Guideline Index

Chapter 2: Limits to Plant Growth

2.8 Weeds

Weeds can have many definitions, but generally in a farming context they are defined as “any injurious or potentially disease causing plant that is relatively useless, whilst competing with established pastures or crops”.

Weeds are usually species that are well adapted to survival with prolific seeding rates; the ability to remain dormant for extended periods of time; as well as excellent methods of seed dispersal – whether by wind, water or by stock movement.

It is often impossible or impractical to eradicate weeds from an area, so the best method of controlling weeds is relying on the management of them. There are many mechanical means by which weeds can be controlled. In the case of annual weeds for example, slashing in order to prevent seed set can be an effective means to control weeds, especially prior to flowering.

The losses to production due to weed infestation are incredibly difficult to estimate. Weeds can result in competition with pasture or cropping species for nutrients and soil moisture; the tainting of milk; stock poisoning; reduction of dry matter yields from lucerne paddocks; as well as reduced quality of the hay due to weed presence.

Grass weeds have prolific fibrous root structures and generally germinate under the more extreme conditions such as cooler soil temperatures. They require soil nutrients as do commercial plants, quickly draining the local soil of available nutrients and moisture, thereby leaving the soil depleted for the following pasture or crop. Table 2.2 clearly demonstrates the effect that weed competition has on pasture establishment and production.

Table 2.2  The effect of fallow weed management on the population of annual summer grasses and the dry matter (DM) production of Bambatsi panic one month after sowing.  Source: http://www.dpi.nsw.gov.au/archive/agriculture-today-stories/april-2008/weed-competition-kills-tropicals
Table 2.2 The effect of fallow weed management on the population of annual summer grasses and the dry matter (DM) production of Bambatsi panic one month after sowing. Source: http://www.dpi.nsw.gov.au/archive/agriculture-today-stories/april-2008/weed-competition-kills-tropicals

One of the best ways of reducing weed infestation in pastures is by maintaining a dense, healthy sward of desirable pasture species. Pasture species will generally have the greater nutritional value so optimum soil conditions should be met to encourage this production. Management strategies to favour desirable pasture species (see Section 2.9) alongside the integrated use of herbicides, offers the best approach to reduce the limitation imposed by weeds.