Pastures or rangeland depleted by drought could become especially susceptible to invasive weeds, partly because of lack of competition from desirable forage plants and also due to potential introduction of weed seeds from outside sources.
Over the past few years, as severe drought first affected the Southern Plains, then the Northern Plains and Midwest, producers shipped huge quantities of hay around the country. Feeding hay from outside sources creates a potential for introducing new weed species to a ranch, Deneke says. In some cases, if the weed seeds came from a significantly different climate, they might not adapt to the new location. In other cases, he says, the seeds just needed to get there to establish a foothold.
Matt McGowin, a DuPont range and pasture specialist based in Flowood, Miss., agrees that purchased hay can introduce weeds. Areas where producers fed hay in pastures is where they typically see the first weeds of the season, McGowin says. Several factors in addition to seeds imported with hay can contribute to the problem, including overgrazing in those areas, along with soil nutrients from cattle and decomposing forage.
In addition to the forage itself, weed seeds can be transported on equipment used to deliver the hay, such as truck or trailer tires. Deneke adds that cattle also can spread weeds to new pastures, as some types of seeds can pass through their digestive systems. Moved to a new location, the animals deliver the seeds complete with moisture and nutrients to help them off to a good start.
Hay-feeding areas are the first places ranchers should scout for weeds in the spring, McGowin says, and Deneke agrees, saying producers should scout and monitor those areas diligently for the emergence of any unfamiliar plants.
Deneke encourages scouting in general, saying ranchers should scout pastures and rangeland for weeds the way farmers scout crop fields, to find weed infestations early and initiate timely control methods. Scouting two to three times during the growing season, such as once in the spring, once in summer and again in the early fall, is ideal he says. But at the least, he says, producers should scout once in the fall.
Systematic monitoring of pasture and rangeland provides managers with a means to identify trends in weed populations and pasture conditions overall, and to plan management accordingly. Photographic records, taken at the same locations at the same time every year, along with notes on moisture and weather conditions, can help ranchers make better-informed grazing and weed-control decisions.
Choose appropriate control methods
While prevention through grazing management serves an important role, on many ranches damage from drought over the past few years was severe enough that additional weedcontrol measures are necessary.
Deneke recommends using an integrated approach toward weed control, incorporating several methods wherever possible.
• Cultural controls are methods that favor desirable plant growth, such as proper grazing management, irrigation and seeding vigorously growing, competitive, desirable plant species.
• Mechanical control physically disrupts weed growth and includes such methods as tillage, mowing, mulching, burning and flooding.
• Chemical control is the use of herbicides.
• Biological control is the use of an organism to disrupt weed growth.
Classical biological control uses natural enemies of weeds, such as insects or disease organisms. Biological control also may include use of sheep, cattle, goats or other large herbivores to control weeds.
Cultural and biological controls, in many cases, provide an eff ective fi rst step in controlling weed outbreaks.
Some invasive or introduced weed species can provide relatively nutritious forage for cattle, especially early in the growing season, Deneke says. Ranchers can use targeted, intensive grazing as an early control method for these weeds, providing more desirable grasses a chance to fi ll in as the growing season progresses. Palatability typically declines as these weeds mature, so timing of grazing is important. Deneke stresses that producers need to learn the characteristics of any non-native weeds in the area and avoid exposing cattle to toxic weeds, as even those with mild toxicity can affect performance and reproduction.
For many annual or biennial weeds, timely mowing, before seedheads mature, can aid control. Deneke says, for example, the best control for biennial thistles often is to keep them from going to seed. Again, timing is critical, as mowing after the plants have gone to seed will help spread seeds and exacerbate the weed infestation.
Perennial weeds typically have deep root systems and compete aggressively for pasture nutrients and water. They often require chemical control, integrated with other control methods, to kill the root system.
Canada thistle, for example, spreads by seed and from vegetative buds in its extensive root system, making it diffi cult to control. According to literature from Colorado State University, one Canada thistle plant can colonize an area 3 to 6 feet in diameter in one or two years.
Likewise, leafy spurge, another invasive plant in much of the North and West, is a creeping perennial that reproduces from seed and vegetative root buds. It has an extensive root system and can reduce cattle carrying capacity of rangeland or pastures by 50 to 75 percent.
Deneke also lists hoary cress, also known as white top, as an example of a perennial invasive weed that invades pastures and rangeland, often after drought, across much of the country.