Charles A. Taylor, Jr., Karen Launchbaugh, Ed Huston and Erika Straka
Juniper infestation of Texas rangelands is an important dilemma because of its impact on forage and livestock production, water yield and quality, wildlife habitats, and rapidly increasing costs of conventional control methods. Fire suppression and long-term overgrazing have caused range deterioration and a significant increase in juniper on millions of acres of Texas rangelands. This increase in juniper has caused a significant reduction in the carrying capacity for both livestock and wildlife as well as a negative effect on the water budget. Many ranches have little grazing space remaining for livestock. Deep percolation of water, essential for replenishing underground aquifers, is halted by dense juniper stands. Wildlife species, including the endangered golden cheeked warbler and the black-capped vireo, are dependent on specific habitats characterized by spacial and structural distributions of both mature and immature trees that facilitate both nesting and feeding. Juniper must be managed to allow for multiple uses of the rangeland to serve the various sections of society claiming actual or philosophical ownership.
For many years Texas’ ranchers have used goats to aid in their efforts to manage brush. Since the 1920’s the Texas A&M University Research Station, located between Sonora and Rocksprings, has been conducting research on the effectiveness of using goats to manipulate vegetation. Results of these studies have revealed that goats spread their grazing pressure more evenly over all kinds of vegetation than do cattle or sheep, which results in light use of grasses (Taylor 1992). This is beneficial for grass vigor and seedling establishment and hence improves the diets of other ruminants. Long-term studies of goat browsing shows dramatic differences between the Research Station grazing treatments, especially with regards to juniper density (Smeins 1990). These studies have shown that goating can significantly reduce juniper seedling recruitment, density and growth rate.
Goats have also compared favorably with all other domestic ruminants in economic production on the Texas A&M Research Station. In a mixed-vegetation complex, their inclusion in the animal mixture has increased efficiency of plant use and helped suppress the growth of brush and extend the life of traditional brush management technologies. Properly managed, the goat is an excellent tool for vegetation manipulation.
This paper will address the unique characteristics of goats, the effects of juniper on goats, the role that goats perform in the management of juniper, and present some new ideas on how to improve the goat’s ability to utilize juniper and hence, its efficiency as a biological control agent for juniper.
Goats Are Ruminants
Goats are ruminants (animals with four chambered gastric region and dependent upon a mixture of bacteria within their rumens to digest forage) but they’re not necessarily the same as other large foraging animals such as cattle, sheep or white-tailed deer. Different species of ruminants exhibit unique grazing and foraging behaviors that often result in large differences in diet composition and nutritional value (Huston 1978). Hofmann (1989) in a review based on detailed comparative morphological studies of all portions of the digestive system of 65 ruminant species from four continents, classified ruminants into a system of three overlapping morphophysio- logical feeding types: concentrate selectors, grass and roughage eaters, and intermediate feeders. Under his classification scheme goats would be grouped as intermediate (opportunistic) feeders while cattle would be classified as grass and roughage eaters and white-tailed deer as concentrate selectors.
The foraging behavior of ruminants is also influenced by the kind of vegetation. Basically, range vegetation is composed of cell contents and cell walls. Cell contents are highly digestible; however, the plant cell wall, composed of cellulose, hemicellulose, and lignin, is much more difficult for ruminants to digest (Van Soest, 1965). As a plant matures, there is a corresponding increase in the proportion of cell walls; therefore, mature forage can limit the rate of food passage through the digestive tract and prevent or restrict consumption of additional forage.
Other components of vegetation that affect its acceptance and digestibility include alkaloids, tannins, and terpenoids, which are collectively referred to as secondary chemicals. Different plant species produce a relatively distinct set of defensive chemicals and these chemical defenses affect different animals in different ways. Juniper is an example of a plant that produces secondary chemicals (i.e., terpenoids) which reduces it palatability and digestibility.
