The weed infestation in a field is defined by three parameters: 1) the number of species present, 2) the density of each species, and 3) the distribution of the species across the field. While the number of species in a field remains relatively constant from year to year, the latter two factors fluctuate widely in response to environment, cultural practices, and weed management tactics. It is the continual changes in weed infestations that make successful weed control such a difficult task to achieve consistently.