• Chemically mediated interactions between woody plants and browsing mammals

      Bryant, J. P.; Reichardt, P. B.; Clausen, T. P. (Society for Range Management, 1992-01-01)
      A diverse array of secondary metabolites deters feeding by mammals on woody plants. However, not all secondary metabolites are equally deterrent and the potencies of these substances as antifeedants is related to their structures. Although the physiological reason underlying deterrence by secondary metabolites is not well understood, the available evidence indicates that toxicity is more important than digestion inhibition. Resource limitation influences the production of secondary metabolites by woody plants. Species that are adapted to unproductive habitats are more chemically defended than species that are adapted to productive habitats. Resource limitation also affects the phenotypic expression of chemical defense with nutrient stress favoring increased production of carbon-based secondary metabolites and reduced production of nitrogen-containing secondary metabolites. Light stress has the opposite effects on the production of these substances. Herbivory by mammals also affects the chemical defenses of woody plants. In some cases browsing results in increased defense and in others decreased defense. Three circumstances under which browsing by mammals can change the chemical defenses of woody plants are discussed.
    • Composition and production of California oak savanna seasonally grazed by sheep

      Bartolome, J. W.; McClaran, M. P. (Society for Range Management, 1992-01-01)
      Seasonal grazing trials, conducted over 3 years at the Hopland Field Station in Mendocino County, Calif., tested the effects of 2 seasonal grazing strategies on within- and between-year production and composition in blue oak (Quercus douglasii H.A.) savanna understory and adjacent open annual grassland. Moderate intensity summer-fall-winter and spring-summer sheep use had few within-year effects. In contrast, production and composition varied considerably between years in both treatments. Forbs (especially legumes) decreased in open grassland and oak understory between years within both seasonal grazing regimes. This change could not have been caused by selective grazing because there were no corresponding within-year patterns. Instead, between-year changes are more likely related to nonselective effects of stocking rate and/or weather. Results from this study suggest that seasonal grazing systems offer little potential for improvement of annual range composition.
    • Continued food aversion: Training livestock to avoid eating poisonous plants

      Ralphs, M. H. (Society for Range Management, 1992-01-01)
      Animals can be trained to avoid eating specific foods by offering them the food and subsequently administering an emetic to induce nausea. The animal associates the taste of the food with the induced illness and subsequently avoids eating that food. Conditioned food aversion (CFA) is a potential tool to prevent livestock poisoning from palatable and abundant poisonous plants. Cattle have been trained to avoid eating tall larkspur (Delphinium barbeyi L. Huth), a particularly troublesome poisonous plant. However, several factors influence the acquisition and retention of food aversions under field grazing conditions. The age and sex of an animal may influence its ability to form and retain aversions. Novelty of the plant and the intensity of the induced illness determine the strength of the aversion. Social facilitation or peer pressure motivates animals to sample the averted food, and the aversion will extinguish if it is not reinforced. Generalizing the aversion created under controlled conditions in a pen, to a complex vegetation community in the field, may be difftcult for some animals. If these obstacles can be overcome, CFA may be an effective tool to reduce the risk of poisoning on poisonous plant infested rangeland.
    • Diurnal measurements of honey mesquite transpiration using stem flow gauges

      Dugas, W. A.; Heuer, M. L.; Mayeux, H. S. (Society for Range Management, 1992-01-01)
      Honey mesquite (Prosopis glandulosa Torr. var. glandulosa) stem flow was measured on days with contrasting environmental conditions during the 1989 growing season in Texas. Midday stem flow varied from near 10 to about 300 g hour-1 and daily totals varied from about 1,000 to 2,000 g day-1. On days with low potential evaporation, regardless of precipitation totals for the previous 20 days, stem flow mirrored potential evaporation. On a day with high potential evaporation and high precipitation totals for the previous 20 days, stem flow was greatest and the diurnal pattern was similar to that of potential evaporation. On a day with high potential evaporation and little precipitation for the previous 20 days, stem flow mirrored potential evaporation until about 1030 and decreased throughout the day, while potential evaporation remained high. Variability of stem flow between stems was large, with a C.V. of about 30% for midday rates and a seasonal average C.V. of 37% for daily rates. Stem flow gauges provide continuous and accurate measurements of honey mesquite transpiration. They respond to changing environmental conditions and are useful for evaluating short-term responses of stem flow to physiological and environmental factors in the field and glasshouse.
    • Germination responses of Lehmann lovegrass to light

