Existing feed evaluation systems for ruminants assess the feed value in a rather empirical way, with a limited ability to integrate metabolism in a meaningful framework. For the quantitative description of the mechanisms, appropriate biological data can be obtained using in vitro methods. The aim of this paper is to examine the use of modelling and in vitro data to predict digestion processes in vivo. Suitable mathematical methods are required to describe and interpret substrate disappearance profiles or gas production profiles. The derivation of such models is important since this allows a clear definition of the underlying assumptions made. Such assumptions are related to the change in fractional rate of degradation (kd) during incubation that will determine the shape of the profile. Furthermore, the value of the fractional passage rate (kp) is of crucial importance in the prediction of extent of degradation in the rumen. The development and application of models, based on classic microbial growth equations, clearly shows that observed variation in microbial efficiency in batch cultures (including the gas production technique) is not necessarily related to that in vivo. Rather, kp is again a major factor contributing to explanation of variation in microbial efficiency. Similarly, the end products of fermentation (VFA) and the VFA molar proportions can be estimated in vitro, but its direct applicability to the in vivo situation is limited. It is concluded that some potential uses of in vitro techniques are ultimately misleading. Mechanistic models indicate that mechanisms governing microbial efficiency and VFA molar proportions in vitro are not necessarily valid for the in vivo situation. Therefore, the in vitro data cannot be used directly for a uniform system of feed evaluation to predict animal responses. Rather, the in vitro data obtained for substrate degradation may be used in whole rumen models as a basal input value to indicate the degradation potential.

The three most important factors describing the nutritive value of ruminant feeds are: feed intake, rumen fermentation, and enzymatic digestion. Animal experiments are usually expensive and time-consuming, so many in vitro techniques have been developed which are supposed to assist in the prediction of nutritive value, usually of rumen fermentation. One of these is the measurement of gas production from the substrate when incubated with a specific inoculum. The cumulative gas production technique measures gas at regular intervals in time, as an indicator of the kinetics of fermentation. The resultant cumulative gas profiles, fitted to specific equations, and in combination with other end-products (i.e. VFA and NH3) can indicate both the rate and extent of fermentability of the test substrates. This information can be used to compare substrates with each other. However, the fact that the technique has not yet been standardized, and that inoculum varies according to time and place, means that comparison both between runs and locations, can be very difficult. Also, there are a number of aspects of the technique which will always mean that its resemblance to the in vivo situation must be limited. Some of these limitations and other important considerations of the technique are described in this review.

In the study of the feeding behaviour and nutrition of free-ranging mammalian herbivores, determining what the animals are eating, its quality and quantity can be difficult to accomplish. The measurement processes themselves may disturb the animals’ normal foraging behaviours which can be a major problem in rangeland, forest and other semi-natural environments. Furthermore, animals are likely to select mixtures of plants and their components which differ from the available vegetation. Quantitative measures of diet composition, digestibility, faecal output and intake in individual grazing or browsing animals have depended on the use of faecal markers. These are materials measurable in faeces that originate from the diet (internal markers), or are absent from the diet, but administered by oral dosing (external markers). The ‘ideal’ faecal marker needs complete recovery in faeces, simple and accurate quantitative measurement, inertness in having no effect on the animal or its diet, and similar physical characteristics (in terms of particle size and density) to the digestive tract contents. No individual material or chemical entity has been found which fulfils all of the ‘ideal’ marker attributes. For example, lignin, indigestible acid-detergent fibre (IADF) and ‘chromogen’, have been used as internal markers, but since they are not discrete compounds, analytical methods are empirical., resulting in inconsistent faecal recoveries.

