Polyethylene glycol influences selection of foraging location by sheep consuming quebracho tannin

Seasonal regulation of condensed tannin consumption by free-ranging goats in a semi-arid savanna

Although condensed tannins (CTs) are known to reduce forage intake by mammalian herbivores in controlled experiments, few studies have tested these effects in the field. Thus the role of CTs on foraging ecology of free-ranging herbivores is inadequately understood. To investigate the effects of CTs under natural savanna conditions, we pre-dosed groups of goats with polyethylene glycol (PEG, a CT-neutralising chemical), CT powder or water before observing their foraging behaviour. While accounting for the effects of season and time of the day, we tested the hypothesis that herbivores forage in ways that reduce the intake rate (g DM per minute) of CTs. We expected pre-dosing goats with CTs to reduce CT intake rates by (1) consuming diets low in CTs, (2) reducing bite rates, (3) increasing the number of foraging bouts, or (4) reducing the length of foraging bouts. Lastly, (5) expected CT to have no influence the number of dietary forage species. In both wet and dry seasons, pre-dosing goats with CTs resulted in lower CT consumption rates compared to PEG goats which seemed relieved from the stress associated with CT consumption. During dry season, the number of dietary forage species was similar across treatments, although goats that were dosed with PEG significantly increased this number in the wet season. Dosing goats with PEG increased the number and length of browsing bouts compared to goats from the other treatments. Pre-loading goats with PEG also tended to increase bite rates on browse forages, which contributed to increased consumption rates of CTs. Based on the behavioural adjustments made by goats in this study and within the constraints imposed by chemical complexity in savanna systems, we concluded that herbivores under natural conditions foraged in ways that minimised CTs consumption. More research should further elucidate the mechanism through which CTs regulated feeding behaviour.

Rumen distension and contraction influence feed preference by sheep

Distension of the rumen limits feed intake by livestock. Ruminal dysfunctions due to bloat, which causes distension by accumulation of excessive gas within the rumen, also reduce feeding. We hypothesized that excessive levels of rumen distension cause feed aversions and that preference increases for feeds eaten in association with recovery from bloat. To test these hypotheses, we determined whether 12 commercial crossbred lambs (average initial BW of 43 +/- 2 kg) could associate ingestion of specific feeds with the consequences of increased intraruminal pressure and its subsidence. Six of the lambs were fitted with rumen cannulas and offered ground alfalfa for 30 min after a rubber balloon was inserted into the rumen of each animal and distended with air to volumes of 1.8, 2.5, or 4.5 L. Subsequently, balloons were deflated and alfalfa was offered again for a second period of 30 min. Feed intake was not affected when the balloon was not distended (P = 0.45 to 0.93), but distension reduced feed intake (P < 0.001) in direct proportion to the magnitude of distension at all 3 volumes (R(2) = 0.70). Relief from distension promoted a compensatory increase in feed intake (P = 0.006). During conditioning to determine if lambs acquired a preference for a feed associated with recovery from distension, fistulated lambs were offered novel feeds: wheat bran (group 1; n = 3) and beet pulp (group 2; n = 3), and the balloon was distended for 30 min. Feeds were then switched and the balloons were deflated (recovery). Control lambs (n = 6) received the same feeding protocol without the balloons. Lambs formed strong aversions to feeds associated with distension and preferred feeds associated with recovery (P = 0.001 to P = 0.10). No preferences or avoidances were observed in control lambs conditioned without rumen distension (P = 0.17 to P = 0.87). Thus, rumen distension and recovery from distension induced feed aversions and preferences, respectively, which may be critical in learning avoidance of bloat-inducing plants and preferences for plants and supplements that relieve the incidence of bloat.

Plant secondary metabolites: antiparasitic effects and their role in ruminant production systems

The purported antiparasitic properties of plant secondary metabolites (PSM) have been the cause of controversy amongst the scientific community. Despite long-standing knowledge of the prophylactic and therapeutic properties of PSM-rich extracts, which comes mainly from ethnoveterinary sources, the scientific evidence of the antiparasitic effects of PSM is inconsistent. In the first part of the present paper the causes of this controversy are addressed, and the evidence available on the antiparasitic effects of PSM is critically examined. The focus is on examples of the antiparasitic activity of PSM against helminth nematodes. The conclusion is that PSM can have antiparasitic properties, which depend on their structure, level of ingestion and availability within the gastrointestinal tract of the animal. The second part is an appraisal of the potential role of PSM for parasite control in ruminant production systems. Despite their antiparasitic properties, PSM consumption does not necessarily have positive consequences in parasitised herbivores, as excessive consumption of PSM can adversely affect herbivore fitness and survival, through their anti-nutritional properties. For this reason it is suggested that the antiparasitic properties of PSM should be assessed at the same time as their anti-nutritional effects. The same measure, e.g. the performance of parasitised hosts, should be used when assessing these properties. The assessment of the costs and benefits suggests that parasitised herbivores can benefit from the long-term consumption of PSM only if the antiparasitic benefits outweigh the anti-nutritional costs of PSM. In addition, it is proposed that parasitised animals might be able to benefit from PSM consumption even if their performance is impaired, as long as the latter is a short-term compromise that leads to long-term benefits.