      Roundy, B. A.; Taylorson, R. B.; Sumrall, L. B. (Society for Range Management, 1992-01-01)
      Lebmann lovegrass (Eragrostis lehmanniana Nees.) is a perennial, warm-season bunchgrass that is native to South Africa and has been seeded and spread naturally in the southwestern United States. Germination of 4 seed lots of varying age was tested in relation to darkness and irradiance with red (R) and far-red (FR) light. Germination was low in continual darkness, but greatly increased after exposure to R. Irradiation with FR after exposure to R reduced germination, confirming phytochrome involvement. Exposure to R after prolonged imbibition in FR did not increase germination of 1-2-year-old seeds and only slightly increased germination of older seeds. An alternating temperature of 16 hours at 15 degrees C and 8 hours at 38 degrees C greatly increased germination of seeds exposed to fluorescent light and slightly increased germination of seeds in darkness compared to a constant temperature of 25 degrees C. Greater seedling emergence of Lehmann lovegrass when the canopy is opened by burning, mowing, or grazing is likely a function of red light stimulation of biologically active phytochrome and increased seedbed temperature fluctuations.
    • High seed retention of Indian ricegrass PI 478833

      Jones, T. A.; Nielson, D. C. (Society for Range Management, 1992-01-01)
      Reduction of Indian ricegrass [Oryzopsis hymenoides (Roem. and Schult.) Ricker] seed shattering losses is of interest because it would result in more economical seed harvest. PI 478833, a genotype with glumes closely surrounding the seed, was compared with 'Paloma' for glume pair angle and seed retention parameters. Our objective was to determine its suitability as a source of seed shattering resistance for genetic transfer to an Indian ricegrass variety. Regrowth from a 19 June 1989 clipping of 'Paloma' and PI 478833 with and without supplemental irrigation was evaluated in late July. Irrigation did not affect glume pair angle or percentage intact seed (number of seeds/number of mature florets). Though Paloma and PI 478833 glume pair angles averaged 66% and 44%, respectively, percentage intact seed was 13% higher for Paloma than PI 478833 because Paloma's more indeterminate flowering pattern allowed it to replace its shattered florets more quickly than PI 478833. In a second experiment we observed individual florets of Paloma and PI 478833 for 7 weeks starting 5 September 1989. Floret opening occurred mostly during daylight hours and on warm days. Shattering events occurred when a storm followed a period of floret opening. Duration of seed retention from glume opening to shattering averaged 91% longer for PI 478833 than Paloma. After 7 weeks 83 and 35% of Paloma and PI 478833 florets had shattered, respectively. In a third experiment seed retention index (seed yield/forage dry weight) on 15 September 1989 was 0.45, 0.19, and 0.11 for PI 478833, 'Nezpar', and Paloma, respectively. PI 478833's acute glume pair angle and resultant increased seed retention make it a valuable source of shattering resistance for introduction into bred Indian ricegrass varieties.
    • Impact of plant toxins on fetal and neonatal development: A review

      Panter, K. E.; Keeler, R. F.; James, L. F.; Bunch, T. D. (Society for Range Management, 1992-01-01)
      Many poisonous plants grazed by livestock on ranges and pastures in the western USA are fetotoxic causing fetrl malformations, embryonic or fetal death, abortion, or early parturition. Decreased incidence of plant-induced livestock malformations may be accomplished through grazing management strategies. To develop these strategies one must understand some basic principles of toxicology and teratology such as susceptible livestock species, type of compound and concentration in the plant and its disposition in the animal, dose response, and the susceptible gestational period. Much of this information is known for certain plants; however, additional information will enhance our ability to control livestock losses from these plants. Certain criteria may be established to maximize grazing management methods to minimize teratogenic effects of poisonous plants. When the suspect plant grows in a restricted habitat, poses a hazard only at certain growth stages or when the susceptible period of pregnancy is relatively short, minor adjustments in management methods can be considerably successful in reducing incidence of malformations and subsequent financial loss.
    • Impact of poisonous plant on the livestock industry

