HERBICIDES in Asian rice - IRRI books - International Rice ...

oping programs that curtail negative environmental impacts, as well as in documenting
improvements in water quality. These benefits support good stewardship in the
use of herbicides.
Regulation of rice herbicides also has increased in the southern United States,
although it is still less extensive than in California. Herbicides that cause environmental
degradation in any rice area of the United States, or with the potential to impair
human health, have been deregistered.
The high cost of herbicide development and the risk of subsequent cancellation
of registration or of obsolescence from weed resistance have discouraged private sector
development of new herbicides for U.S. rice. Rice hectarage in the United States is
less than 2% of the world’s total, making it a minor, and high-risk, market for agricultural
chemicals. Understandably, multinational companies are focusing on the discovery
and registration of herbicides for Asian rice. In a 1994 survey, only four experimental
herbicides were under investigation in public programs in the United States
(J.E. Hill, University of California at Davis, unpubl.). Such programs are usually
good indicators of pending commercial development, but registration and actual use
of the herbicides being investigated is still at least 3 yr away.
Currently, five herbicides—bensulfuron, molinate, thiobencarb, propanil, and the
phenoxys-are available in California, and two of them have severe hectarage restrictions.
Rice farmers in the southern United States have a wider choice of herbicides,
primarily because of better crop safety when they are applied on drill-seeded rice. But
in the absence of new herbicides, even fewer herbicides are likely to be available for
U.S. rice in the foreseeable future. Maintaining current rice herbicides will require
better understanding of how best to use them for weed control and how to minimize
or eliminate their negative impacts on the environment.
Continuing experiments on the influence of water management, crop rotation,
and other cultural practices indicate that without herbicides, yields cannot be maintained
at the high levels now possible. Commercial, nonchemical (organic) rice that
has been produced for more than 20 yr by some California farmers provides some
indication of expectations for rice grown without herbicides. First-year yields are
somewhat lower than those of conventionally grown rice. Summer fallow rotation
becomes necessary in 1 out of every 2 or 3 yr, to reduce weed seed banks and prevent
severe weed populations in subsequent rice crops (National Research Council 1989).
On soils suited only for rice production, fallow rotations essentially reduce yields by
one-third to one-half compared with continuous rice.
Of all the cultural practices used by U.S. rice farmers to control or suppress weeds,
water management has been the most important. Precision field leveling with
laser-directed equipment has greatly improved a farmer’s ability to manage water. In
recent studies on water depth, 20 cm deep water suppressed barnyardgrass, but could
not entirely replace herbicides for grass weed control. Yields in deep (20 cm) water
treatments without herbicides dropped in successive years to 80, 73, and 60% of yields
with herbicides, as infestations of the aquatic sedge ricefield bulrush Scirpus
Herbicides in United States rice production 49