Mar. 13, 2009 The greater duration of heat in a summer-prescribed burn provides more effective management of encroaching woody or cactus species on rangeland, a Texas AgriLife Research scientist said.
Controlling encroachments of prickly pear, mesquite, juniper and other rangeland plants that compete with grass can be pretty expensive without the use of fire in controlled burn situations, said Dr. Jim Ansley, AgriLife Research range management expert.
"The longer period of high temperature in a summer burn is what helps control the prickly pear the best," Ansley said.
Summer burns are normally scheduled in August and September, just before the rainy season.
"Use of summer burning as a management tool is still something being explored and is on the edge of being an accepted practice," Ansley said. "The question is whether we can adopt it as a safe and manageable practice."
Summer fires are not necessarily hotter than winter burns, but they produce a much longer duration of high heat, he said, so if they get out of control, it can be very hard on fire-fighting crews. Also, the fatigue factor is much higher for the fire fighters working in the heat of the summer.
"But we're finding some biological evidence that they are more effective," Ansley said.
Once a prickly pear motte (collection of pads) grows to 4- to 5-feet in diameter, the inner core will have no grass at all, he said. This not only creates a void in the pasture for grazing animals, but it also leaves less grass fuel that is needed to carry a controlled fire.
"There is nothing to ignite and kill the interior pads," Ansley said. "You can get to the point where you can't get enough heat from a winter fire to penetrate those mottes. But a summer fire can overcome this because the preheating and duration of high heat is so much greater."
He said some prickly pear is valuable in a pasture; it serves as a protective area for quail nesting and other wildlife. But for livestock managers, it has mostly negative effects. For example, a 30 percent canopy cover of prickly pear basically removes that ground area from grass growth, leaving only 70 percent of the land where grass even has a chance to grow.
Ansley and other researchers are conducting a study on the R.A. Brown Ranch near Throckmorton where they are chaining the pasture first to knock down the mesquite and cactus and following up with a summer fire.
While he hasn't looked at the economics, Ansley said they can already see a much more productive stand of grass on the chained/fire treatment than the burn-only treatment or the untreated controls.
The chaining increases grass growth because it reduces competition for water by the mesquite, he said. Chaining in February and March followed by a fire within a year is recommended in most cases, with the exception of heavy juniper-covered areas.
The juniper would be too volatile and combustible immediately after chaining, Ansley said. About four years are needed for the juniper leaves to fall off the branches and make the fire controllable.
"Chaining followed by fire appears to be extremely effective," he said. "If you've chained an area that has a lot of prickly pear, you need to have burning in your long-term plan. Otherwise the pads are spread and will put down roots and establish more mottes."
Mottes grow rapidly, Ansley said. A motte about 2 square feet expanded to about 10 square feet in only three years.
"That's a frightening rate of growth for those small mottes, especially if you've chained and spread the pads. It could take over a pasture very quickly," he said.
"We found that the growth rate will slow after a motte gets larger. Medium mottes, about 15 square feet to start grew to 30 square feet in the same three-year period, and large mottes, about 100 square feet, did not get much bigger."
Studies so far have found that summer fire usually delays grass regrowth, taking about a year or two to get back to pre-burn levels, he said.
With a winter fire, grass returns to pre-burn levels by the end of the first growing season in August, while a summer fire it might take two growing seasons before similar recovery occurs, Ansley said.
"But that's not saying it can't be grazed prior to that," he said. "One just needs to be very careful the first two years after a summer fire and not overgraze."
Ansley said planning is a must with prescribed burns, and there are several things to keep in mind when considering a summer controlled burn. The first caveat he gave was that no inexperienced person should try a summer prescribed burn first.
"First start with winter burns working with some experienced people and then work up to a small-scale summer burn," he said.
Also, having pre-burned black lines on downwind sides of the pasture is a must for summer fires, Ansley said. The black lines can be burned up to a month in advance of the prescribed burn under cooler temperatures and higher humidity.
Any grass that comes back during that time should be green and won't fuel the fire.
Another important concept is that summer fires may be applied on smaller burn units, in the hundreds of acres instead of the thousands of acres, just to increase the safety and control, Ansley said.
He said the idea of summer fires still needs to be run through the various prescribed burn associations around the state and agencies like the U.S. Department of Agriculture's Natural Resources Conservation Service to get more people thinking about it as a management tool and to help develop safe burning prescriptions.
"We know they can be applied safely, and we are beginning to gather evidence that there are a lot of ecological benefits to summer fire, but it is still in the experimental stage," Ansley said.
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