Weed control seed rates

Nonchemical or traditional practices, such as weed seed removal, optimal crop seeding rates, crop selection, enhanced crop competitiveness, crop rotation, and mechanical weed control are all important components of an effective weed management program (458,459). In the context of modern intensive chemical herbicide appHcation, nonchemical practices may even represent an innovative approach to weed management and should receive careful consideration. 

Herbicide safeners (also referred to as herbicide antidotes or protectants) fulfill an important role in crop protection. Safeners are chemicals that protect crop plants from unacceptable injury caused by herbicides. Either by placement on the crop seed or by way of a physiological selectivity mechanism, safeners in commercial use do not negatively impact the weed control of the herbicide. Although many herbicides have been developed for use without a safener, some of the strongest and most broad-spectrum herbicides tend towards border-line crop selectivity, which may completely preclude use in a particular crop or at least limit maximum use rates or the crop varieties that can be safely treated. It is for such situations that safeners have been developed. Several books and reviews of safeners have been written over the past 20 years [1-3]. It is not the intention of this chapter to cover in detail older safeners, but rather to focus on more recently developed commercial safeners as well as some of the older compounds still in wide commercial usage. 

Rates vary between 0.5 and 4.5 kg/ha. The lower amount is used with precision drilhng. Pelleted seed is sown 7-15 cm apart in the row at row widths of 17-35 cm. This is particularly applicable where overall chemical weed control is carried out and/or the crop is to be folded. The aim is for a high plant population of up to 100000/lia. 

There are a number of possibilities for using plant secondary chemistry to control herbivory in crop plants. One possibility is to select for insect resistant lines and though it has been done in only a few cases, select for specific allomones. There are, however, some potential problems with this approach. There is a cost for the production of the secondary compounds which may be useful for defense ( ). Insect resistant soybean cultivars produce lower yields of seeds and accumulate nitrogen at a slower rate than insect susceptible varieties in the absence of herbivores 3 ). Conversely, varieties of crop plants selected for high yield are often more susceptible to insects, pathogens, and weeds (35). 

The first ALS-resistant weeds were reported in 1987 when prickly lettuce (Mallory-Smith, 1990 Mallory-Smith et al, 1990b) and kochia (Primiani et al, 1990) control failures occurred in Idaho and Kansas, respectively, after 5 consecutive years of chlorsulfuron use. The kochia biotype proved to be cross-resistant to six other ALS-inhibitor herbicides, including sulfonylureas and imidazolinones. Within 5 years, sulfonylurea-resistant kochia had been identified at 832 sites in 11 states of the United States and in three Canadian provinces (Saari et al, 1994). ALS inhibitor-resistant kochia and Russian thistle have become widespread problems in cereal-producing regions of northwestern United States and Canada. The mobility of these tumble weeds as plants with mature seeds or pollen carried by wind has undoubtedly contributed to the rate at which resistance has spread. 

Butachlor is a pre- and early postemergence selective herbicide for the control of grass weeds and a few broad-leaved weeds. Depending on the organic components of the soil, it is used at a rate of 1.5-3 kg active ingredient/ha on seeded and transplanted rice in Asia, and in Central and South America. 

DNOC is used as its ammonium- or sodium salt, which are water-soluble. The salts are contact herbicides for the control of broad-leaved weeds in cereals, onion and garlic at rates of 3-5 kg active ingredient/ha. In emulsifiable concentrate formulation DNOC can be used for the preharvest desiccation of potatoes and leguminous seed crops (Worthing, 1979). 

Molinate is a selective pre- and postemergence soil herbicide mainly for the control of grass weeds and particularly for Echinocloa spp. (Smith and Fox, 1965). It is applied either before planting to water-seeded or shallow soil-seeded rice, or post flood on other types of rice culture. Applied at its recommended rate of 2-4 kg active ingredient/ha, its action lasts over the whole crop period (Swain, 1974). 

Benazolin is a selective postemergence herbicide translocated in the phloem. Combined with phenoxy herbicides, such as MCPA and MCPB, 2,4-DB and 2,4-DP, it efficiently controls many broad-leaved weeds, particularly Stellaria media and Galium aparine, in undersown cereals and direct sown leys. Alone it is used against wild mustard, chickweed cleavers and oil seed rape at a rate of 0.9-2.S kg active ingredient/ha (Shafer and Stobbe, 1973a, Rea et al. 1976). Used in the form of its water-soluble sodium or potassium salt it is not adsorbed in the soil and is rapidly leached. 

---