HerbicideResistant Crop Management

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Herbicide-Resistant Crop Management

• Jeffrey L. Gunsolus
• Weed Scientist
• University of Minnesota

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Herbicide-Resistant Crop Management

• Why Herbicide-Resistant Crops (HRCs)
• How are HRCs developed
• How Resistant are HRCs
• Inheritance of herbicide resistance
• HRCs potential to become a weed

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Why HRCs

• Allows the introduction of previously registered
  herbicides into once susceptible crops.
• Increases the margin of crop safety to a
  particular herbicide.

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How are HRCs Developed

• Selection of naturally varying populations
• Metribuzin (Sencor/Lexone) tolerant soybeans
• Selection of tolerant mutants
• STS soybeans
• IMI corn
• SR / PP corn
• Gene insertion
• Roundup Ready crops
• Liberty Link crops

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How Resistant are HRCs

• Is it resistance or is it tolerance?
• Does it really matter?
• The minimum degree of HRC tolerance is generally
  in the range of 2 to 3 times the field use rate
  in the conventional crop.

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How Resistant are HRCs

• Two primary means to resistance
• Enhanced metabolism
• Liberty Link crops
• STS soybeans
• Altered site of action
• Roundup Ready crops
• IMI corn
• SR / PP corn
• Via over expression or alteration of enzymes

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How Resistant are HRCs

• In HRCs, both enhanced metabolism and altered
  site of action are single gene traits.
• The gene is a unit of inheritance that determines
  what a given trait will be.
• Promoters associated with the herbicide
  resistance gene can modify the expression of
  herbicide resistance in the crop.

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How Resistant are HRCs

• The promoter determines when, where, and how much
  a given trait will be expressed.
• Promoters are influenced by the environment.
  Therefore changes in temperature, growth stage,
  etc. can influence expression of the trait.

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How Resistant are HRCs

• HRCs are very herbicide specific
• A HRC often has a different level of response to
  the other herbicides that are chemically similar
  to it and affect the same site of action.
• Poast tolerant corn example.
• IMI corn example.

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How Resistant are HRCs
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How Resistant are HRCs

• IMI corn comes is two forms
• IR
• IR has resistant gene from both parents
• IR has some level of tolerance to sulfonylurea
  herbicides
• IR is not labeled for direct application of
  Scepter (an imidazolinone related to Pursuit)

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How Resistant are HRCs

• IMI corn comes is two forms
• IT
• IT has resistant gene from either parent
• IT is not labeled for tolerance to sulfonylurea
  herbicides
• IT is not labeled for direct application of
  Scepter (an imidazolinone related to Pursuit)

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How Resistant are HRCs

• Watch out for tank or package mix partners
• Example Resolve plus Prowl on IMI corn
• Resolve Pursuit Banvel
• Injury occurred in areas of low organic matter
  and / or shallow planting
• Complaint was in regards to purity of IMI line
• Injury symptoms were Banvel and Prowl related

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How Resistant are HRCs

• Make sure you get the right herbicide on the
  right crop.
• Example What do Liberty and Lightning have in
  common?

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Inheritance of Herbicide Resistance

• Yield Drag / Lag - Breeding is a numbers game
  and time is needed to rectify this issue
• Possible reasons for lower yields of some biotech
  varieties
• Older varieties used for backcrossing
• Insufficient backcrosses made
• Detrimental genes carried with biotech gene
• Genetics of variety inferior to top varieties
• Biotech variety lacking in needed traits
• Jim Orf - University of Minnesota

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Inheritance of Herbicide Resistance

• Gene flow
• Cytoplasmic inheritance
• Relates to genes in the cell cytoplasm rather
  than the chromosomes. As a result these traits
  do not follow Mendelian genetic ratios.
• Genes are transmitted through the female parent.
  Therefore, the gene stays with the seed.
• Atrazine resistance is cytoplasmicaly inherited.

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Inheritance of Herbicide Resistance

• Gene flow
• Nuclear inheritance
• Relates to genes in the cell nucleus. These
  traits follow Mendelian genetic ratios.
• Genes are transmitted through both the female and
  male parent. Therefore, the gene can be
  transferred with pollen.
• Most HRCs are in this grouping.

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Inheritance of Herbicide Resistance

• Inheritance of HRCs currently follows fairly
  simple Mendelian genetics and is transferable
  between cultivars through conventional breeding
  methods.
• Dominant genes are expressed even if inherited
  from only one parent.
• Recessive genes are expressed only when inherited
  from both parents.