      James, L. F.; Nielsen, D. B.; Panter, K. E. (Society for Range Management, 1992-01-01)
      Livestock poisoning by plants is one of the serious causes of economic loss to the livestock industry. Losses can be classified as either direct or indirect. Direct losses include deaths, weight loss, abortions, lengthened calving intervals, decreased efficiency and other effects on the animals. Losses from death and some reproductive losses in the 17 western states are estimated at 340,000,000. In addition to these are the indirect losses such as fencing, herding, supplemental feeding, medical costs, management alterations, and loss of forage which are associated with efforts to prevent or minimize poisoning of livestock by plants. Nearly all plant communities include poisonous plants, thus, most grazing animals are exposed to intoxication. However, the presence of these plants does not cause poisoning. Poisoning is usually associated with management errors, lack of forage due to range conditions, drought, and other events that would cause Livestock to consume vegetation normally unacceptable. Often a sequence of events, such as storm, frost, cold, and other occurrences can influence an animal to where it will eat too much of a toxic plant too fast.
    • Larkspur chemistry: Toxic alkaloids in tall larkspurs

      Manners, G. D.; Pfister, J. A.; Ralphs, M. H.; Panter, K. E.; Olsen, J. D. (Society for Range Management, 1992-01-01)
      Three species of tall larkspur (Delphinium barbeyi (Huth), Delphinium occidentale (Wats.) Wats, and Delphinium glaucescens) that are toxic to cattle were chemically analyzed to determine “total alkaloid” content. D. barbeyi and D. occidentale contained more “total alkaloids” than D. glaucescens. The “total alkaloid” content of all plant tissues in the 3 species declined as the growing season progressed. Variation in the occurrence of specific diterpenoid alkaloids was established by gas chromatographic analysis of D. barbeyi plant tissues at different phenological growth stages. Highest yields of specific alkaloids were found in early growth stage plant tissues. Deltaline was the most prominent diterpenoid alkaloid in D. barbeyi and 14-0-acetyldictyocarpine is a new diterpenoid alkaloid with high occurrence in this plant. The toxicity of specific diterpenoid alkaloids obtained from the tall larkspurs evaluated in a mouse bioassay showed methyllcaconitine to be highly toxic. Other diterpenoid alkaloids isolated from the 3 Iarkspurs showed much lower levels of toxicity compared to methyllycaconitine
    • Measuring cottonseed hull passage by white-tailed deer

      Schultz, S. R.; Johnson, M. K. (Society for Range Management, 1992-01-01)
      Cottonseed hull fragments were identified in feces collected for the first 14 of 17 days after ingestion of 1 g cottonseed hulls by each of 4 adult nude white-tailed deer (Odocoileus virginianus). Acclimation periods for deer forage evaluation trials should extend at least 14 days prior to food item evaluations to ensure passage of previously consumed fibrous plant materials.
    • Mechanisms of learning in diet selection with reference to phytotoxicosis in herbivores

      Provenza, F. D.; Pfister, J. A.; Cheney, C. D. (Society for Range Management, 1992-01-01)
      Our objective is to develop explanations for why herbivores ingest poisonous plants by first discussing how herbivores learn to select diets, by then considering mechanisms that enable herbivores to ingest phytotoxins, and by finally developing hypotheses about why herbivores overingest phytotoxins. Animals learn about foods through 2 interrelated systems: affective and cognitive. The affective system integrates the taste of food and its postingestive feedback; this system causes changes in the intake of food items, depending on whether the postingestive feedback is aversive or positive. The cognitive system integrates the odor and sight of food and its taste; animals use the senses of smell and sight to select or avoid specific food items. We further divide cognitive experiences into 3 categories: learning from mother, learning from conspecifics, and learning through trial and error. Physiological and conditional responses enhance the ability of animals to ingest phytotoxins. Physiological mechanisms include binding the compound before it can exert its action, metabolizing the compound so it cannot exert its action, and tolerating the compound. Conditional responses complement physiological responses and further decrease herbivore susceptibility to toxins by preparing the animal for the effects of the toxin. Herbivores are likely to overingest poisonous plants when any of the aforementioned systems fail. For example, the affective system is likely to fail when phytotoxins circumvent the emetic system, when aversive postingestive consequences are delayed temporally and positive consequences during the delay are pronounced, and when toxicosis is accompanied by a change in environmental context. Likewise, cognitive systems are likely to fail when animals are unable to distinguish subtle molecular changes that render nontoxic plants toxic, when toxins in 2 or more plants interact, and when herbivores are unable to differentiate nutritious from toxic plants as a result of being placed in an unfamiliar environment. We conclude that a thorough understanding of affective and cognitive systems, and the specific conditions under which these systems fail, will be necessary in order to understand why herbivores ingest foods that do them harm.
    • Metabolism and absorption of toxic glycosides by ruminants

