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?