Browsing Journal of Range Management, Volume 45, Number 1 (January 1992) by Title
Now showing items 19-22 of 22
Plant toxins and palatability to herbivoresA 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 managementStocking 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 digestionTannins 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 overviewImproved 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.