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Inheritance of Herbicide Resistance

• HRCs with dominant herbicide resistance genes
• IMI corn
• Liberty Link
• Poast Protected (PP/SR) corn
• Roundup Ready
• STS soybean

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Inheritance of Herbicide Resistance

• Herbicide resistant gene flow and expression is
  possible, especially within cross-pollinated
  crops.
• Cross-pollination - Is the transfer of pollen
  from one flower to a flower on a different plant.
• Self-pollination - Is the transfer of pollen
  within the same flower or another flower on the
  same plant

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Inheritance of Herbicide Resistance

• Normally self-pollinated crops
• Wheat
• Barley
• Oats
• Soybean
• Field peas
• Potato
• Vetch
• Medic
• Less than 5 natural cross-pollination

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Inheritance of Herbicide Resistance

• Normally cross-pollinated crops
• Corn
• Rye
• Sugarbeets
• Alfalfa
• Trefoil
• Clover
• Sunflower
• Canola
• Less than 5 natural self-pollination

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Inheritance of Herbicide Resistance

• Often cross-pollinated crops
• Sorghum
• Foxtail millet
• Proso millet
• Self-pollination predominates

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HRCs Weed Potential

• Volunteer crops
• Gene flow
• Within the species to nonresistant neighbors
• To weedy relatives

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Volunteer PP / SR Corn
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Volunteer PP/SR Corn

• Volunteer corn control options in soybean
• Liberty in LL soybeans
• Roundup in RR soybeans
• You must anticipate and plan for this
• What about volunteer corn that came from the
  interface between RR and PP/SR corn fields?

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Gene Flow

• Maximum gene flow potential exists for
  cross-pollinated plants with traits that are
  nuclear (pollen transfer), homozygous, and
  dominant in expression.
• Limits to corn gene flow
• Pollen movement (less than 660 feet id the
  standard)
• Timing of pollination
• Potential for gene flow in corn does exist, at
  least to a limited extent.

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Gene Flow

• The incorporation of genes from one species into
  another is favored by
• crops and wild types within pollen flow distance
• rate of hybridization
• Fitness of hybrids
• Cross-pollinated crops with weedy relatives in
  the midwestern U.S.A.
• Sunflower - wild types
• Canola - wild types
• Sorghum - wild types

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Gene Flow

• Cultivated Sunflower - 12 marker genes
• Where the crop was grown for 10 years, the
  frequency of the marker genes in wild sunflowers
  growing near the cultivated field averaged 28.
• Where the crop was grown for 35 years, the
  frequency of marker genes was 38.
• Science, Oct. 11, 1996.
• Vol. 274 180-181.

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Gene Flow

• Transgenic Herbicide Resistant Oilseed Rape
• First generation crosses of rape with field
  mustard (Brassica campestris) retained some
  degree of herbicide resistance and had fertile
  pollen.
• Science, Oct. 11, 1996.
• Vol. 274 180-181.

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Gene Flow

• Gene flow beyond the initial hybrid can occur
• Neutral or favorable cultivar genes can invade
  and persist in wild type populations
• If the herbicide-resistant gene escapes into the
  wild population it could render the herbicide
  obsolete.
• Is this really a superweed or a lost weed control
  option?

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NCWSS Survey of HRCs Risk

• Ranking from most (1) to least risk (6)
• 1. Crop injury due to misapplication and drift.
• 2. More rapid selection for resistant weeds.
• 3. Controlling HRC volunteers.
• 4. Species shifts to more tolerant weeds.
• 5. Transfer of resistance trait to weedy
  relatives.
• 6. More reliance on or excessive use of
  herbicides
• Hart. 1996 NCWSS Proceedings 51175-176

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Packaging Herbicide Resistance

• Stacked traits
• Most HRC traits are dominant and the resistance
  gene can come from either parent (not both) to be
  expressed.
• Allows for faster cultivar development time and
  makes the stacking of multiple traits much
  easier.
• For examples Bt stacked with IT Bt / LL

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Packaging Herbicide Resistance

• Marker traits
• Some seed companies linked the glufosinate
  (Liberty) tolerant gene to the Bt gene to
  facilitate the selection process for
  incorporating Bt into corn.
• In this case the Liberty (LL) trait was initially
  used as a selective marker to ensure the presence
  of the Bt gene.
• In some but not all hybrids both Bt and LL are
  fully expressed.

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References for HRCs

• www.aphis.usda.gov
• Click on Ag Biotech
• www.agron.iastate.edu/extweed
• Click on Weed Management and Weeds in the News