Behavioral strategies of mammal herbivores against plant secondary metabolites: the avoidance-tolerance continuum

We review the evidence for behavioral avoidance of plant secondary metabolites (PSMs) and identify how, and the circumstances under which it occurs. Behavioral strategies of avoidance of PSM can only be fully understood in relation to the underlying physiological processes or constraints. There is considerable evidence that animals learn to avoid PSMs on the basis of negative postingestive effects. The extent to which this process determines foraging choices is limited by the ability of animals to experience the consequences of their behaviors and associate particular cues in foods with their specific effects in the body. The proposed learning mechanisms require that animals must at least "sample" plants that contain PSMs. They do not completely avoid PSMs, but there is evidence that they restrict their ingestion to within limits that they are physiologically able to tolerate, and that the amounts of PSM ingested result from a balance between toxicological considerations and the nutrient content of the plant material. These limits are influenced by the kinetics of PSM elimination, which underlies patterns of bite and patch selection from plant parts to landscapes. We suggest that altering spatial location of feeding (to alternative food patches or alternative foods within patches, including plant parts), and temporal distribution of feeding activity, by either cessation of feeding or by continuing to feed, but on alternative foods, can both lead to reduction of the intake and toxic effects of PSMs. We propose that the strategy of avoidance or reduction of intake of PSMs coevolved with the animal's ability to physiologically tolerate their ingestion, and that avoidance and tolerance are inversely related (the avoidance-tolerance continuum). The animals' propensity and ability to seek alternatives also vary with the dispersion of their food resources. Further work is required to test these proposals and integrate temporal and spatial aspects of foraging behavior and its nutritional consequences in relation to PSMs.

Plant Volatile Compounds: Sensory Cues for Health and Nutritional Value?

Plants produce many volatile metabolites. A small subset of these compounds is sensed by animals and humans, and the volatile profiles are defining elements of the distinct flavors of individual foods. Flavor volatiles are derived from an array of nutrients, including amino acids, fatty acids, and carotenoids. In tomato, almost all of the important flavor-related volatiles are derived from essential nutrients. The predominance of volatiles derived from essential nutrients and health-promoting compounds suggests that these volatiles provide important information about the nutritional makeup of foods. Evidence supporting a relation between volatile perception and nutrient or health value will be reviewed.

Can animals use foraging behaviour to combat parasites?

Host-parasite interactions are often seen as an arms race, with parasites attempting to overcome host resistance to infection. Herbivory is a common route of transmission of parasites that represents the most pervasive challenge to mammalian growth and reproduction. The present paper reviews the foraging skills of mammalian herbivores in relation to their ability to exploit plant properties to combat parasites. The starting point is that foraging behaviour may ameliorate the impact of parasitism in three ways; hosts could: (1) avoid foraging in areas contaminated with parasites; (2) select diets which increase their resistance to parasites; (3) select for foods containing anti-parasitic properties (self-medication). Details are given of the pre-requisite skills needed by herbivores if they are to combat parasitism via behaviour, i.e. herbivores are able to: (a) determine their parasitic state and alter their behaviour in relation to that state (behaviours 1, 2 and 3); (b) determine the environmental distribution of parasites (behaviour 1); (c) distinguish plant species or plant parts that increase their resistance to parasites (behaviour 2) or have anti-parasitic properties (behaviour 3). Mammalian herbivores cannot detect the presence of the parasites themselves and must rely on cues such as faeces. Despite the use of these cues contacting parasites may be inevitable and so mechanisms to combat parasitism are necessary. Mammalian herbivores have the foraging skills needed to exploit the heterogeneous distributions of nutrients and parasites in complex foraging environments in order to avoid, and increase their resistance to, parasites. Current evidence for the use of plant secondary metabolites (PSM) by herbivores for self-medication purposes remains equivocal. PSM have both positive (anti-parasitic) and negative (toxic) effects on herbivores. Here details are given of an experimental approach using tri-trophic (plant-herbivore-parasite) interactions that could be used to demonstrate self-medication in animals. There is strong evidence suggesting that herbivore hosts have developed the foraging skills needed to take advantage of plant properties to combat parasites and thus use behaviour as a weapon in the host-parasite arms race.