We often think of pastures as monocultures or simple mixtures of improved cool-season grasses and legumes. In reality, mature pastures are complex mixtures of many different species including “weedy” grasses and forbs (non-leguminous broadleaves). So what makes a plant a weed? Many people consider a plant a weed if it is growing where it was not planted. For example, a corn plant is a weed if you find it in a soybean field. For pastures, I like to define a weed as a plant that you cannot get a cow to eat.
We often think of weeds as lowering the nutritive value of pastures, but grazed at the correct growth stage, some weeds can be relatively high in nutritive value (Table 1). For example, in the vegetative growth stage, pigweed is 73% digestible with a crude protein concentration of more than 20%! So are weeds really the unsung heroes of pastures? I am not sure that I would go that far, but having a few weedy grasses or forbs is not the end of the world and may in some cases even be beneficial.
A great example of a beneficial weed is crabgrass. When I worked at Virginia Tech, I used to say that if it wasn’t for crabgrass and wiregrass (bermudagrass) we would have a lot of hungry cows in Southside VA. Crabgrass is a summer annual grass that is commonly found in closely grazed cool-season pastures during the summer months. Although it is an annual, it acts like a perennial through prolific reseeding. This grass is both highly digestible and palatable. Recent work in Georgia showed that adding crabgrass to pearl millet increased average daily gain by approximately one-third of pound per day.
It is very important to recognize that some weeds are simply NOT palatable and may even be toxic.
Examples of toxic weeds that can be found in pastures include perilla mint, jimsonweed, and poison hemlock. Normally, animals will avoid grazing toxic plants unless forage availability is very low. In cases where toxic or unpalatable weeds are present at high levels in pastures, an application of the proper herbicide at correct the time of the year for the targeted weed species is likely warranted. However, it is important to recognize that herbicides that are really good at killing broadleaf weeds, will also kill or injure desirable forbs and clover.
Next time you are out in your pasture take a few minutes and watch what your cows are grazing. They will let you know what they consider is a weed and what is a forage! ~Dr. Chris Teutsch