Weeds are most important pests of the world and affect overall crop and plant productivity, including health, owing to their competitive nature for light, space, and organic and inorganic nutrients. Weeds can protect the soil by minimizing erosion but because of shading effects and competition in nature, it may be harmful to the associated trees and crops in terms of reducing yield, overall biomass, and productivity. The deleterious effects of understory weeds in forest are not specific but also affect the health and productivity of harboring wild animals, livestock (in rangeland and grazing lands), forest-fringe-residing humans, and even terrestrial soil pools. Weeds have some chemical compounds that are added to the soil through litter and leaf fall and affect the decomposition and decaying of organic matter by disturbing the health of soil-inhabiting organisms (bacteria, viruses, protozoa, and earthworms). Ruminants and lactating cattle (especially cows) are more prone to poisonous weed exudate chemicals such as oxalate than nonruminant animals. Thus, more attention should be given to controlling and managing weeds, either in the forest or in forest-based nursery areas in the tropics. Also, the global impacts of herbicides and other toxic chemicals are a major concern today that needs much more attention for further study in the ecological and environmental context.