Methods for increasing the competitive ability of crops over weeds include critical or primary weeding periods, the use of cultivars, an understanding of weed and crop biology, preferred soil conditions, crop sensitivity to particular weeds as in the case of allelopathy, and identification of threshold levels at which weeds have minimal or no effect on reducing crop yield, thereby making control measures uneconomic.
In crop-weed competition the aim is to provide conditions which are more favourable to crops than weeds. For instance a study in Italy on tomato crops found that drought increases weed biomass and decreases crop yield. It is projected that drought will increase in the Mediterranean region as a result of anthropogenic climate change which will have a direct effect on crop-weed competitive ability. However the study found that by targeting irrigation at the tomato crop and avoiding watering weeds the reverse was the case, that is, tomato yield increased and weed biomass decreased.
Critical or primary weeding periods could include pre and post emergence weeding through cultural, mechanical, biological or thermal methods. Critical or primary weeding periods aim to give crops a head start over weeds. There are a number of ways this can be done ranging from stale bed preparations, to advancing plant growth in greenhouses prior to transparent, to seed priming and drilling as well as the use of cultivars. Cultivars are plants which are recognised and breed for their highly competitive and/or allelopathic effect on weeds. Allelopathy is a bio-chemical process producing allelochemicals which have the effect of being either beneficial or harmful to neighbouring organisms.
An understanding of crop and weed biology allows for the recognition and development of cultivars as well as crop sensitivity in relation to allelopathic responses and preferred soil conditions. Understanding crop and weed biology is take an ecological systems approach. An ecological systems approach allows for identification of threshold levels at which weeds have minimal or no effect on reducing crop yield and which weeds may be beneficial for crops allelopathtically or as pest deterrents, thereby making control measures uneconomic.
Knowledge of plant morphology is important in determining how competitive the weeds are with the crop. Weeds which are a tall and leafy such as docks and thistles can be a competitive advantage for light as can bracken which forms a dense leaf canopy. Weeds which do not from a canopy are less successful at competing with
Annual ryegrass (Lolium rigidum Gaudin.)
a dense crop. Established crops are more successful at suppressing weeds; however weeds which form a taproot may be at an advantage in subsequent crops if allowed to get a head start.
Annual ryegrass is highly competitive for nitrogen when allowed to emerge before crops but a poor competitor when it germinates after crops.
Annual ryegrass becomes a serious problem in winter cropping. It is a weed of winter fallows and crops due to its soil moisture preference and effect on crop yield loss. It is well adapted to most soil types in the winter rainfall regions of southern Australia, which are characterised by hot, dry summers and mild, wet winters.
