Controlling one-seed juniper saplings with small ruminants: What we’ve learned

Authors: Estell, Richard - Rick; CIBILS, ANDRES - New Mexico State University; UTSUMI, S - Michigan State University; STRICKLAN, D - New Mexico State University; BUTLER, E - Brigham Young University - Idaho; FISH, A - Brigham Young University - Idaho; GANGULI, A - New Mexico State University

Submitted to: Rangelands
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/1/2018
Publication Date: 10/1/2018
Citation: Estell, R.E., Cibils, A.F., Utsumi, S.A., Stricklan, D., Butler, E.M., Fish, A.I., Ganguli, A.C. 2018. Controlling one-seed juniper saplings with small ruminants: What we’ve learned. Rangelands. 40:129-135. https://doi.org/10.1016/j.rala.2018.07.002.
DOI: https://doi.org/10.1016/j.rala.2018.07.002

Interpretive Summary: Shrub encroachment onto rangelands causes management issues for ranchers and land managers in the western United States. One-seed juniper has invaded millions of acres in the southwest. Targeted grazing by small ruminants may have potential to reduce the spread of juniper and other woody plants. However, plant secondary metabolites (PSM) may limit the intake of encroaching species. A variety of supplements, feed additives, and management practices have been used in an attempt to overcome the negative effects of PSM on intake by browsing ruminants. A series of studies was conducted to examine methods to increase juniper intake and the effectiveness of targeted browsing on juniper. Season, sapling size, animal species, stocking density, nutrient supplementation, and feed additives were evaluated in terms of their relationship with juniper intake and sapling mortality. Key findings from these studies were that protein supplementation and polyethylene glycol (PEG) both increased juniper intake by sheep and goats (particularly goats) in all seasons but fall. The highest concentration of PSM in juniper occurred during the fall. Goats and sheep had reduced concentrations of amino acids in the bloodstream when consuming juniper, which were partially restored by protein and/or PEG supplementation. Terpene concentrations varied with season and sapling size, and were inversely related to juniper intake. The highest level of browsing occurred on small saplings during summer, while the highest amount of branch debarking occurred on taller saplings during spring. Two years later, sapling mortality after browsing was about 5% and branch mortality from debarking ranged from 13-22%. Ten years later, juniper kill ranged from 5-14%, and saplings exposed to browsing averaged 13 cm shorter than controls. Strategies to implement targeted browsing of juniper have a greater chance of success if aligned with periods when PSM are lowest.

Technical Abstract: Shrub encroachment onto rangelands causes management challenges for ranchers and land managers throughout the western United States. One-seed juniper (Juniperus monosperma) dominates millions of hectares in the southwestern United States. Juniper removal methods have been used to encourage transition back to savanna grasslands, but sapling reinvasion hinders these practices. Targeted grazing by small ruminants may reduce the reinvasion of woody plants by decreasing seedling establishment. However, plant secondary metabolites (PSM) in shrubs may reduce the intake of woody species such as one-seed juniper. A variety of supplements, feed additives, and management practices have been explored to overcome the negative effects of PSM on intake by small ruminants. A series of studies was conducted to evaluate methods to increase juniper intake and to examine conditions in which targeting juniper use is most effective. Season, sapling size, animal species, stocking density, nutrient supplementation, and feed additives were examined with respect to juniper intake and sapling mortality. The objective of this synthesis is to 1) summarize key findings and make recommendations for increasing one-seed juniper browsing by sheep and goats, and 2) to reevaluate juniper mortality 10 years after the original studies were completed. Cumulatively, phenolics and terpenes represented 8-10% of needle dry matter, and concentrations peaked in the fall and were lowest in the summer. In pen studies, goats consumed about 2.5-fold more juniper than sheep. Overall, goats and sheep consumed about 40-60% less juniper in the fall when PSM concentration was greatest than during the other three seasons. Protein supplementation nearly doubled juniper intake by goats in all seasons except fall, while sheep increased intake by about 50% when supplemented. Basal diet intake decreased when animals were fed juniper and was partially restored when polyethylene glycol (PEG) was fed. Juniper intake doubled from 10 to 20% of the total diet when PEG was fed. Juniper consumption reduced the plasma concentration of several amino acids. Some of these depleted amino acids were partially restored by feeding protein supplements and/or PEG. In a grazing study, goats spent 3-fold more time browsing juniper than sheep (~8 vs. 25%), and goats spent more time browsing under high-density stocking than low-density stocking. Short saplings were browsed more during summer, while tall saplings were used more in spring. In general, sapling size and amount of herbivory were inversely related, with more than 60% of short saplings in the heavy use category vs. approximately 45% of tall and medium saplings. Across treatments, degree of juniper defoliation was inversely related to terpene concentration. Two years after the grazing study was conducted, sapling mortality averaged about 5% and was not affected by treatment. Ten years the completion of the study, sapling mortality was higher (P < 0.05) in targeted grazing plots vs. controls and ranged from approximately 5% for goats in the high-density treatment to 14% in the mixed-species high-density treatment.