Herbicides were evaluated alone, in combinations, or as sequential treatments on direct-seeded tomato each year from 1982 through 1984. Diphenamid at 4.5 kg ai/ha or napropamide at 1.1 to 2.2 kg ai/ha alone are registered but frequently do not control weeds adequately, particularly when yellow nutsedge and/or Florida pusley is common. Combinations or sequential treatments are necessary to overcome these weed problems in cropping systems. Metolachlor at 0.8 to 3.4 kg ai/ha injured direct-seeded tomato in proportion to the rate applied. Pendimethalin at 0.6 kg ai/ha also injured tomatoes. Fluazifop at 0.3 to 0.6 kg ae/ha and sethoxydim at 0.2 to 0.5 kg ai/ha effectively controlled annual grasses without injuring tomato seedlings. These herbicides would reduce competition and would increase harvesting efficiency where grass weeds are a problem.

A seed remover using a modified water-spray system was constructed to separate weed seeds rapidly and effectively from a silt loam soil A 1-kg soil sample can be washed through the seed remover in 6 min. Using a 500-μ mesh sieve, only 70% of small shepherdspurse seeds were recovered, while 100% of larger seeds of henbit, curly dock, wild oats, and downy brome were recovered. The modified seed remover system reduced the required man-hours by more than 50% compared to previous systems.

The growth of two mallow family weeds, velvetleaf and prickly sida, was inhibited 50% when seedlings were treated with less than 200 g ai/ha of thidiazuron. Spurred anoda was unaffected by treatment with up to 1120 g/ha of thidiazuron. Cotton, okra, and hibiscus, all in the mallow family, also were susceptible to thidiazuron treatment. Leaf development in 2-week-old cotton seedlings was inhibited 20% by less than 35 g/ha thidiazuron as were seedling okra and hibiscus. Plants of all these susceptible species were more resistant when grown for 2 months before treatment. Seedlings of soybean displayed localized injury after treatment, but plant height, biomass, and seed production were not significantly affected 10 weeks after treatment with up to 1120 g/ha thidiazuron. Seedlings of corn appeared unaffected by treatment with up to 1120 g/ha thidiazuron.

Field studies were conducted from 1984 to 1986 to assess cabbage injury and yield response to several acetanilide herbicides. When incorporated, metolachlor significantly injured cabbage and reduced yields while post-transplant applications caused foliar flecking but did not reduce yields. Crop injury from pretransplant applications of 4.4 kg ai/ha was significant for all herbicides in both 1985 and 1986. Although total yields in 1985 and 1986 were comparable, maturity was delayed in 1986. With few exceptions, the acetanilides did not differ in either injury or yield-reducing potential. However, compared to trifluralin, metolachlor reduced yields in both years and at both rates.

Metribuzin and the ethyl ester of chlorimuron were evaluated alone and in combination for preemergence broadleaf weed control in soybeans. Neither herbicide alone controlled all broadleaf weeds tested, but combinations showed both complementary and additive action. Two field studies quantified these interactions on broadleaf weeds and showed that low rates of either herbicide alone controlled Pennsylvania smartweed and redroot pigweed. Metribuzin was more effective than chlorimuron in controlling prickly sida and hemp sesbania, while chlorimuron was more effective on common cocklebur, sicklepod, and ivyleaf and pitted morningglories. Additive action was most important on velvetleaf, sicklepod, annual morningglories, and hemp sesbania. Because the components were both additive and complementary, a range of mixture rates and ratios were more effective for weed control than either herbicide alone.

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.

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 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.

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.