Matney assesses impacts of grazing on grass, emphasizing plant growth mechanismsWritten by Saige Albert
Grazing is a widely used practice across the West, and Casey Matney of the University of Alaska-Fairbanks says assessing the impacts of grazing can help producers to ensure their rangelands see good regrowth and are healthy.
“When cattle are grazing, they remove material from the landscape,” he says. “When grass is tall, they grab, on average, three inches above the soil surface in the first bite.”
He continues that they may only eat that much, but it is also possible that they graze the grass all the way to three-quarters of an inch above the ground.
Grazing results in removal of photosynthetic material, reduction of growth and reduction of seed production. Each impact may result in reduced plant performance, depending on the frequency and intensity of the grazing.
To understand the impacts of grazing, Matney says it is important to first understand how plants grow.
“During the first half of leaf growth, a lot of times, plants push all of their carbohydrates into the leaves,” Matney explains. “Most of the energy of the plant is going to the leaves to grow and expand.”
Matney compared leaf growth to installing solar panels.
The growth of leaves allows plants to progress to their next stage, which is maintenance. Through the maintenance phase, plants collect enough energy to sustain themselves.
“When we get past the maintenance point, the rest of the energy goes into seed production, root production, tillering and whatnot,” he says. “The three periods of plant growth are investments going into the leaves, leaves maintaining themselves and leaves maintaining the roots.”
Response to grazing
Grass varieties respond to impacts differently, depending on their growth patterns, Matney says. Golf course grass, for example, has a different morphology in that the grass is bent, growing sideways, making it more tolerant to mowing. Rangeland grasses typically grow up, making them more susceptible to grazing and damage.
“Plants can respond with declining, equal or decreased growth,” Matney says.
He explains that decreased growth may result from any of several factors, including lack of time left in the growing season, removal of growing points, drought, competition, lack of root reserves or other factors.
“Equal growth is the response by a grass plant where there appears to be no increase or decrease in seed or biomass production,” he comments. “Increased growth may be realized where grazing increases plant growth and/or see production.”
Matney continues, “Compensatory factors, such as timing of grazing, amount of material removed by grazing, high water availability, high soil fertility, lack of plant competition, optimum temperatures and other factors contribute to plants being able to have increased growth with grazing.”
To see increased growth in grasses, optimal conditions and multiple growth points are necessary, Matney explains.
“Some grasses have to have leaf material,” he says. “Some send regrowth from the roots.”
Matney also notes that timing is important in removing plant material, based on the growth stages of the plant.
“We want to try and give the grass the opportunity to grow while still grazing it in a pattern so not all plants take all of the impact every year,” he says.
“We have to think about how much we are taking based on when we graze,” he says. “If we take 30 percent of some plants off at the right time, it won't matter how much we take later, because the plant is already dead.”
Matney encourages producers to graze at different times during the year.
“When we ground-level clip the same time every year, we see total reduction for these years,” he says. “This is because we were impacting the grass during stem elongation. When we do that, it reduces the chances for the plant to regrow.”
At the other end of the growing season, Matney says chances for the plant to regrow are better, but the strategy may negatively impact grazing plants.
“We always have to sacrifice somewhere, and we do that by moving our grazing to different pastures,” he continues.
Rainfall and soil nutrients are all important in assessing how much growth is possible, and competition from other plants also affects grass growth.
“If we look at defoliation-based studies and competition between other plants, which grazing scenario produces more forage in the end?” Matney asks. “We can maximize grass growth by not having competition from weeds.”
Frequency of grazing is another important factor to consider.
Matney adds, “If we are allowing the pastures to be grazed once while the plants are in the growth stage, allow it to come back and graze when it is dormant, we are still getting the forage.”
Matney spoke during the 2015 Range Beef Cow Symposium, held in Loveland, Colo. in mid-November.