Pre-crop effects

In general, triazines are pre- and post-emergence selective herbicides particularly effective on annual and perennial broadleaf and grassy weeds in corn, sorghum, cotton, soybeans, sugar cane, and a host of other fruit and cereal crops. Some have anti-fungicidal properties (e.g., anilazine), and some (e.g., simazine) can be used for... 

Rice. Rice, a major food crop in the tropics, is highly resistant to 2,4-D at most periods of growth. Although chemical weed control is widely used in the United States, relatively little has been done in the tropics. Some chemical weed control in rice has begun in Cuba (18), Philippines (39), Malaya (6), and Venezuela (16). Both dinitrophenol and pentachlorophenol were effective as pre-emergence treatments in Venezuela, and CMU gave very promising preliminary results. 

The Imazethapyr Area. As Figure 3 shows, thione 16 is less effective overall than imazethapyr (2) in preemergence weed control on soybeans, particularly in the broadleaves, where 16 is inactive on morning glory and ragweed. Imidazolinthione 16 is also more injurious to soybeans than imazethapyr and shows no other crop selectivity. Thione 16, postemergence at the same rate, is essentially inactive compared to imazethapyr. The methyl ester (19) of thione 16 is essentially inactive both pre- and postemergence at 63 g/ha. 

In individual tests the meta- and para-toluic acids 37 and 38 were shown to be injurious to crops both pre- and postemergence. However, imidazolinthione 23 is selectively detoxified by cereals and is more effective in controlling selected weeds on cereals, especially postemergence (Figure 7). Unfortunately, 23 fails to control green foxtail and thus fails to provide a useful new range of activity on cereals. 

Herbicide resistance is currently less of a problem than with the other pesticide groups, although, once it occurs, the effects are profound. Instead, the major interest centres around the control of off-target drift (i.e. application issues), specificity and the use (and side-effects) of certain herbicides with genetically modified crops. The examples listed in Table 8.1 indicate the continuing importance of pre-1980 molecules. For instance, glyphosate has become the most important herbicide, with its recent fall in price, broad spectrum of efficacy, low mammalian toxicity and (where GM crops are acceptable) its compatibility with Roundup ready crops. 

Compounds of the group are pre- and postemergence herbicides, effective against mono- and dicotyledonous annual weeds. They seem promising in monocotyledon-ous crops such as barley, wheat, maize and beet. Examples of efficient compounds are 3-(N,N-dimethylureido)phenyl N-(4-chlorophenyl)carbamate... 

Dedicated pre-screens for protectants would best be developed with in vitro cultures of the crop to be protected. This can be problematic with herbicides which affect photosynthesis when green photosynthetic cultures are not available. A pre-screen for protectants against diquat was set up using a duckweed mini-plant system (Figure 3). Such a system has the advantage that potential protectants can be seen by their effects on growth and color. They can also be quantifled by measuring 002 fixation or by chlorophyll extraction (2f). 

Application of pesticides in China should follow Guidelines of Safety Application of Pesticides. There are over 50 laboratories that can conduct pesticide held trials certihed by 1C AM A. The good agriculture practices (GAP) for the safe and effective use of the product must be established. Over 1,000 held trials with over 300 active ingredients in more than 30 crops have been conducted in China. On the basis of these held trials, a series of GAP have been drawn up and new revisional MRLs have been proposed. The main contents in this GAP are common name, crops, application rate and method, maximum number of application, PHI (pre-harvest interval), and recommended MRLs. 

Thiazopyr is a pre-emergence herbicide that is currently used in non-crop areas, tree, vine and Oryza crops and has demonstrated selectivity in Medicago spp., Gossypium spp., Arachis spp.. Glycine spp., and Saccharum spp. [13-15). It is effective on most annual grasses and certain broadleaf weeds. Thiazopyr is presently registered in 13 countries in North America, Latin America, Europe, Australia and Asia. 

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