when weeds win the canopy, crops lose - mu...
TRANSCRIPT
When weeds win the canopy, crops lose
Reid J. Smeda, Professor of Weed Science University of Missouri
Do you ever feel trapped by the rat race? Life is competitive
Competition also occurs among plants • Intraspecific competition (with each other) • Interspecific competition
– WEEDS!
How long do plants compete? • Season-long if life cycles match • Winter annuals will compete early in the life
of summer annual crops • Summer annuals will compete late in the life
of winter annual crops
Henbit in leeks can be a big problem
Definition • Competition occurs when the combined
demand of 2 plants for a growth factor is below the immediate supply
Plants compete for 5 five factors
• Big three – Water – Light – Nutrients
• Little two – Oxygen (important for rice) – CO2 (C4 plants more competitive than C3)
Plants do not compete for space! • They compete for the stuff in the space
How do we examine the competitive impact of a weed on a crop?
• Depends upon the particular weed species and density
• Depends upon the particular crop, planting design, and date of establishment
Research at the University of Guelph: 1 pigweed per 2 square yards caused a 5% reduction in corn yield when emerging with corn; 40 plants for the same area were needed when pigweed came up after corn was established
• Combined influence of weed and crop is reflected in what we call the critical weed-free period
It is a time when the crop yield can be reduced from its optimum
Critical weed-free period
Period during crop production when weeds are most likely to reduce yield or harvest
Shorter critical weed-free period means:
• Slow development of weed • Rapid growth or high competitiveness of crop
Common purslane
Longer critical weed-free period means: • Rapid development of weed • Slow growth or low competitiveness of crop
Waterhemp Bell pepper
Are newly emerging weeds a threat in this situation?
Using bullets (tools) to control weeds at the wrong time is:
…..not going to accomplish your goal and will just cost you more money!
What does this boil down to?
Data provided by Clarence Swanton
With competition, things on the surface may not be as simple as they appear
2 years after establishment
Canada thistle
Competition for water • >97% of water absorbed by plants is lost via
evapotranspiration (cooling process)
Why do you feel cool coming out of pool?
• Without sufficient water, plants wilt and internal temperature increases; plant growth stops
We do not seem to have a water shortage right now!
Clarksville, MO; April 22, 2013
Weeds are inefficient water users • 1 pound dry weight of barnyardgrass requires
80 gallons of water
Competition for water begins before light and nutrient competition
• Water is drawn some distance from roots and can be stored
Water is also needed for ………. • Nutrient uptake
Nutrient uptake depends on availability and soil pH
• Macronutrients N, P, K, Ca, Mg, S
• Micronutrients B, Mn, Fe, Zn, Mo, Cu, Cl
Nutrients can be stored in plants?
• Phosphorous in Arabidopsis – Continuously recycled at cellular level
Low P Sufficient P
Can we add sufficient nutrients for both crop and weeds?
Data generated by University of MN
Competition for nutrients and water depends upon relative root volume
• Grasses – fibrous • Broadleaves – often
modified taproot
Light is the most critical growth factor
• Only certain spectra are used (and removed by the tallest plants)
• There is a finite amount per acre (based upon area)
• Light cannot be stored in plants (use it or lose it)
Far red (730 nm)
What happens when plants compete for light? • Red light is removed by taller plants and
far red light is passed through • Change in ratio of R:FR light (reduced) • This stimulates production of a PIN3
protein that regulates movement of auxin; more auxin stays in the upper shoot of plants
• Shaded plants elongate to compete for available light
Weeds have different methods to optimize light usage:
Vining growth habit Movement toward
available light Plant height
Vining growth habit - annuals
• Morningglory
Some plants adjust leaf angle to optimize sunlight usage
Most competitive weeds adjust internode length to grow taller
• Weed will want to optimize light utilization, but no need to waste resources on stems
Some plants are like people……..
• They want to be taller
Key to avoiding problems!
How does that translate to weed control?
• Pay most attention to weeds early in the growing season
• Do not let weeds get established or they will take available water and nutrients
• Use mulches to shade weeds • Narrow crop rows restrict light and decrease
weed emergence
Continued use of the same practice for weed control is like repetitive use of a bird deterrent; if birds can figure it out, so can weeds!
QUESTIONS?