By Dwight Lingenfelter
Buttercup management:
Pastures that are yellow because of buttercup infestations are a common sight during spring. However, this should not be considered a badge of honor. Do not wait until the pasture is "yellow" with buttercup flowers (as pictured above) to spray. A combination of mowing and herbicides will provide the best management of buttercup species. Mowing fields as low as possible during the early spring before they start to flower can help reduce seed production. Routine mowing can also occur during the rest of the growing season to thwart weed seed production and encourage pasture growth, but a well-timed herbicide application is important for best control. The question about when the optimal time is to spray herbicides — the short answer is early spring. The use of herbicides during early spring (Mid-March thru April) is the most effective time to control buttercup populations. Herbicides such as 2,4-D (2-3 pints/acre), 2,4-D (1 quart) + dicamba (1 pint), metsulfuron/MSM (0.2-0.3 oz), or Crossbow (2-3 quarts) are commonly used to control buttercup. Since herbicide spray coverage is important, it is best to use higher spray volumes (at least 15 gallons/acre, but 20 gpa or more would be better) to allow the application do get into the sward and onto the weeds. Spraying when buttercup is flowering usually does not provide effective control. When spraying in the spring make sure the air temperatures are at least 50F or more for a few consecutive days for effective control. Keep in mind, these treatments will severely injure or kill any legumes in your stand. If you are concerned about keeping white clover in the mix, 2,4-D (1 pint) could be used. However, by using this tactic, buttercup will likely only be suppressed, and clover growth can be temporarily stunted. So be sure to consider all the tradeoffs when using this practice. On a sidenote, herbicides applied in fall (mid-September thru October) are not as optimal as a spring application but can still provide some control of buttercup.
Regarding buttercup species that are common to PA, these include: bulbous buttercup (Ranunculus bulbosus), creeping buttercup (Ranunculus repens), tall buttercup (Ranunculus acris), and small flower buttercup (Ranunculus arbortivus). Most of these have a perennial lifecycle, however some can behave as a winter annual.
Keep in mind, that pastures with dense populations of buttercup and other weed species in most cases are lacking proper management. Often it is a sign of over-grazing and a poor stand of desirable forage. Good agronomic practices are key to a healthy, persistent forage stand and should include proper fertility and soil pH levels and grazing and mowing management, along with effective and properly timed herbicide applications.
Japanese stiltgrass management:
Japanese stiltgrass is an aggressive annual grass species. It thrives in moist, shaded areas and can spread into pastures from woodlands, but it also does well in full sun. Most livestock and wildlife will not graze it, so it often remains and overtakes desirable grass forages. Since it is a grass, it is difficult to eradicate it without harming desirable pasture grasses. It is a summer annual, so it germinates from seed each year and dies in the fall. Therefore, make sure to prevent seed set. Mowing Japanese stiltgrass can provide some suppression but once routinely mowed it tends to grow shorter and sets seed heads lower than the mower height. Unlike crabgrass, stiltgrass flowers and sets seeds much later, from mid-September through October. The best way to use this method is to allow the grass to grow naturally until late August or September when it starts developing seed heads. When you start seeing seed heads but before they drop their seeds, mow the tops off or hand-pull the plants. This method may take several years to be effective because there may be a considerable seed bank in the soil waiting to germinate.
Japanese stiltgrass in late summer. Image Credit: D. Lingenfelter, Penn State Weed Science.
Certain herbicides can be used to control stiltgrass. Prowl H2O or Satellite HydroCap, (both contain pendimethalin) can be applied preemergence in early spring to control germinating seedlings. These herbicides can be applied to established grass forages (with at least 6 tillers) in the spring to prevent seedlings from emerging. For best results, this should be applied at the higher end of the rate spectrum and twice in the spring (see Spring Weed Control in Grass Hay and Pasture for more details), and will also control foxtail, crabgrass, and some broadleaf weeds. Keep in mind, Japanese stiltgrass can germinate in early spring a few weeks before crabgrass, therefore Prowl H2O or Satellite must be applied before germination otherwise, it will not control it. One major limitation with this method is that these products have a 10-month reseeding restriction for pasture grasses and 6 months for alfalfa, so areas of the field might have limited forage until the following spring. Prowl H2O and Satellite have no grazing or harvest interval restrictions for any type of livestock.
Currently, there are essentially no labeled, post herbicides that selectively control stiltgrass without injuring the desirable forage grasses. Facet and Quinstar (quinclorac) do not control Japanese stiltgrass but have some activity on other weedy grasses. Herbicides such as Plateau (imazapic) and Pursuit (imazethapyr) can control stiltgrass and both of their labels contain inferences to use in forages that include grasses. However, their exact utility in pasture settings is still tenuous. We, at Penn State, are working on field studies to find best ways to potentially utilize these products to control stiltgrass postemergence while not causing too much crop injury. Another option for smaller, dense infestations is to spot spray (up to 1/10th of the total paddock) with glyphosate from mid-May through August.
Glyphosate is non-selective and will kill all plants that it contacts but does not have soil residual activity to control germinating seeds. Milestone at 5-7 fl oz/acre (or other aminopyralid products, GrazonNext or Chaparral) provides suppression of stiltgrass and controls several other broadleaf weeds but it will severely injure or kill forage legumes. Also, when using aminopyralid-containing products consideration must be given to issues with forage grass tolerance and herbicide residues in the forage and manure.
In many situations Japanese stiltgrass invades pastures from woodlands or other non-croplands that are adjacent to them. It is best to manage it before it invades. Some herbicides that can be used in non-cropland areas include: quizalofop (Assure II); fluazifop-p (Fusilade II); fenoxaprop-p (Acclaim Extra in turf settings but not pastures); sethoxydim (Segment); clethodim (Envoy); imazapic (Plateau); imazamox (Clearcast). Refer to the product label for application guidelines. Keep in mind some of these products can kill desirable species as well.
Regardless of which method is selected, it will be essential to reseed the infested area with cool-season perennial forages in the fall (after mowing or applying glyphosate, or before applying Prowl H2O or Satellite). Japanese stiltgrass grows so densely that there is often no other grass left. Leaving a bare spot after the Japanese stiltgrass is killed allows erosion of your topsoil and germination of other weed seeds. However, a thick stand of pasture grasses can shade out the Japanese stiltgrass seedlings (and many other summer annual weeds) that emerge later in the spring. Seed with a pasture forage mix between mid-August and mid-September for the best chance of a successful establishment.
Source : psu.edu