By Cassandra Schroeder AAg, Range Management Extension Specialist, Kindersley
Pasture weed management is important because weeds can decrease the productivity of rangelands. Weed management can impact not only forage productivity, but also animal and ecological productivity. Animal health can be affected by decreased forage quality or palatability, by toxic compounds in some weeds, or by physical features of the weed such as rough awns that damage soft tissues. Soil quality is negatively impacted when aggressive invasive weeds form a monoculture. When there is only a single weedy species with a certain root type contributing to soil structure and biology, soil health is compromised. Weed monocultures also limit vegetative diversity and consequently the wildlife that it supports. Many of these factors are interrelated and contribute to decreased production of desirable forages. Since weeds can impact the productivity of rangelands, pasture weed management deserves some forethought.
The impact that weeds can have on an operation, and to what extent they occur, depend largely on the weed species and environment. For example, downy brome robs spring moisture with its early growth, but then matures early, leaving an unpalatable grass with rough awns for the rest of the season. Downy brome also increases the risk of fire on rangelands because of its early maturity. Leafy spurge is a very competitive weed with a strong root system that can quickly out compete other desirable vegetation. Since cattle avoid eating leafy spurge, this weed can bring the grazing capacity of heavily infested land to near zero. Both of these are examples of invasive weeds that are not native to Canada. Because they are growing in a new environment without their natural enemies, they spread more rapidly and can overtake large areas. The weed species and its environment should be taken into consideration when thinking about weed management.
There are also some important weed management considerations that are unique to rangelands. These include land quality, polyculture crops, perennial plants, and preferential grazing. Firstly, pastures are often lower quality lands that are not well suited to annual cropping regimes. They can be sandy, hilly, and stony, and such characteristics can limit weed control methods. For example, topography may prevent the use of large equipment and poor soil can mean that using tillage to control weeds is not a good option. Also, some rangeland herbicides cannot be used on sandy soils. Secondly, pastures are usually a polyculture of species, which makes chemical selection challenging. In many cases, spraying a pasture weed will come at the cost of killing desirable legumes in the treatment area. Thirdly, pastures are perennial systems which are expensive and time consuming to terminate and re-establish, and fourthly, cattle are preferentially grazing within the pasture. The weeds typically are avoided by cattle, while the nearby desirable species are more heavily grazed. This gives the weeds an advantage and facilitates their growth over the desirable vegetation. The sum of these four challenges is that once weeds are established in a pasture they can be difficult to control. This calls for proactive weed management to prevent weeds in pastureland.
What does proactive weed management look like? The first step is to prevent the introduction of weeds. Weeds can be spread in bales, on equipment, in soil or gravel and by cattle and other animals. Some ideas to prevent weed spread are to use weed free bales or to feed contaminated bales in areas where weeds can be easily monitored and controlled. In other words, feeding contaminated bales on the edge of a ravine is not advisable. When moving animals from weedy areas give them a chance to clean out before moving them to a new pasture and watch for weeds like burdock that stick to their hides. Be mindful of equipment and vehicles that may be spreading weeds and clean them when possible. Implementing biosecurity practices that help prevent the spread of weeds is usually cheaper than relying on reactive control measures.
Preventing weeds is not just about preventing weed spread. Weed establishment can also be prevented by keeping a healthy pasture stand. Although it cannot completely stop all weeds, keeping a healthy pasture stand makes a difference. When the plant community growing in a pasture is strong and healthy, weed seeds have less chance of germinating and developing. In contrast, an overgrazed pasture with bare ground and weak plants invites weeds to establish themselves. This is because there are extra resources, such as light and soil nutrients, that are available to the weeds in overgrazed areas. Some of the keys to maintaining a healthy pasture include leaving sufficient litter cover and allowing plants sufficient rest to grow their roots. Preventing weed establishment is only one of many benefits of keeping a healthy pasture stand.
If weeds are established it is still best to be proactive and control weeds as soon as possible. It is easiest and cheapest to manage weeds while they are just a few plants in a small area. If they are left to spread and become more established, they become more expensive to control. But, a few plants in a small area are not always easily noticed, so this warrants some intentional scouting. Controlling a small patch of weeds before it becomes obvious across the field will save on control costs. For help recognizing invasive weeds, check out the Saskatchewan Forage Council’s pocket guide. Watch for these weeds and make note of any sightings so that weeds can be relocated for prompt control.
Proactive pasture weed management can help keep weed control costs low. Proactive management includes preventing weed introduction and keeping a healthy pasture stand to prevent weed establishment. Being proactive is important because control options on pasture land can be very limited. Although different weeds have different impacts and severity, remember weeds do have negative impacts so a proactive weed management strategy is important for ranchers, not just grain famers.
For more information contact your local Range Management Extension Specialist.