Herbicide resistance is the result of an evolutionary process where a population gradually changes from being susceptible to being resistant to an herbicide. The resistance mechanism is usually a change in the biochemistry of the weed, but in some cases changes in morphology or growth rhythm. The change is caused by random mutations. Two resistance mechanisms are found in black-grass; target site resistance and metabolic resistance.Black-grass is a tufty, winter annual grass weed, which is mainly found in milder climates. Black-grass is a big problem in Southern Europe.