Across the Western United States, grasslands that ranchers, their livestock, and wildlife all depend on are disappearing. This important ecosystem is being shaded out and out-competed for resources by juniper forest, which can support far fewer cattle and less biodiversity than grasslands. Currently people reclaim land by cutting down the trees or setting prescribed fires, but these methods can be expensive or risky. But, what if we could entice the livestock already grazing there to eat juniper? Like getting a child to eat his or her vegetables, could we get these animals to eat a plant they don’t normally like?
Juniper slowly encroaching on rangeland originally dominated by grasses.
Junipers, like many plants, produce compounds to discourage animals from eating them. In the case of junipers, the main class of defensive compounds are terpenes. Terpenes are a family of similar compounds that are responsible for the aroma of juniper and can be toxic to animals. However, there are some animals that are better able to break down these toxic compounds and use the tree as a food source.
A research team co-led by Andres Cibils at New Mexico State University and Rick Estell at The Jornada Experimental Range has found that sheep and goats, under the right conditions and circumstances, will graze and damage the junipers while also gaining valuable nutrients from them.
A goat grazing on the tender branches of a young juniper tree.
To identify these optimal conditions, the researchers first allowed sheep and goats to graze in an enclosed area that included live juniper trees. Some of the trees were grazed heavily by the animals, while others weren’t at all. They then took samples from the trees the animals enjoyed the most and the least, and analyzed the chemical compounds within them. They found that it wasn’t any one particular compound that the animals avoided, instead they avoided trees with the highest total concentrations of terpenes. And they found that goats, famous for eating just about anything, did the majority of the damage to the trees.
They also identified the factors that lead to a juniper having higher and lower concentrations of terpenes. They found that small junipers have lower concentrations than tall, which is unusual since many plants have higher concentrations when young as an additional defense since they are more vulnerable. Junipers also have the highest concentration in fall, when the animals would avoid eating them altogether. The lowest concentrations occurred in the summer, and the best grazing happened on the smallest plants (less than .5 m tall), which led to many of the trees dying or having their growth stunted. Spring was the next best time to graze when, due to higher concentrations, the animals would eat mostly the bark on taller trees, resulting in less of the trees dying off, but much more dead branches.
Juniper tree loved by goats, most likely due to
relatively low concentrations of terpene.
Juniper tree avoided by grazing animals, probably
due to high terpene concentrations.
When the animals consume juniper, the terpenes are broken down by enzymes in the liver. As a result, the limiting factor in how much juniper the animals will eat depends on how quickly the terpenes can be broken down. The researchers gave the animals supplements of protein and polyethylene glycol (a non-toxic compound used in many medicines for people), and found that it allowed them to better metabolize terpenes and thereby eat more juniper. This was especially true for the goats.
So, it is possible to use livestock to eat the unwanted junipers at a time when they are most vulnerable by targeting use of seedlings and small saplings, thereby potentially reducing establishment and slowing further encroachment. Prescription grazing techniques like this can help ranchers out by using juniper as a food source for their animals, rather than it cost time and money to remove juniper. This is especially important now that over a hundred million acres of the Western US are covered in juniper. Although these studies have only just begun and on a small scale, it is a potential new tool for ranchers and land managers in helping restore the grassland ecosystems, and help out those who depend on the land economically.
A special thanks to Rick Estell of The Jornada Experimental Range, Las Cruces, NM, for the information.
- Johnny Ramirez
"Juniper Dunes" Bureau of Land Management Oregon, (CC BY 2.0)
"Grazing Goats" "Grazed Junipers", https://jornada.nmsu.edu/files/Estell_ASAS-2013.pdf, USDA ARS The Jornada Experimental Range