Grassland damage and yield depression can be caused by a variety of causes. In addition to less than ideal crop management, climatic causes such as dry periods or animal pests can also lead to negative changes in the vegetation cover and plant population. If the important forage plants fail, the population becomes gappy and undesirable grasses and herbs can establish themselves in the resulting gaps. The feed value of the gap fillers is usually very low and the plants are not eaten by the animals or only with great reluctance. In addition to the native weeds and grasses such as the blunt-leaved dock and the common panicle, neophytes, some of which are poisonous, are also increasingly becoming a problem.