      Majak, W. (Society for Range Management, 1992-01-01)
      The cyanogenic glycoside prunasin is the toxic component in a number of browse species (serviceberry, Amelanchier alnifolia, and chokecherry, Pnmus virginiana); and glycosides of 3-nitropropanol (NPOH), such as miserotoxin, are the poisonous principle in a number of Astragalus species such as timber milkvetch, A. miser var. serotinus. Hydrolysis of the glycosidic bond in rumen contents is the first step in the bioactivation of the glycosides. Diet influences populations of rumen microorganisms and diet may induce the proliferation of bacteria that function in glycoside hydrolysis and detoxification. The absorption of NPOH from the reticulo-rumen was examined in cattle on alfalfa hay and corn silage diets. 3- nitropropionic acid (NPA), the lethal metabolite of NPOH, was detected in both plasma and urine. The plasma levels of NPA were reduced when the diet enhanced the rate of NPOH detoxification in the rumen. The enhancement was achieved with a feed supplement containing nitroethane, a synthetic analogue of NPOH that is much less toxic than the natural toxin. The high levels of NPA in urine (>30 ppm) suggested a procedure for detecting livestock poisoning by nitro-bearing plants. The absorption of hydrogen cyanide (HCN) from the reticulo-rumen was also examined in cattle given sublethal doses of prunasin from serviceberry. Metabolites of HCN in blood and plasma were detected at low levels (<5 ppm) during the initial 6-hour sampling period. High levels of thiocyanate (<20 ppm), a metabolite of HCN, were detected in urine samples collected at 24-48 hours and this also suggested a diagnostic procedure for detecting HCN poisoning in cattle.
    • Oak seedling establishment on California rangelands

      Adams, T. E.; Sands, P. B.; Weitkamp, W. H.; McDougald, N. K. (Society for Range Management, 1992-01-01)
      Factors responsible for poor recruitment of blue oak (Quercus douglasii H. & A.) and valley oak (Q. lobata Nee) need to be determined on California hardwood rangelands so that management strategies for enhancement of recruitment can be developed. To examine selected factors, exclusive of large herbivore impacts, a series of acorn seeding experiments was initiated in 1985 in 6 counties on representative sites. At each site, the experimental treatments were the factorial combination of herbs vs. no herbs and screen protection vs. no protection. The experimental design was 4 randomized complete blocks of the herb treatments with each main plot split for the 2 levels of protection. Rainfall and edaphic factors were used to help interpret measures of seedling emergence, survival, and growth. Herbaceous plant control for reduction of moisture stress was the most important factor examined. Emergence was significantly improved by control in nearly 80% of blue oak seedings and in 33% of valley oak seedings. Average first year survival, expressed as a percent of acorns sown, was significantly improved with control in seedings of both blue oak (33% vs. 18%) and valley oak (45% vs. 21%). Limited data suggest the difference in survival remains consistent over time as overall survival declines. With few exceptions, the addition of screen protection discouraged predation and significantly enhanced survival and growth. Window screen cages also may have contributed an unmeasured shade effect.
    • Plant toxins and palatability to herbivores

      Molyneux, R. J.; Ralphs, M. H. (Society for Range Management, 1992-01-01)
      A complex relationship exists between the presence of toxins in a plant species and the palatability of that plant. The nature of the toxin and its concentration within the plant can generally be precisely defined, given a reasonable amount of research commitment, but the measurement of palatability, especially in livestock, is much more difficult to achieve. We hypothesize that analysis of possible roles of toxins in plants, their metabolic activity in animals, and physical and temporal distribution within the plant can be used to examine whether or not such compounds may significantly increase or reduce palatability to mammalian herbivores. Thus, if the toxin is effective in preventing predation of the plant or plant part by insect herbivores, or if it provides the plant with a competitive advantage versus other species, but does not produce adverse effects upon large mammals until significant quantities of biomass are consumed, then the toxin-palatability relationship is not significant. This concept is illustrated by examination of the toxicity produced in livestock by consumption of alkaloid-containing groundsel (Senecio) and locoweed (Astragalus and Oxytropis) species. The prevention of predation by localization of the toxin, mobilization to the site of attack, or production at a particular stage of growth provides opportunities for the application of management techniques designed to reduce exposure of livestock to natural plant toxicants.
    • Reducing livestock losses from poisonous plants through grazing management

