Weed control in established pastures

by Dow AgroSciences

Most pastures in New Zealand consist of mixtures of both grasses and clovers. While perennial ryegrass is the most common grass species sown for permanent pasture, other species sown may include tall fescue, cocksfoot and phalaris. There is no difference in weed control methods using herbicides for pastures containing these grass species. White clover is the most common clover species sown, however, mixes with red and subterranean clovers are also popular. The dominant clover species present will have some influence over herbicide selection. In terms of the herbicide treatment, an established pasture is more than 12 months old and both grasses and clovers are tillered and well developed.

Pasture renovation is both time consuming and expensive. Two very good reasons to try and maintain both new and existing pasture for as long as viable. Sensible grazing management and suitable fertiliser applications will go a long way to achieving pasture longevity. Unfortunately, even the best laid plans may not have been sufficient to prevent established pasture swards from 'opening-up' during this very dry summer experienced by many regions however. Pastures with low grass/clover populations at present are likely to succumb to severe weed infestations once the autumn rains arrive.

The best time to use herbicides for weed control in established pasture is during autumn, early winter or early spring when weeds are actively growing. During winter (especially colder regions) weeds may become stressed, with less active growth likely to cause variable control. A range of herbicides are available for use in established pasture. Used correctly they will give a high level of weed control, no damage to pasture grasses and minimal damage to clover species. Common, difficult to control weeds are listed below. Herbicides that control these troublesome weeds also control many other broadleaf weeds. Check the label and/or with your local Dow AgroSciences technical representative if in doubt about which herbicide to use.

• Californian thistle: Where Californian thistle is the predominant weed, Select* herbicide (a new product that does not cause clover damage) or Dow AgroSciences MCPA either alone or in mixture with Versatill Herbicide is the preferred treatment. Repeat applications will be required. Carpet wiper applications using Versatill Herbicide give a very high level of control with little if any clover damage.
• Nodding and winged thistles: Pasture-Kleen* Herbicide, Select or Dow AgroSciences MCPA are the recommended treatments for nodding and winged thistles, but, where they have become “multi-crowned” plants or crown size is greater than 4 cm, the addition of Versatill Herbicide is required to achieve control. Where these thistles have become resistant to Pasture-Kleen Herbicide and/or Dow AgroSciences MCPA applied alone, the addition of Versatill Herbicide is required irrespective of plant size.
• Ragwort: Where ragwort is the predominant weed, applications of Pasture-Kleen Herbicide should be followed by spot treatments of either Tordon* Gold or Tordon 2G Herbicides for control of multicrowned plants not killed by Pasture-Kleen Herbicide.
• Buttercups: Preside Herbicide is the preferred treatment for buttercups because control is achieved without damage to clovers. Repeat spring (or autumn) applications of Preside Herbicide over consecutive years will be required for a high level of control. Select or MCPB can be added to Preside Herbicide in order to control thistles.
• Scotch Thistles: These thistle are generally, relatively easy to control. Scotch thistle is an annual weed which germinates through the autumn/winter/spring months. Apply Pasture-Kleen Herbicide, Select or Dow AgroSciences MCPA for control of "Scotties."

Remember, once weeds have been eliminated, a dense, vigorous pasture should be maintained by correct use of fertilisers, appropriate fencing, and stock management to avoid over grazing - if successive weed germination's are to be minimised and pasture longevity maximised!!