Influence of nutrients, toxins, and grazing strategies on the utilization of one-seed juniper by small ruminants

Targeted grazing with small ruminants is being used increasingly in weed control programs on rangelands of the western United States. Application of targeted grazing prescriptions to control one-seed juniper (Juniperus monosperma Engl. Sarg.) saplings in New Mexico has been limited by the lack of knowledge about the interactions between the chemical ecology of juniper saplings, and the feeding behavior of small ruminants. This dissertation addresses a number of basic aspects of plant-herbivore interactions in one-seed juniper which are critical to developing effective grazing strategies to control its encroachment.;First, the essential oil profile of one-seed juniper leaves in relation to sapling size and plant material handling protocols was determined (Chapter 2). Next, the influence of protein supplements and plant secondary metabolites on seasonal changes in voluntary intake of one-seed juniper in goats and sheep was investigated (Chapter 3). Later, the relationship between the ingestion of juniper, protein supplements, condensed tannins, and polyethylene glycol (PEG) and rumen fermentation parameters and plasma amino acid levels was studied to determine potential modes of action of juniper toxins (Chapter 4). Then, the knowledge acquired in controlled experiments described in Chapters 2-4 was used to conduct a grazing study at the Corona Ranch, to determine the influence of mixed species grazing and two stocking densities on one-seed juniper sapling voluntary intake levels of goats (Chapter 5). Finally, the area that had received targeted grazing treatments was monitored to document subsequent use of grazed patches by cattle and mule deer (Chapter 6).;Fifty one mono- and sesqui-terpenes were identified in leaves of one-seed juniper saplings. The controlled pen studies reported in Chapters 3 and 4 showed that concentrations of total phenolics, condensed tannins (CT), and terpenes co-varied seasonally, and affected seasonal patterns of voluntary intake of goats and sheep. Greater juniper intake was observed in goats than in sheep, and in animals fed supplements with rumen-degradable and undegradable protein. Concentrations of volatile fatty acids in the rumen rose after initial intake of juniper and dropped with addition of CT and PEG. A number of plasma AA declined when animals were fed juniper, or juniper + CT, regardless of protein and PEG supplements.;The Corona Ranch grazing study described in Chapter 5 showed that highest utilization of juniper saplings was achieved in plots grazed with goats or goats + sheep at high stocking density. Utilization of herbaceous understory was highest in plots that were grazed with a low stocking density of goats + sheep. Summer grazing was associated with greater depletion of leaves and twigs of small saplings while spring grazing favored utilization of bark on taller saplings. Monitoring of subsequent use of the targeted grazed area, described in Chapter 6, showed that the time cows spent in the treated area was 2.8 times higher than that of alternative random sites in the pasture. Presence of mule deer in grazed patches was negatively related with cattle presence. When cattle were absent, mule deer selected small treated patches, whereas cattle selected large grazed patches that deer avoided.;The results of this dissertation suggest that targeted grazing with sheep and goats could be used to synergize current one-seed juniper control methods used in New Mexico. Manipulation of season of grazing, supplement programs with mixed sources of protein, livestock species, and stocking density could boost juniper utilization and induce levels of herbivory likely to suppress juniper sapling growth. In addition, targeted grazing prescriptions could also be used to improve cattle and mule deer habitat.