Effect Of Juniper On Goats
Junipers contain monoterpenoid oils which are volitable. These oils are composed of terpene compounds which are five-carbon rings with alcohol, ketone, and hydrogen side groups. (Fig. 1) The kind of side group makes a big difference in the properties of each oil. Terpenoids are chemicals that are produced by the plant and that have no known metabolic use in the plant (not used for growth), other than defense against herbivores that may eat the plant’s leaves, and as an attractant to specific insect pollinators. The volatile oils in juniper give juniper wood many properties that make it a desirable industrial material. Juniper “cedar” posts are used for fencing because the oils make the wood more resistant to insects, bacteria, and fungi. The oils, because of their volatility, give cedar its strong and lasting characteristic smell. Each plant species that contains volatile oils, has a distinct “fingerprint” of oils. This “fingerprint” or oil composition is commonly used to identify different species of plants. For example, blueberry juniper and redberry juniper each has its own individual monoterpene pattern (Fig 2). The terpenoids in juniper affect its taste and a number of the animal’s metabolic processes. Taste is the most important sense used by domestic livestock in diet selection. When determining to eat a plant, the animal smells the plant first for recognition, then takes a bite of it. The experience of tasting juniper, or anything else, is actually a complex electrochemical interaction that is enacted instantly. Taste buds are chemical receptors which send electrical signals to the brain regarding the chemistry of the food tasted. Individual chemicals in juniper can serve as electrochemical triggers to these taste buds. Two chemicals of similar but different structure could cause the animal to experience different tastes. The emetic system is the part of the brain which, when signalled, will evoke an experience of nausea in the animal. It is possible that this is the area that is signalled by the volatile oils, leading the animals to feel ill after eating a certain amount of juniper. Which oils are present and in what proportion probably influence how much juniper is consumed by livestock. An example is that blueberry juniper is eaten more readily by goats than redberry juniper, probably because of the characteristic profiles of volatile oils (Riddle et al. 1996, Straka 1993).
Terpenes from both blueberry and redberry juniper are bacteriocidic (destroys bacteria) (Brattsten 1979). As such, they serve as chemical defense mechanisms to discourage destructive herbivory and encourage survival of the plant. Since goats are ruminants, the terpenes in juniper could potentially kill some of the ruminal bacteria. Possibly, this could reduce digestive efficiency and cause digestive upset if enough juniper were consumed.
In reality, juniper in livestock diets may not cause high levels of bacterial death. Generally juniper is consumed in relatively small quantities; and when it is consumed, the total terpenoid concentration is diluted during repeated chewing and rumination.
Once in the gastrointestinal tract, terpenes are likely absorbed to a small degree through the rumen wall and to a larger degree from the small intestine. Terpenes then enter the blood supply and are transported to the liver where they are detoxified by multifunctional oxidase enzyme systems (MFO’s) (Brattsten 1979). MFO’s act somewhat like antibodies, where their actual synthesis and production is in direct response to contact with the toxin itself. This process acts as a positive feedback system where initial contact with terpenes stimulates or increases the production of more enzymes capable of handling more terpenes. MFO’s add molecular groups that break structural bonds to turn terpenes into more polar, water soluble compounds. This process allows the terpenes to be harmlessly excreted by the animal through its urine.
Optimizing Juniper Intake
Since we know that juniper intake is limited by the presence of terpenoids, we can overcome this limitation through two different management schemes. We can manage juniper to reduce terpenoid concentration in the foliage and/or we can manage goats to increase their tolerance of the terpenoids. In order to decrease production of terpenoids in the trees we need to understand why juniper produces this secondary chemical. Terpenoid production is influenced by the species of juniper, individual tree and site (the kind of soil the tree grows on).
Considering site first, we know that in a nutrient-limited environment, it is not advantageous for juniper to lose nitrogen and carbon that is tied up in its foliage (i.e., plants growing in nitrogen deficient soils appear to produce increased quantities of terpenes; Bryant et al. 1983, Mihaliak and Lincoln 1985). Physiologically the tree protects its resources with these secondary chemicals that keep the foliage from being browsed off.
