complete or between the veins
4. Leaf mottling with or without necrotic spots
5. Leaf distortion – small, curling, strap-like, wrinkling, or ‘mouse ear’
6. Abnormal bud break, stem proliferation – witches broom
7. Retarded, slow regrowth after cutting or running (alfalfa, perennial plants)
8. Lower yields, lower mineral value – vegetative parts and reproductive (grain, seeds)
9. Early fruit, bud, or leaf drop
10. Early maturity, death before physiological maturity, tip die-back
11. Predisposition to infectious diseases and extended infection/susceptible period– numerous
12. Predisposition to insect damage
13. Induced abiotic diseases – drought, winter kill, sun scald, bark cracking (perennial plants)
14. Root stunting, inefficient N-fixation and uptake
15. Poor root nodulation in legumes
Proceedings Fluid Fertilizer Forum, Scottsdale, AZ February 14-16, 2010. Vol. 27. Fluid Fertilizer Foundation, Manhattan, KS.
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