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

severe the weed infestation, the more likely herbicides will be more economical than
hand weeding.
In Peninsular Malaysia, only 46% of transplanting farmers use herbicides, but
95% of direct seeding farmers do. The wide use of herbicides in direct-seeded fields
is to be expected, given the weed problems that accompany the practice. Without
herbicides, yields would be seriously reduced (Wong 1993). US$4.25 ha -1 is spent on
herbicides for the transplanted crop; US$29.96 ha -1 for the direct-seeded crop.
In 1993, in Iloilo Province, Philippines, a wet-seeded rice farmer using pretilachlor
+ fenclorim, the most widely used herbicide in the province, at the recommended rate
of 0.3 kg ai ha -1 , would spend US$24.20 ha -1 to control weeds. With a farmgate price
for rice of US$0.20 kg -1 , a yield increase of only 121 kg ha -1 would cover the cost of
the herbicide. Hand weeding once would cost US$l28 ha -1 (40 people × 2 d × US$1.60
d -1 ), more than five times the cost of herbicide.
A similar situation occurs in wet-seeded rice in the Mekong Delta, Vietnam:
hand weeding is at least five times more expensive than herbicides (Moody 1992). In
West Java, Indonesia, hand weeding of transplanted rice costs about US$33 ha -1 , equivalent
to 280 kg of rough rice, or 16% of average yield. This is three to four times the
cost of herbicides (Burhan 1993).
The cost advantage of wet seeding more than compensates for herbicide cost,
and use of herbicides is increasing even with higher herbicide prices (the real price of
butachlor increased 15% between 1979 and 1986) (Erguiza et al 1990). In the Philippines,
rice herbicide prices increased an average of 48% within 10 yr, while labor
costs for weeding increased 78%.
In Thailand, rice herbicide prices have not increased appreciably for the last 6 yr;
the price of thiobencarb + 2,4-D has not increased since 1982. The cost of labor,
however, has more than doubled (P. Vongsaroj, 1993, National Weed Science Research
Institute, Department of Agriculture, Bangkok, Thailand, pers. commun.).
In Muñoz, Nueva Ecija Province, Philippines, in 1994, the cost of pretilachlor
(US$21.50 L -1 ) was equal to 1.7% of the value of the crop (5 t ha -1 at US$0.25 kg -1 ).
Butachlor cost only $10.80 L -1 and piperophos + 2,4-D $12.30 L -1 . Clearly, the cost of
herbicides is a relatively small part of the total value of a rice crop.
IMPACTS OF INCREASED HERBICIDE USE
Weed shifts
Weed control does not simply remove one species from within a plant community, it
alters the relationships among constituent species. Disturbing the environment, altering
competitive interactions, and creating stress disrupt the natural pattern of development
(Cook 1990). Weeds that were of secondary importance may emerge as primary
problems.
In Japan and Korea, weed management in rice is herbicide-based (Kim 1993).
Overreliance on herbicides has not only failed to give long-term control of weed prob-
Economic and technological influences on herbicide use 235