W6, Plant competition (& canopy management) Flashcards
True or false?
Even a small difference in plant height can have a large influence on light interception by shorter plants.
True.
In one study, even a 2cm difference in height resulted in a 75% reduction in light availability to the shorter plants.
Which processes of ion movement/uptake (diffusion, interception, or mass flow) are most important for competition between crops and weeds? Provide some examples of nutrients that are competed for the most.
Mass flow.
It is unlikely that roots are going to be growing in such close proximity that the other processes will have much effect.
N, Ca, Mg, S all move mostly by mass flow (and so are strongly competed for).
Explain what a knockdown control is and why it’s important.
Knockdown weed control = controlling the weeds that are present at sowing.
Important for minimising competition with the crop and for controlling weed pressure. Mallow, for example, is almost impossible to kill once the crop is up.
Define allelopathy and explain why its important in cropping systems.
Allelopathy is the direct negative influence of one crop on another by the release of chemical compounds that inhibit the growth and development of neighbouring plants.
Cereal rye has allelopathic activity, hence its weed supression capabilities when used as a mulch (common in organic systems).
Allelopathy can also be used as a tool for breeding more competitive crops (e.g. rice).
What factors determine the level of yield loss caused by weeds?
- Density
- Species
- height
- root growth
- leaf area index
- water use efficiency
- Emergence time
True or false?
Annual ryegrass is now resistant to all post-emergent herbicide options.
True.
What is the ultimate goal for managing weeds?
To reduce the amount of seed set (and therefore the size of the soil seed bank).
List wheat, barley, and oats in the order of competitiveness (greatest –> smallest).
barley > oats > wheat
What management options are available for increasing crop competition to improve weed management?
- select competitive species (barley > oats > wheat) and varieties
- ↑ crop density
- ↑ sowing rate
- ↓ row spacing
- band fert below seed (reduced accessibility for weeds)
- limit early weed growth
- i.e. support early vigour of the crop
- plant large (plump) seed
- = ↑ establishment and early vigour
- prevent disease and insect damage to the crop
Which is the best direction to sow in (north-south or east-west) for increasing a crop’s ability to compete with weeds?
East-West provides greater shading and weed supression.
How can farmers use strategic grazing management to improve the competitive ability of pasture legumes and improve pasture composition?
By resting (i.e. not grazing) the pasture over winter, the height of reproductive structures in grass species is raised, meaning they have a larger probability of being grazed off, thus reducing seed set and the amount of competition with pasture legumes (i.e. pasture composition improves).
Planting bitter-tasting forages is another way of using grazing to manage weeds, because livestock will preferentially graze the sweeter-tasting weeds before moving on to the forage (thus reducing weed seed set and maintaining good pasture composition).
How has modern wheat breeding affected it’s competitiveness?
Overall, it’s made wheat less competitive by:
- reducing crop height
- reducing early vigour
- producing thinner leaves
- smaller root systems
What agronomic tactics can be used to improve the competitive ability of crops?
- ↓ row spacing
- ↑ crop density
- choose competitive crop type (barley > rye > wheat, allelopathy)
- sow east-west rather than north-south
- manage N to improve early vigour
- plant large seed
- use knockdown herbicides
