– Victor Shelton, Retired NRCS Agronomist/Grazing Specialist
Ironweed is a pasture weed but is also a good pollinator plant.
What defines a weed? The most fitting definition is simply a plant out of place. For instance, pigweed in a garden is typically unwanted and considered out of place. Similarly, a volunteer corn plant in a soybean field might be seen as a weed, since it disrupts the desired crop. In a pasture setting, defining a weed can be nuanced, often requiring careful consideration of its impact on the ecosystem, its usefulness, desirability of the livestock present, and its compatibility with desired vegetation.
A weed can sometimes be beneficial to a pasture system when it aligns with the grazing preferences of livestock and does not overly compete with other desirable perennial vegetation. For instance, certain weeds may provide additional forage options during different seasons or conditions, supplementing the diet of grazing animals. Moreover, they can contribute to biodiversity and soil health by adding organic matter and improving nutrient cycling. When managed properly, these weeds can enhance the overall resilience and productivity of the pasture ecosystem without compromising the growth of desired vegetation.
Crabgrass is often considered a weed due to its rapid growth and ability to invade lawns and gardens. However, in a pasture setting, crabgrass can have significant forage value, especially when it is immature. Young crabgrass plants are highly palatable to grazing livestock and can provide nutritious forage during certain times of the year when other grasses may be less abundant or nutritious. This characteristic makes crabgrass a valuable supplementary “forage” that can support the nutritional needs of livestock in a pasture system, demonstrating how a plant labeled as a weed can offer benefits when managed appropriately. Most tall fescue pastures are riddled with crabgrass which often helps to meet nutritional needs during the summer slump period when most cool-season grasses usually take a bit of a siesta.
Some people prioritize aesthetics over anything else. You need to ask three questions: From what view are you looking? What is the true economic and forage impact threshold? And, are the plants toxic or invasive?
Personal thresholds of weeds, especially some particular weeds, is often very low for some people. A single specimen in some cases might cause weed anxiety. Too many times the roadside view of a field appears a lot worse than reality. Before taking action, walk around the field and see up close what the situation is really like. Quite often, it is not nearly as bad as you thought.
Look at the canopy. In other words, how much sunlight are the weeds intercepting and preventing from getting to desirable forage? In cool-season grass pastures, the typical weed threshold for canopy coverage is often set at around 10% to 20%. This means that weeds are managed to ensure they do not exceed 10% to 20% of the total ground cover within the pasture. Frequently, they are a lot lower than this threshold.
This threshold allows for some weed presence without significantly impacting the growth and productivity of the desired forage, thereby maintaining the desired balance between forage quality and weed control. The bigger those weeds get, especially ones that can grow tall and wide, the fewer of them that are tolerable. Two or three small plants per square yard are normally under the above mentioned threshold. If not removed by grazing/browsing or perhaps a hoe, then they can quickly grow to be above that threshold and require removal by some means. When canopy approaches 30%, forage yield is almost always impacted unless the “weed” is somewhat desirable forage for the livestock present.
Depending on the species of the weed, high density, short duration grazing will take out quite a few weeds, especially if those weeds tend to be desirable species for the type of grazing livestock. Small ruminants such as goats and sheep quite often do a better job of weed control than cattle on a regular basis. This is especially true when the weeds are small and young. Some “weeds” then just add to the diversity of the salad bar.
Cattle do a better job when they are concentrated on a smaller alllotment for a short period of time, ideally less than 24 hours. This creates an environment where there is more competition between animals for the forage present that results with more even grazing and less selectivity. Recovery of desirable species is then key to help them provide good competiton over undesirables for the next grazing.
Quite often the bushhog is utilzed a little prematurely. You must assess that weed threshold and also the amount of grazable forage present prior to mowing. Sometimes, and mainly due to not being able to mow high enough, valuable, grazable desirable forage is impacted. In most cases, you are better off grazing it first and taking to a desirable stop grazing height and then clipping it post grazing. Just make sure to do so prior to seed set on the undesirable species. Spot mowing or spraying also disturbs less grazable forage.
Generally, you can not graze your way out of a a weedy situation if those plants are not at least somewhat desirable to the grazing livestock. Animals can be conditioned to eat some species. Plants that are considered toxic should never be forced to be consumed.
Some species that most would consider weeds can be desirable and nutritious for some grazing livestock. Giant ragweed is sought after by most goats and sheep, crabgrass like mentioned earlier is highly desirable to cattle. Tick-trefoil (Desmodium), which most people see as a nuisance because of the “stick-tight” seed pods, is extremely good forage and desirable to most grazing livestock with a relative feed value of up to 120 until flowering. Leguminous plants like tick-trefoil can fix significant amounts of nitrogen, often contributing 50 to 200 pounds of nitrogen per acre per year.
Some “weedy” appearing pastures can be extremely productive because of increased diversity. There must be a healthy balance and action taken when undesirable plants exceed the threshold of productive and nutritious grazing and negatively impact production or animal health. Toxic and invasive species should always be controlled. Remember, it is not about maximizing a grazing event, but maximizing a grazing season! Keep on grazing!