Pasture Weed Management after a Drought
Mark Renz Extension Weed Scientist, University of Wisconsin-Madison
Although spring precipitation has alleviated some concern about a continued drought, we can expect some lingering effects in 2013. Many pastures last summer were overgrazed, and only the weeds remained green until the late rains in September. I expect the combination of slow regrowth this spring and overgrazing of pasture forages last year will result in significant changes in pasture plant composition in 2013, with the potential for weed species to increase. Below are several management practices to consider in pastures related to weed management.
Pastures that were overgrazed in 2012 had little residual cover present over the winter (<4 inches). Because of this we will likely see increased seed germination this spring. These species germinating may be weeds or desirable plants such as clover, so early identification will be important to determine what species are present. Once this information is available, a management plan can be developed based on the weed and its density in your pasture. Unknown plants should be identified as these could be toxic or regulated invasive plants, both of which should be controlled immediately. There are several weed identification resources identified at the end of this article.
If poisonous plantsare found, avoid animal contact with plants, especially when limited forage is available (early spring). We recommend removing animals from areas with highly toxic plants and controlling populations with the appropriate management at the correct stage of growth. This could be well into summer depending on the species. If using an herbicide to control the poisonous plant, make sure to keep animals off the treated area for at least 14 days to allow the foliage to senesce. Herbicides can increase the palability of many poisonous plants, increasing their intake, and resulting in animal toxicity, a situation that can be avoided by delaying turnout. Not sure what the common poisonous plants in Wisconsin are, or what the symptoms of poisoning might be? See: http://www.uwex.edu/ces/crops/uwforage/PoisonPlants8-12.pdf
Canada thistle and other perennial weeds will likely also be more common in 2013. Canada thistle is of the greatest concern as this plant can greatly reduce forage productivity and utilization of forage grasses. I suggest intensive scouting and management of Canada thistle and other perennial pasture weeds (e.g. horsenettle, hoary alyssum) to prevent spread. See weed management resources below for more information on control options for perennial weeds.
Biennial weeds like plumeless thistle, wild carrot (Queen Anne’s lace) and burdock will likely germinate in high numbers this spring. As biennials require a year of overwintering to flower, I don’t expect to see dramatically larger flowering populations until 2014 as plants will be seedlings and rosettes in 2013. While competition with desirable forages may not be critical this year, plants should still be targeted for management as controlling these plants as rosettes is the most effective strategy. See weed management resources for more information on control options for biennial weeds.
Annual weeds that are common in annual row crops will be more common in pastures in 2013 (e.g. lambsquarter, ragweed, yellow foxtail). These will start germinating from mid-April through June and have the potential to continue germinating through August. See the weedometer for germination timings of specific weed species: http://weedecology.wisc.edu/weedometer/ . Annual weeds are most problematic in continually grazed pastures, as they are rarely eaten in continuously grazed situations and deter animals from feeding on desirable forage growing among the weeds. Rotationally grazing your animals can alleviate many of the negative effects of these species if timed correctly. Most broadleaf weeds have good forage quality if eaten before they flower. Encourage animals to feed on broadleaf plants before they flower to prevent seed production and maximize forage quality. Broadleaf herbicides can also be used to suppress populations. If the correct herbicide is selected and applied at the correct time, one can expect effective removal without harming established grasses. However, these herbicides will also injure desirable legumes in the pasture, so if desirable legumes are common in the pasture, avoid broadcast spraying.
Annual grasses will also be more common, especially yellow foxtail. While annual grasses can be grazed like broadleaf weeds (before they flower), their forage quality is low. No herbicides are registered for use to control annual grasses in Wisconsin pastures, therefore the best approach to manage these plants is to prevent emergence and promote the growth and competitiveness of the desirable forage present. To prevent emergence of annual grasses and broadleaf weeds leave a minimum of 6 inches (8 inches is ideal) of forage in the pasture (residual or actively growing) when germination of the weed occurs. This can be difficult to accomplish for some species as they can emerge throughout the entire growing season.
Clovers: In addition to weeds, I expect this year to have high levels of clover emergence due to limited residual cover and reduced competition from existing forage. While many graziers add clover seed to pastures periodically, these legumes have a hard seed coat that allows them to survive in the soil for many years. So even if clover seed was not added for several years there is a chance that clover will germinate and appear in your pasture. Some of these clovers will likely be desirable (e.g. red clover), while others may not (e.g. prostrate, feral white clover). If desirable clovers are present, manage weeds and desirable forage so that they do not out-compete establishing clovers. This can be done with grazing, or clipping/mowing. Typically clovers readily establish as long as they get an opportunity to emerge and develop a root system. I expect the slow regrowth of pasture grasses and low residual cover will be enough for good establishment in most pastures. Avoid over-grazing these areas, especially in spring, as this can reduce establishment. Also avoid broadcasted herbicides as they will injure or kill establishing clovers.
In addition to these weed management tips, several agronomic practices will help alleviate some of the effects of the drought. Please refer to this excellent publication from UWEX grazing specialist Rhonda Gildersleeve for information on assessing pasture condition, soil fertility, grazing management, and pasture renovation options: http://fyi.uwex.edu/grazres/files/2013/03/Spring-Pasture-Mgt-Tips2013final.pdf
WEED IDENTIFICATION RESOURCES:
- Weed Identification website : http://weedid.wisc.edu
- The dirty dozen and beyond: http://fyi.uwex.edu/weedsci/2004/10/30/the-dirty-dozen-and-beyond/
- Invasive plant identification videos: http://www.youtube.com/user/uwweedscience
WEED MANAGEMENT RESOURCES
- Pest Management in Wisconsin Field Crops: Forages and Pasture weed management section pages 182-191: http://learningstore.uwex.edu/Assets/pdfs/A3646.pdf
- Weed Control in Pastures Without Chemicals: http://www.uwex.edu/ces/crops/uwforage/nonchem.pdf
- Canada thistle Management in Pastures: http://fyi.uwex.edu/weedsci/2013/04/18/canada-thistle-management-in-pastures/
- Thistles in Pastures and Beyond: http://fyi.uwex.edu/weedsci/2007/09/30/thistles/
- Multiflora rose: http://fyi.uwex.edu/weedsci/2007/06/30/multiflora-rose/
- Specific Invasive plant control factsheets: http://fyi.uwex.edu/weedsci/category/invasive-plants-of-wisconsin/
- Brush Management in Wisconsin: http://fyi.uwex.edu/weedsci/2007/08/30/brush-weeds/