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  4. 1997
  5. February

Grass Burs

Posted Feb. 1, 1997

Why are there so many grass burs in the bermudagrass pastures this year? What can be done to inhibit the grass burs this spring? These two questions were posed by many producers this past fall. Not only is this a good time to ponder these questions, it is also the right time of year to develop a plan of action.

This past year, our area observed one of the most abnormal rainfall patterns in a long while. We had drought conditions throughout the spring followed by the best early fall precipitation on record. Bermudagrass fertilized in the spring never reached its potential production because of the lack of moisture. Subsequently, much of the soil that was usually covered with a thick mat of bermudagrass going into the summer remained exposed.

Grazing pastures were short and the hay fields were thin. This made for ideal conditions for the opportunistic grass bur seed, which are always present and awaiting moisture for germination. When the rains came in late summer the pastures greened up. However, the bermudagrass was past its peak production period and had already used the fertilizer applied in the spring. The exposed grass burs sprouted and rapidly produced seeds, thus insuring a good supply in the seedbank for this spring.

There is something we can do to inhibit grass burs: apply Ammonium Nitrate + Herbivory. Unfortunately, this is not a new herbicide that is as effective as Atrazine was when it could be used on bermudagrass. (Atrazine is a restricted use herbicide and is not labeled for bermudagrass.) Ammonium Nitrate + Herbivory, or nitrogen fertilizer + grazing, simply means to fertilize the bermudagrass in order to out-grow grass bur plants and flash-graze the grass bur plants periodically to keep them in a vegetative state for as long as possible.

Cattle do not usually prefer bermudagrass if there is something else available. On the other hand, grass bur plants are highly preferred by cattle until they become reproductive. By repeatedly grazing the grass bur plants through the early spring, we are increasing the competitive advantage of the bermudagrass, especially if it's well fertilized. The key is to allow the cattle access to the bermudagrass pasture only long enough to graze the grass bur plants.

This could be only a couple of hours or less flash graze and then repeat every five to seven days until the bermudagrass has the opportunity to overtake the majority of the grass bur plants in the pasture. Although nitrogen (N) stimulates the growth and production aspects we associate with fertilizer, it is important to know the phosphorus (P) and potassium (K) levels of the soil.

For the greatest efficiency in nitrogen utilization, there are minimal levels of P and K that should be maintained. It is also important to know the pH of soil. Bermudagrass can grow in a wide range of pH's but optimal range is between 6.0 and 7.0. If the pH falls below this range (becomes acidic), commercial lime can be applied to effectively elevate the pH.

On bermudagrass pastures, lime should be applied during the fall and winter seasons to allow it time to become incorporated into the soil before the spring application of fertilizer. It would be advantageous to consult your soils specialist for nutrient guidelines for your specific soils.

So, if you had a healthy crop of unwanted grass burs this fall and do not want a repeat performance this spring, you might have the soil in your bermudagrass pastures analyzed very soon, and prepare a grazing plan for the coming spring. As always, feel free to contact me or another forage specialist if you have questions.

We are always glad to help.