      Taylor, C. A.; Ralphs, M. H. (Society for Range Management, 1992-01-01)
      Stocking rate, multi-species grazing, and grazing systems are 3 components of grazing management that can be manipulated to minimize losses in animal production due to consumption of poisonous plants. Our study evaluated 3 case studies where either all or some of the above components of grazing management were the experimental treatments. For study 1 the grazing treatments included 3 rates of stocking; a 4-pasture, 3-herd grazing system; and combinations of different kinds of livestock that were measured for 21 years. For study 2 the grazing treatments included 2 rates of stocking, 4 different grazing systems, and combinations of either all sheep or a ratio of 3:2 cattle to sheep (au equivalents) for 11 years. Study 3 measured cattle poisoned by locoweed prior to and following the implementation of a 3-herd, 4-pasture grazing system over 6 years. Sheep death losses to bitterweed (Hymenoxys odorufa DC.) poisoning occurred in 13 of the 21 years on continuously grazed pastures heavily stocked with sheep and only 8 years under both moderate and light stocking rates. Regardless of the stocking rate, death losses were greatest on pastures stocked with sheep only and least with the combination of livestock species on conjunction with a 4-pasture, 3-herd grazing system. Stocking rate, multi-species grazing, or grazing system seemed to have little effect on goat losses due to oak (Quercus spp.) consumption. Cattle and sheep were not affected by sacahuista (Nolina texauo Wats.) in this study; however, their inclusion in the grazing herd reduced goat death losses from 5% with goats only to 2.5 and 1.5% for combinations of cattle and goats and cattle, sheep, and goats, respectively. In study 2 sheep death losses from bitterweed poisoning under continuous yearlong grazing treatments averaged 5.2% vs 3.7% for grazing treatments with some type of grazing system. Death losses were greatest under yearlong continuous grazing stocked at 10.4 ha/auy with 100% sheep and least under yearlong continuous grazing stocked at 15.2 ha/auy with 4% sheep. In study 3 the number of sick calves declined from 20% to about 3% with the implementation of a new grazing system. The reduction in sickness and loss was attributed to the reduction in grazing pressure and the shorter grazing season. It is concluded that for these case studies tactical management decisions such as proper stocking rate, combinations of animal species to be grazed, and grazing system used played an important role in minimizing livestock death losses to poisonous plants.
    • Tannin chemistry in relation to digestion

      Hagerman, A. E.; Robbins, C. T.; Weerasuriya, Y.; Wilson, T. C.; McArthur, C. (Society for Range Management, 1992-01-01)
      Tannins are a diverse group of compounds which precipitate protein. The impact of tannins on herbivory has been difficult to assess because of diversity in tannin chemistry and in animal physiology. We have evaluated the effects of tannin on large ruminants (deer, sheep) using artificial diets containing well-defined tannins, and have compared the results to those obtained with natural forages. The different effects of condensed tannins and gallotannins on herbivores are related to the chemical stability of the tannins. Commercial tannic acid does not have the same effects on herbivores as gallotannins in natural forages. Molecular weight apparently determines the metabolic fate of gallotannins from various sources.
    • Toxification and detoxification of plant compounds by ruminants: An overview

      Smith, G. S. (Society for Range Management, 1992-01-01)
      Improved usage of rangelands for livestock production requires better ways to reduce losses caused by poisonous plants, such as management practices to minimize ingestion and treatments to improve animal tolerance of ingested poisonous plants. In ruminants, gastrointestinal microbes can detoxify plant compounds, and this capacity has been enhanced in a few cases by deliberate modification of rumen microbial populations. Some plants are poisonous because ingested plant material is made toxic by microbial fermentation in the rumen, and better understanding of such toxifications will provide opportunities to diminish poisonings of that type. Absorption of toxic substances from the gastrointestinal tract into blood and lymph may be modified by feeding binding agents such as clay, resins, and indigestible fibers, or by pharmaceuticals that interfere with absorption of toxicants. Agents that induce or inhibit biotransformational enzymes in tissues of the host animal might modify animal tolerance of some plant toxicants. Provision of substances that serve as co-substrates of detoxification can enhance animal tolerance of other types of plant toxicants. Some reports that illustrate these approaches have been reviewed, and questions have been raised to stimulate further research.