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Climate Change & Weed Management: C3 vs. C4 Weeds

How to use the tables:

Check if the table has data for a weed species you’re trying to manage (e.g. Horseweed)

You can then see how the performance of the herbicide against the weed will change under different climate conditions.

  • A red “-“ means the herbicide will be less effective as the climate factor increases
  • A green “+” means the herbicide will be more effective as the climate factor increases
  • A blue “=” means there will be no change in efficacy
  • A white “ND” means there was no research data for that interaction
  • S and R refer to susceptible or resistant biotypes (e.g. "-R" means efficacy is reduced against resistant biotypes)

C3 vs C4

Weeds can be grouped by how they capture light energy through photosynthesis to fuel their growth:
  • C3 pathway plants: Temperate plants, more efficient under elevated CO2 and cooler temperatures.
  • C4 pathway plants: Tropical species including many monocots and grasses.  More efficient at higher temperatures than C3 plants.
More information and species lists below:

Glyphosate Efficacy on C3 Weeds

Photosynthetic pathwayCommon name 
(Scientific name)		CO2TemperatureDroughtReference Number
C3​Velvetleaf
(Abutilon theophrasti)		NDND–16​
C3​Common ragweed
(Ambriosia artemisiifolia)		–+ (S+R) | – (R)ND​14, 9, 8
C3​Giant ragweed
(Ambriosia trifida)		ND​+ND​9​
C3​Slender wild oat
(Avena barbata)		–ND​ND​17​
C3​Rescuegrass
(Bromus cartharticus)		=ND​ND​17​
C3​Downy brome
(Bromus tectorum)		=–ND​3​
C3​Common lambsquarters
(Chenopodium album)		––ND2, 10, 18
C3​Canada thistle
(Cirsium arvense)		–ND​ND​2, 4, 5, 19
C3​Horseweed
(Conyza canadensis)		––ND​10​
C3​Hairy fleabane
(Conyza bonariensis)		ND​–ND​13​
C3​Quackgrass
(Elymus repens)		–ND​ND​2, 4, 5
C3​Prickly lettuce
(Lactuca serriola)		=ND​ND​3​
C3​Ryegrass
(Lolium rigidum)		–ND​ND​2
C3​Annual sowthistle
(Sonchus oleraceus)		ND​ND​– (R) | = (R)* | + (S)*11, 13

Glyphosate Efficacy on C4 Weeds

Photosynthetic pathwayCommon name 
(Scientific name)		CO2TemperatureDroughtReference Number
C4​Smooth pigweed
(Amaranthus hybridus)		ND​+ND​12​
C4​Palmer amaranth
(Amaranthus palmeri)		–ND​ND​2​
C4​Redroot pigweed
(Amaranthus retroflexus)		=ND​ND​18​
C4​Yellow nutsedge
(Cyperus esculentus)		=ND​ND​7​
C4​Purple nutsedge
(Cyperus rotundus)		=ND​ND​7​
C4​Goosegrass
(Eleusine indica)		+ (R) | – (S)ND​ND​1, 2​, 20
C4​Weeping lovegrass
(Eragostis curvula)		–ND​ND​2, 6, 8​
C4​Johnsongrass
(Sorghum halepense)		–ND​ND​2​

Photosynthetic Pathway

All plants photosynthesize to produce food. The basic requirements are sunlight, atmospheric CO2, chlorophyll in plant leaves, and water. The two main photosynthetic pathways are C3 and C4. There is also CAM photosynthesis, but this is found primarily cacti species and will not be discussed in this fact sheet.  

C3 Pathway (for “temperate” species)  

  • Rubisco captures CO2 and produces two 3-carbon molecules, hence the name C3 
  • If oxygen is involved, photosynthesis is inefficient resulting in photorespiration 
  • CO2 to O2 ratio is important and temperature sensitive 
  • Optimum temperature: 59-86°F (15-30°C) 
  • If CO2 levels are elevated, the C3 pathway is more efficient 

C4 Pathway (for “tropical species”, many monocots and grasses) 

  • Biochemical and morphological modification of the original C3 pathway 
  • Increases photosynthesis rate by using an additional enzyme (PEP carboxylase) to capture more CO2, resulting in a 4-carbon acid 
  • Photorespiration is reduced at the cost more cellular energy (ATP) 
  • Optimum temperature: 68-95°F (20-35°C) 

Photosynthetic pathways may have some influence on herbicide efficacy in a changing climate.  But thus far studies have not shown trends consistent enough to draw hard and fast conclusions.

C3 Weeds

C4 Weeds