Four keys to an effective weed management program are knowing the types of weeds present, the problems they cause, estimating their economic thresholds and knowing their available management options.
The first key is to identify the types of weeds that may be present. Most references use flowers or reproductive features that develop too late for effective weed management so early identification requires using seedling or vegetative features. Start by identifying the weed as a grass, broadleaf or sedge. Grasses have linear leaves with parallel veins and rounded stems. Some identifying vegetative features for grasses include ligules, auricles, pubescence, stem shape and leaf margins. Of these vegetative features, the ligule (a hairy or membranous structure between the leaf sheath and blade) is often the most distinctive. Broadleaves are those with typically wider leaves and branched veins. Identifying broadleaf vegetative or seedling features include cotyledon shape and size; leaf shape, size, or arrangement; pubescence; stem shape; and growth habit. The cotyledon (seed leaves) and leaf shapes are often the most useful features for early identification of broadleaves. Sedges are similar to grasses with narrow leaves and parallel veins, but have triangular stems. Identifying features include leaf cross section, leaf shape and plant origin (seed or tuber). The Noble Research Institute's Plant Image Gallery and a Field Guide to Oklahoma Plants are a couple of the many sources available to assist in weed identification.
The second and third keys, estimating the losses a weed may cause and its economic treatment threshold, are closely related. The economic threshold is the infestation level that causes an economic loss greater than the cost of management. The treatment threshold is the level where management must be applied to avoid reaching the economic threshold. In pasture or range situations, a general rule of thumb is reducing 1 pound of weeds will result in 1 pound of additional forage production. For example, if herbicide treatment will control 1,000 pounds of weeds per acre, according to this general rule of thumb, it will result in an additional 1,000 pounds of forage per acre produced. If the herbicide treatment costs less than the value of 1,000 pounds of forage, then it meets the treatment threshold. If the weed population or level of control obtained does not result in enough additional production to cover the cost of the herbicide, then the treatment threshold has not been met and you should not spray. Reduced forage utilization, harvest losses, weeds acting as a host or breeding ground for insect pests, and the potential for increasing the weed populations should also be considered when estimating the economic threshold.
The fourth key is choosing a management option that fits your production system and weed spectrum. Start with cultural practices that maintain profit potential while reducing losses to weeds. These may include crop selection, tillage system, grazing system and prescribed burning programs, to name a few. If cultural practices do not provide acceptable results, herbicides may be required. If necessary, utilize the most inexpensive herbicide that will control the weed spectrum. In pasture or range situations, this usually starts with 2,4-D for broadleaf weeds and then changes to other herbicides based on the weed spectrum. In row or cereal crops, the herbicide choice usually starts with a pre-emergence or early postemergence material that will keep weeds below the economic threshold.
Most weed problems are easier to manage early in the growing season while they are small and weather conditions are better for weed germination and growth. Early management will often allow the crop to get ahead of the weeds and prevent them from reaching an economic threshold.
As input prices like seed, fertilizer and diesel continue to soar, reducing losses to weeds or other pests will continue to increase in importance.