The spread of perennial weeds, such as foxtail barley, is a prime deterrent to adoption of no-till cereal production in the Northern Great Plains. In a 3-yr study, the isopropylamine salt of glyphosate at 0.56 kg ai/ha plus nonionic surfactant at 0.25% (v/v) did not control established perennial foxtail barley in early spring. However, adding ammonium sulfate at 2.8 kg/ha to glyphosate plus surfactant improved the effectiveness and year-to-year consistency of glyphosate for killing this weed. This spring treatment fits into no-till farm practice in the Northern Great Plains, providing no-till farmers with a defined strategy to manage this perennial bunch-grass weed.

Biologically active natural products of microbial origin are attractive candidates for possible use in agriculture. They may be obtained by fermentation, used in their natural state, or subjected to synthetic modification for specific uses. These natural products are characterized by high specific activity and high selectivity, and they are biodegradable. The structures are extremely diverse and represent many classes of compounds ranging from very complex to simple. Cyclocarbamide A and B, from Streptoverticillium sp., have marked preemergence herbicidal activity. Nigerazine A and B, from Aspergillus niger van Tieghem, also inhibit root growth in certain plants. Citreoviridin, from Penicillium charlesii Smith, preferentially controls the growth of monocotyledonous plants, as does a synthetic derivative of cladosporin, from Aspergillus repens DeBary, which bleaches chloroplasts. The 12-membered fungal macrolides (macrocyclic lactones) also inhibit root growth in many test plants and offer templates for further synthetic work. Herbicidins, from Streptomyces saganonensis, are particularly effective against barnyardgrass, goosegrass, tufted mannagrass, and green panicum. Alternaria eichorniae Nag Raj et Ponnappa produces a toxin that is active against waterhyacinth and represents one of the more exotic structures. The macrocyclic trichothecenes are a significant class of natural products that tend to concentrate against a gradient in seeds of certain plants, which resist these microbially derived metabolites thereby producing seed with “built-in” natural herbicides.

Mitotic disrupters from higher plants have been studied for their effects on animal cells, but their effects on plant cells have been studied less. The plant-derived compounds have counterparts among herbicides with respect to cellular effects and, in some cases, mode of action. Cellular effects in common are arrested prometaphases (colchicine, dinitroanilines), multipolar divisions (Vinca alkaloids, carbamates), and production of binucleate cells (caffeine, dichlobenil). The potential of these natural compounds, either directly or as the basis of new chemistries for herbicides, has remained largely untapped.

Mediterranean saltwort, a member of the family Chenopodiaceac, is a dominant perennial species in the Mediterranean arid zone. Propagation is by seed only, and those collected from an infestation in California were used for various germination studies. Seed with the persistent calyx removed germinated 2 to 4 days earlier than those with the calyx present. The germination percentage was also greater (82 vs 62% at Day 13) than seed with the calyx. Those seed stored at room temperature germinated less than seed stored in the refrigerator or freezer. Brown (mature) seed germinated earlier and more than green seed. Greenhouse-grown plants cut or burned off at the ground level failed to regrow. Plants cut at 1 cm or more above ground level regrew from side shoots. Mediterranean saltwort could become a serious noxious weed in areas of the United States that have a Mediterranean climate.

The effectiveness of oxyfluorfen and bromoxynil for Russian thistle control in onion was measured in a 2-yr study conducted in the high desert region of southern California. Oxyfluorfen, bromoxynil, or combinations of the two applied one at the 2-leaf growth stage of onion did not control Russian thistle. When a first application was followed 14 days later with a second herbicide application, a first application of oxyfluorfen plus bromoxynil was most effective. Oxyfluorfen applied alone as a second application was least effective, but bromoxynil alone plus oxyfluorfen improved control. Adding petroleum oil to oxyfluorfen plus bromoxynil improved Russian thistle control.

The potential for use of compounds isolated from diverse sources, ranging from microbial to higher plants, has received increasing attention. These natural products represent a vast array of chemistries and mechanisms of action often having the requisite specificities to be ideal herbicides. Tentoxin, a cyclic tetrapeptide, has potent chlorosis activity on a variety of weeds of soybean and corn while not affecting these crops. Investigations into this toxin's mechanism suggest several enzyme target sites for the development of herbicidal analogues, each having the inherent specificity of the parent compound. Structure/activity relationships determined so far may permit a directed synthesis of efficacious analogues. New insights into biosynthesis of the toxin suggest possible means of increasing production to allow economical exploitation of the toxin itself as a herbicide.

Many natural and synthetic plant compounds become toxic when irradiated by ultraviolet or visible light. These compounds may be considered for use as herbicides, remembering the requirement for light. Concurrently, methods for blocking metabolic pathways so that phototoxic intermediates accumulate and thus kill the plant also have been studied. The mechanisms of action of potentially useful plant phototoxins are described.

The thistle invasion in the United States which began over 3 centuries ago included plumed thistles, pasture thistles, swamp thistles, and lance thistles, comprising more than 120 species.

Common lambsquarters or fat hen (Chenopodium album L. # CHEAL) was classified by Linneaus in 1753. The generic name is from the Greek chen, a goose, and pous or podos, a foot; the leaf shape of plants in this genus are reminiscent of goose feet. The goosefoot family, Chenopodiaceae, includes many vegetables: table beets, sugar beets, spinach, and mangold. The name ‘fat hen’, used for several plants of the Goosefoot family, was first published in English in 1795.

Weed Science is knowledge about unwanted plants and about methods to control such plants. The concept “weeds” probably was born when man first learned that desirable plants would grow better if surrounding plants were killed. Knowledge of elementary weed control began when man learned that unwanted plants died if they were separated from the soil. Thus rudimentary Weed Science is as old as man's efforts to grow crop plants.

Plants are components of ecosystems (Figure 1). The primary or sole target of an agro-ecosystem is still production. Production systems are centered around two points: the ecosystem and the socio-economic system (Figure 2). Weeds, except parasitic weeds, are primary producers and compete with the crop plants. They may have beneficial effects besides their detrimental ones. Weeds are, therefore, plants which are more harmful than beneficial.