9 Matching Annotations
  1. Apr 2021
    1. Animals possess several mechanisms to negate or restrict the toxic or negative effects of plant compounds once i ngested. If a t oxin is eaten, it is in the animal's best interest to quickly get r id of it. Sheep, goats, a nd cattle can and will vomit in response to eating toxins, but it is rarely observed. Horses probably do not vomit except when near death, but commonly experience diarrhea. Diarrhea aids in rapid elimina- tion of toxins from the gut which can reduce absorption. In some episodes of diarrhea, there is a decrease in intestinal motility, further reducing t he absorption of toxins.
    2. With continued consumption of a plant containing a specific phytotoxin, the animal may gain an ability to overcome its negative effects because enzyme systems in animal tissue can increase their detoxification capacity and efficiency. Rumen microbes may also facilitate the ability of animals to adapt to diets high in phytotoxins. Microbial populations can change rapidly depending on th e substrates available fo r degradation. These "inducible defenses" could explain why herbivores often appear less sensitive to toxic or low quality plants with continued exposure. Nonetheless, adaptation does not develop to all toxins. The effects of many toxins are cumulative and animals may get progressively more poisoned as they continue to ingest plant material containing these toxins
    3. Once plant toxins are absorbed fr om the gut into the blood, they are often trans- ported to the liver. T he liver primarily, and secondarily the kidney, intestinal mucosa, lungs, a nd skin contain enzyme systems that metabolize or alter toxic compounds, rendering them inert. Ability to metabolize or reduce sensitivity to specific phytotoxins varies by herbivore species a nd individuals. For example, sheep can tolerate and detoxify more pyrrolizidine alkaloids than cattle, therefore it takes five times more tall larkspur (Delphinium occidentalis) to poison sheep compared to cattle
    4. Chemical reactions during ingestion may provide protection against the effects of some plant toxins. The ruminant's large forestomach is generally well adapted to bind, sequester, degrade, or detoxify plant toxins. The neutral p H of the rumen environment may modify a plant toxin or the toxin may be quickly diluted in the large volume of the rumen (e.g., ar ound 60 gallons for cattle). Of great significance for ingesting toxic plants is the massive number of rumen microbes that transform most phytotoxins into inert or less-detrimental compounds. For example, leucaena (Leucaena leucocephala) is a tropical forage legume that contains mimosine, a toxic amino acid. Mimosine i s detoxified by a group of rumen microbes and animals susceptible to mimosine toxicity can be cured by receiving a dose of the "mimosine- metabolizing" microbes. Rumen microbes usually reduce the toxic effects of plant compounds. However, in some cases, such as nitrates or cyanogenic glycosides, the rumen microbes convert a harmless compound into a deadly toxin.
    1. This idea that plants become desirable or aversive depending on their digestive consequences is simple. But, how do grazing animals figure out exactly which plants made them fee l good or ill? One way herbivores apparently accomplish this task is by regarding unfamiliar plants with caution. Animals associate positive or negative effects of nutrients or toxins with novel foods when offered meals that contain novel and familiar foods. When foraging bouts include several novel plants, plants that dominate the diet are probably 'weighted' more than less-consumed plants, even if the minor foods were primarily responsible for the positive or negative feedback. Furthermore, digestive feedback begins within 10 to 15 minutes of consumption which could help animals attribute digestive benefits or liabilities to specific plants. Finally, livestock grazing on rangelands usually become familiar with the forage resource and may seldom encounter truly novel plants. This allows greater opportunity to 'sort out' feedback from individual or similar groups of plants
    2. When a grazing animal smells and tastes a plant, the flavor is either pleasing or distasteful depending on the animal's previous grazing experiences. When a plant is eaten, it provides feedback during digestion. If consumption of a plant improves the nutrient or energy status of the animal, the plant flavor becomes more desirable or pleasing. If eating of the plant yields illness, the flavor becomes aversive and distasteful (Fig. 2). These flavor-consequence relationships form the basis for dietary likes and dislikes, and the animal then seeks highly palatable foods and avoids aversive foods. The resulting behavior patterns generally lead to increased consump- tion of nutritious foods and limited consumption of toxic or low quality plants.
    3. The digestive consequences of forage consumption are determined by plant forage quality and animal digestive and detoxification abilities. This interaction, in turn, affects the nutrients and energy available for animal growth and maintenance.
    1. Successfully navigating the seasonal and spatial variation of forage quality in grazing environments can be accomplished by knowing how much to eat, when to eat, and what else to eat. Grazing animals have a strong natural tendency to select diets composedof several plant species and sample available plants on a regular basis. This behavior may increase the likelihood of ingesting necessary nutrients and reduce the potential of over-ingesting toxins. The toxic effects of a plant are determined largely bythe amount eaten, but the ingestion rate may also be important. Grazing animals can avoid toxicosis by limiting their consumption of a specific toxic plant each day to allow sufficient time for detoxification, and to limit potential cumulative effects ofspecifictoxins
    2. When an animal eats a plant, it receives digestive feedback in the form of energy, nutrients, illness, or toxicosis. If the feedback is positive, preferences are formed to the plant and if the feedback is negative, aversions are formed. The strength of the preference or aversion is determined by the magnitude, nature, and timing of digestive feedback