A promising area of research is measuring monoterpenoid changes in relation to young vs old plants, or to look at the effect of aging on monoterpenoid production and the resulting effects on palatability. Research at the A&M Research Station at Sonora has revealed that monoterpenoid composition for young juniper growth is lower than for old juniper growth and that goats prefer juniper seedlings and regrowth over mature growth. Goats will regularly return to utilize the same juniper trees, harvesting the young regrowth. When the tips are browsed off, regrowth sprouts from lateral buds. This pattern is seen on a larger scale when redberry regrowth sprouts up from a top-killed plant. When these observations were tested in a lab, we found that physiological age of the leaf material influences the amount and kind of monoterpenoids. Young seedlings and sprouts were much lower in monoterpenoids and more palatable than older plants (Fig. 3). There appears to be a threshold after which leaf material becomes significantly less palatable as the juniper foliage ages and monoterpenoid composition increases. This has important management implications. If juniper can be maintained below this threshold with control methods such as fire, consumption by goats can be increased. Combinations of mechanical treatments, fire, and goating are also beneficial in that they keep juniper canopies within reach of goats and help maintain a higher goat:juniper ratio which is critical for effective goating, since goats can’t consume large volumes of juniper.
Our second approach to juniper management is to increase the tolerance of goats to the monoterpenoids. Monoterpenes are thought to deter goat browsing of juniper plants by being toxic or by reducing nutrient assimilation, or by influencing forage selection at subtoxic levels by imposing high detoxification costs post absorption (increases nutrient demand of animals in order to neutralize terpenoids) ( Freeland and Janzen 1974, Guglielmo et al. 1996). Because of this additional demand for nutrients, adequate nutrition is important to meet the demands of detoxification. A protein rather than energy supplement appears to be more beneficial to goats consuming juniper. Feeding goats cotton seed meal and alfalfa as a supplement increased redberry juniper intake 40% compared to goats fed a corn supplement and 30% greater for goats in the control treatment (no supplement) in feeding trials on the Sonora Research Station (Fig. 4). Eventhough juniper intake can be significantly increased by feeding a protein supplement, individual goat consumption is still relatively low (.8 lbs/hd/day maximum intake for an 80 lb goat). For example, if an average 3 foot high juniper tree has 10 pounds of consumable foliage and an 80 pound goat consumes .8 lbs/hd/day then it would take over 12 days for all of the foliage to be consumed. Multiply that by 200 to 300 trees per acre and it would take a number of goats to have a major impact on the juniper. Obviously, early treatment of juniper is critical for effective biological control.
Previous research at the Sonora Research Station has shown that Spanish goats have the potential to consume more juniper than Angora goats (Riddle et al. 1996). Further work in this area has determined that goats crossed with the Ibex breed (wild goats) consume more juniper than Spanish goats. We believe this idea of selecting goats for increased juniper consumption has merit. It’s a new approach to livestock management (i.e., manipulate diet selection through selective breeding). This would create a more “ecologically-friendly” animal and allow land managers to increase the harvest efficiency of noxious plants that cause deterioration of the range resource.
Our approach to increasing the efficiency of goating for juniper management involves decreasing terpenoid concentration in the plant and increasing tolerance of these terpenoids in the animals. Listed below are suggestions that may help you to achieve more effective “goating” for juniper management:
1) We recommend frequent browsing with goats to take advantage of the window of palatability that seedlings and regrowth experience before they cross over the threshold and become less palatable.
2) Don’t overstock your range: Year-round goating does not mean year-round overgoating. Use of desirable browse plants should be monitored.
3) Increase grazing pressure (concentration of goats) on target pastures in the winter. Hit the juniper hard when goats will most likely consume it and harm to other plant species can be minimized (i.e., when the warm-season grasses are dormant). Also, initiate a close monitoring program for early detection of juniper seed germination and seedling emergence. Concentrate goats on the young seedlings to attack the juniper when it is in its most vulnerable life stage and lowest in terpenoids.
4) Provide a high-quality protein supple-ment. Adequate nutrition is important to meet the demands of detoxification.
5) Goating alone will generally not solve a juniper problem. A sustainable juniper management system must be developed for each situation. This may include a combination of treatments (i.e., mech-anical, fire, chemical and goating). Goating should be viewed as one component of the overall juniper management system. Goating is a unique management tool in that it can directly generate income in the short term to help pay for the other methods and to extend the effective treatment life of the more expensive, conventional control methods.
6) Finally, goating will be most effective where juniper density and biomass are very low because of the limited intake of juniper leaves.
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Comments: Allan McGinty, Professor and Extension Wildlife Specialist