The following material was adapted from Pasture Management for Reduced Weed Problems, L. Burrill and R. Parker, Oregon State University Extention Crop Science Report 92

Because of the many ways that weeds can be transported and spread, some weed problems in pastures are inevitable. When considering how to deal with a weed problem in a pasture, a grower should first evaluate the economic impact and animal health impact of the problem. In some instances the weeds present may not be toxic to animals and may even provide somewhat nutritious forage. In addition, the economic return in controlling weeds may not justify the time and expense involved in using herbicides. The cost per acre of treating a pasture with herbicide treatment may range from a few dollars per acre up to over $50 per acre ($20 per hectare). As long as the weeds present don't constitute a serious threat to animal health or violate local weed control regulations (e.g. noxious weed laws), the grower may decide not to treat the weed problem.

If a serious weed problem is present in a pasture and treatment is desired, the first step in the treatment process is to determine why a weed problem is present. Has the condition of the pasture been neglected to the point that there are significant areas of bare soil? This gives weeds a prime opportunity to invade the pasture. If this is the case, pasture renovation or overseeding may be necessary. Other reasons that weeds gain a significant foothold in pastures include overgrazing, lack of water, the movement of weed seeds into the pasture via contaminated equipment, the movement of weeds into the pasture via irrigation ditches, the movement of weed seed via moving animals, and the use of less competitive forage species.

Once it has been decided to implement weed control, probably the first places to be treated are areas where there has been the maximum disturbance of the soil. In a grazing situation this is typically areas around water supplies, roadways, livestock trails, gates, and salt blocks. Weeds that gain a foothold in these areas are then able to move out into other areas of the pasture. If economical, the pasture manager may want to reseed a competitive grass in these areas each year.

In some pastures or rangelands there will be problems with poisonous plants and/or noxious weeds. In these cases the pasture manager will be under a legal and ethical obligation to initiate some type of control. It is always the responsibility of land managers to be aware of local regulations concerning noxious weed control. In some areas the local government may even pay for part of the cost of control of some weed species. Consulting with the nearest extension agent and/or weed control specialist is essential. These specialists will advise you on the best choice of weed control techniques to control the specific weed problems in your area.