Virtually all higher plants have an outer surface layer of wax, which is usually a complex mixture of aliphatic lipid compounds whose composition differs between plant types, and different parts of the same plant. Plant waxes can be analysed as discrete compounds, are relatively inert, and because the patterns of individual compounds tend to differ between plant species, they offer the potential of enabling measurement of the contributions of specific plant species to the diet. Leaves and floral parts tend to have the highest concentrations; roots have very low levels. The main classes of plant waxes are straight and branched chain alkanes, alkenes, long–chain fatty acids and esters, long–chain fatty alcohols; long–chain fatty aldehydes and ketones and b–diketones. Analysis is usually carried out by a stepwise process of solvent extraction, purification and gas chromatography (GC). Straight–chain alkanes (n–alkanes) have been the most commonly used marker to date, being present as mixtures with chain lengths ranging from 21 to 37 carbon atoms. Over 90% of n–alkanes have odd–numbered carbon chains, with C29, C31 and C33 alkanes being dominant in most pasture species. Recovery in faeces of plant waxes is high but not complete, and is related to chain length, the longer the chain, the higher the recovery. Correction factors have been measured in a number of herbivore species.

The first application of plant-wax n-alkanes as faecal markers was to determine herbage digestibility in ruminants. Subsequently it was realised that dosed synthetic alkanes could be used to determine faecal output, and hence dosed and herbage alkanes could be concurrently used to estimate intake. This offered substantial advantages over other methods. Furthermore, differences between plant species and parts in their patterns of individual alkanes can be exploited to enable quantitative determination of diet composition from the patterns found in faeces. Since differences in the relative faecal recoveries of individual markers could modify the faecal marker pattern, recovery corrections may be necessary. This approach was first used for measuring the composition of simple dietary mixtures and the intake of dietary supplements to be determined. The use of long-chain fatty alcohols and fatty acids as additional markers offers the potential for more complex diets to be evaluated. The fact that plant-wax alkanes remain attached to particulate dietary residues throughout the ruminant gut, means that they are also good markers for determining the rate of passage of material along the digestive tract

Estimates of digestibility and faecal output obtained from respective natural and dosed n-alkanes will be biased, unless corrections are made to account for incomplete faecal recoveries. However, intake estimates will be unbiased if the faecal recoveries of the two markers are the same. Studies in sheep, cattle and goats have shown plant C33 and dosed C32 alkanes to have very similar faecal recoveries and thus give unbiased estimates of herbage intake. The alkane method for estimating intake offers advantages over other techniques. It gives individual-animal intakes and can be used where animals are receiving feed supplements. Also, GC analysis allows both plant and dosed markers to be determined at the same time, which limits analytical time, error and bias. Since the ratio of the concentrations in faeces is used, it is not necessary to obtain absolute faecal concentrations.

Alkenes and branched-chain alkanes have been investigated as additional markers for diet composition estimation since they can be quantified in the same GC analysis as n-alkanes. Although alkenes, which tend to be associated with floral plant parts, have low recoveries (25-40%), they can be useful diet composition markers since their recoveries are little affected by chain length. Faecal recoveries of the branched-chain alkanes, from Agrostis capillaris herbage, were slightly lower (60-65%) than the respective n-alkanes (C30 and C32) of equivalent carbon number (85-90%). These alkanes are rare in forage species, and their practical usefulness as markers for quantitative composition estimation has yet to be tested.

Long-chain fatty alcohols have been shown to be effective diet composition markers. In most plants fatty alcohol concentrations are higher than those of hydrocarbons, and there can be profound differences in composition between species. They may be of particular value for diets containing plants with low alkane concentrations. Faecal recoveries in sheep, like n-alkanes, increase progressively with chain length from about 60% to 90%. It has been shown experimentally that the use of alcohols, together with n-alkanes, is likely to give a better estimate of diet composition in a given situation than n-alkanes alone.

The very long-chain fatty acids of plant cuticular wax (C20-C34), originally suggested as digestibility markers, may also have potential as diet composition markers. Like n-alkanes and long-chain fatty alcohols, the faecal recoveries of plant-wax fatty acids in sheep increase with carbon chain length. Comparisons with n-alkanes and fatty alcohols suggested that the fatty acids were inferior as diet composition markers. This may have been due to the fact that the fatty acid extracts analysed by GC were relatively impure, containing a number of unidentified compounds. The reliability of plant-wax fatty acids as markers may be improved with more effective analytical procedures.

