Excessive weed growth forces crops to compete for sunlight and
nutrients, often leading to significant losses. Because herbicides cannot
differentiate between plants that are crops and plants that are weeds,
conventional agricultural systems can only use 'selective' herbicides.
Such herbicides do not harm the crop, but are not effective at
removing all types of weeds. If farmers use herbicide resistant crops,
'non-selective' herbicides can be used to remove all weeds in a single,
quick application. This means less spraying, less traffic on the field, and
lower operating costs.

'Non-selective' herbicides: not always useful

'Broad-spectrum', or non-selective herbicides are effective at killing
a wide range of weeds. The problem is, they can also kill valuable crops.
Therefore, broad-spectrum herbicides are only useful before seedlings emerge
or in special cases like fruit orchards, vineyards, and tree nurseries.

Herbicide resistant crops are changing weed managment

Several crops have been genetically modified to be resistant to
non-selective herbicides. These transgenic crops contain genes that enable
them to degrade the active ingredient in an herbicide, rendering it
harmless. Farmers can thereby easily control weeds during the entire growing
season and have more flexibility in choosing times for spraying.

Herbicide resistant crops also facilitate low or no tillage cultural
practices, which many consider to be more sustainable. Another advantage is
that farmers can manage weeds without turning to some of the more
environmentally suspect types of herbicides.

Critics claim that in some cases, the use of herbicide resistant crops
can lead to an increase in herbicide use, promote the development of
herbicide resistant weeds, and damage biodiversity on the farm. Extensive
ecological impact assessments have been addressing these issues.

Among the field trials conducted on herbicide resistant crops, studies
in the United Kingdom have shown that different herbicides and different
herbicide application practices can affect the amount of wild plants on the
farm. In comparison with conventional cropping systems, weed and animal
populations were negatively affected by herbicide tolerant sugar beet and
rapeseed, but biodiversity was increased with the use of herbicide tolerant
maize.

Currently, two herbicide resistant cropping systems are common for
soybean, maize, rapeseed, and cotton: RoundupReady (active agent:
glyphosate) and Liberty Link (active agent: glufosinate).


[www.gmo-compass.org]