Weed control in grass pastures is limited to controlling broadleaf weeds and is generally accomplished with postemergence, translocated herbicides. These herbicides are absorbed by the foliage and move within the plant. As a result, they may produce a toxic effect a considerable distance from the point of entry. As might be expected, postemergence applications are greatly affected by the age of the weeds and the growing conditions. As a general rule, postemergence herbicide applications should be made when the weeds are young and/or actively growing because they are easiest to control then. Adverse environmental conditions such as hot, dry weather before spraying make postemergence applications less effective than when applied during warm, moist weather. In addition, rainfall shortly after postemergence applications may reduce effectiveness of the herbicides.
For control of summer annual weeds such as common lambsquarters, translocated herbicides should be applied to the foliage of seedling plants in the spring or early summer. The rosettes of winter annual weeds such as shepherdspurse should be treated in the fall or early spring.
Most problem weeds in grass pastures are either biennial or perennial broadleaf weeds. Postemergence treatments for biennial weeds such as common burdock, or simple perennials such as dandelion, should be applied to the rosettes, or other fall growth, in the fall or early spring before these plants bolt (send up a flower stalk).
Foliar treatments for creeping perennial weeds such as common milkweed must be made when they are actively growing and have a large leaf area. The ideal time for treating them is after they have reached the bud stage in mid- to late summer. During this period they have their maximum leaf area and are storing food reserves for the winter. Translocated herbicides applied during this period are absorbed by the leaves and moved into the underground reproductive and storage organs with the food reserves. Because the herbicides recommended for broadleaf weed control in pastures will kill legumes, they should not be used as broadcast treatments if legumes are present. In all cases, grazing and haying restrictions in Table 4.14.1 and on the labels must be followed carefully.
Herbicide Resistance Management
Herbicide resistance management involves the use of crop rotation along with herbicide rotation and/or use of herbicide combinations that include herbicides with different sites of action (how they affect weeds). These practices will help manage existing herbicide resistant weed populations and delay development of new resistant weed populations.
To effectively utilize herbicides with different sites of action, everyone involved in decisions about weed management must have site of action classification readily available. The Weed Science Society of America (WSSA) has approved a numbering system to classify herbicides by their site of action (Mallory-Smith, C.A. and Retzinger, E.J. 2003. Revised classification of herbicides by site of action for weed resistance management strategies. Weed Technol. 17:605-619).