Extension Education: Why Weeds are Problems in Range and Pasture

Weed scientists often discuss the importance of managing weeds to maintain or improve rangelands for livestock production, wildlife habitat or other ecosystem services. These implications may make intuitive sense because we have heard them repeatedly, but the details of how or whyweeds are able to have these impacts are often not discussed as frequently.

Carrying capacity

Weed infestations can reduce carrying capacity of rangelands by using resources that desirable plants need. 

Many of the problematic rangeland weeds are highly competitive against desirable plant species for limited resources. Light, soil nutrients and water are needed by plants to grow and reproduce. When one of these resources is limited in availability and neighboring plant species each need the resources, competition takes place. 

For multiple reasons, weeds are often better able to obtain these resources than native plants. Some potential reasons for the superior competitive ability of weeds are lack of grazing pressure from herbivores or insects, lack of pathogens leading to “healthy” plants, and an evolutionary history that resulted in highly competitive populations. Another potential explanation is allelopathy – where weeds produce chemicals that inhibit the growth of neighboring plants. This plant “chemical warfare” is often cited as a primary explanation for weeds forming very dense stands to the exclusion of other species, but the science to support widespread allelopathy in weeds is inconsistent. 

Regardless of the mechanism for competitive ability, the result for livestock producers is similar – increased proportion of non-palatable weeds in a management unit is often associated with a decrease in available forage. Several research studies suggest that only moderately dense infestations of weeds such as leafy spurge or Russian knapweed can greatly reduce grass production. As these weeds spread across pastures, increasing their area, total available forage in the pasture decreases. 

Ranchers facing this situation that do not manage their weeds may experience reduced weight gains in the pasture, or they may need to reduce the stocking rate in the infested pasture. Maintaining historical, pre-weed stocking rates may increase grazing pressure beyond sustainable levels in the non-infested portions of the pasture, thereby increasing susceptibility to further spread of weeds.

Rotational grazing

Weeds may limit flexibility in rotational grazing systems. 

Many current grazing best management practices are related to rotational or rest-rotation grazing strategies – try not to graze the same pasture at the same time every year, target approximately 50 percent utilization for plant health, limit the number of times a grass plant is grazed in the same year, etc. Most of these practices, with the exception of moderate utilization, are achieved by manipulating livestock distribution throughout the year using fences, water sources, herding or other methods.  Logistical constraints, such as shipping corrals or calving barns, also affect how a grazing rotation is implemented. 

Another factor that may limit which pastures can be used at a specific time of year is the presence of specific weeds. 

Toxic plants like larkspur and death camas have a higher probability of causing livestock deaths when they are palatable to livestock, usually in the early spring. Lupines can cause birth defects if bred cattle consume them during the middle of the first trimester. 

Direct losses associated with poisonous plants can cause significant economic impact to a livestock producer. Managing aroundthese toxic windows in plants limits, or precludes, the use of those pastures during dangerous times of year and also reduces the number of timing options to graze other pastures in the same grazing system. 

Nutritional value

Other weeds may alter the seasonality of nutritious forage. Heavy infestations of early-maturing weeds, such as cheatgrass or possibly bulbous bluegrass, may reduce the time that palatable green forage is available in a pasture. Such plants use early spring moisture to complete their life cycle in a short time period, and in so doing, reduce soil moisture for plants that may stay greener longer into the year. This effect is particularly evident in years with limited precipitation in later spring to early summer.

The impacts of invasive weeds in rangeland systems are varied and manifold. Thinking through how weeds may affect grazing decisions will enable land managers to better develop a strategic approach to their rangeland weed challenges.

Brian A. Mealor is an Assistant Professor and Weed Extension Specialist at the University of Wyoming and can be reached at bamealor@uwyo.edu. More information about ongoing weed science research at the University of Wyoming can be found at weedcontrolfreaks.com.