There are a number of ways of calculating the diet composition from marker patterns in the faeces and potential dietary components. A simple approach is to determine a solution from a matrix of simultaneous equations; the number of dietary components must equal the number of markers used. Because, for simple dietary mixtures, there may be more available markers than dietary components, difficulties may arise in making the best choice of marker. Least-squares optimisation methods allow the number of markers to exceed the number of diet components, and thus (in theory) make better use of available information.

Since the concept of using faecal marker patterns for making quantitative diet composition estimates is relatively new, the associated mathematical and statistical procedures used to date have been rather crude and simplistic. There is potential to make more effective use of the marker data by using more sophisticated computational approaches. These include a range of multivariate techniques, including: a) Principal component and discriminant analysis; b) The weighting of the contribution of different markers, since with the leastsquares optimisation procedure, markers with the highest overall concentrations contribute most to the composition estimate, even though some markers with low concentrations may have large relative differences between dietary components. It would logical to weight markers in favour of those having the greatest relative variation across dietary components, and those providing the least compositional information could be weighted against; c) Statistical procedures are needed to evaluate the quality of diet composition estimates. The minimisation procedures described earlier take no account of any within-component variation in marker composition, and the effect of such variation on the quality of resultant diet composition estimate is not known. Attempts are being made to develop statistical procedures which will provide details of confidence intervals of compositional estimates resulting from particular plant species mixtures of known within- and between-species variability in marker composition.

For reliable estimation of digestibility, intake and diet composition, the feed sample must be representative, with respect to its plant wax marker concentration, of the material ingested by the animals under investigation, not necessarily all of the material present. Since marker concentrations can differ for different plant parts and plant species, care must be taken in sampling the vegetation for analysis. Although oesophageal-fistulated animals have been used to collect samples of ingested vegetation, hand-plucked grass samples have been found to be adequate for uniform grass swards,. In heterogeneous vegetation environments, especially when browse species are present, herbivores are likely to be highly selective. It is thus wise to make preliminary observations of animals’ ingestive behaviours, so appropriate parts of the dietary plants are sampled.

The use of plant waxes, initially with alkanes, as markers in the study of the diets of domestic ruminants is finding increasing use in other herbivores, both domesticated and wild (moose, fallow deer, mountain hares, pigs, rabbits, horses, donkeys, giraffes). Such methodologies have been applied in non-mammalian herbivores, including birds (pigeons, and ostriches) and reptiles (tortoises). Synthetic alkanes have been used as markers to estimate digestibility and intake in fish. Although tests have not yet been carried out, plant wax marker methods may even be applicable to non-vertebrate herbivores, such as caterpillars, slugs and snails. Expansion of the technique to include plant wax compounds other than alkanes will broaden the spectrum of animals and systems which can and could be studied. Alkanes can be used to estimate the botanical composition of plant mixtures, including mixed root mats, and since these compounds can remain in soil for a considerable period, they may also be used to describe the vegetation history of an area by analysis of soil strata. There may be potential for this approach to be extended into archaeological and forensic studies. Insects and spiders contain hydrocarbons (mainly branched-chain alkanes) in their cuticular wax, and preliminary tests have indicated that these compounds are recoverable in the faeces of bats and insect-eating birds; thus they could be used to determine the insect species composition of the diets of insectivores. Analysis of these waxes is relatively simple, and although good GC equipment is required, there is scope for laboratories which do not posses the equipment to make the initial extractions and purification (simple but laborious), with the final GC analysis undertaken by a collaborating laboratory. Thus there are many uses of plant and animal waxes as markers, and potential applications are probably limited only by our imagination.

Diets and habitats that allow animals to select among alternatives enable individuals to better meet needs for nutrients and to better cope with toxins. All plants contain toxins, and the amount of toxin an animal can ingest depends on the kinds and amounts of nutrients and toxins in the forages on offer. Nutrients and toxins both cause animals to satiate, and excesses of nutrients, nutrient imbalances, and toxins all limit food intake. Thus, individuals can better meet their needs for nutrients and regulate their intake of toxins when offered a variety of foods that differ in nutrients and toxins than when constrained to a single food, even if the food is “nutritionally balanced.” Food intake and preference also depend on differences in how individual animals are built morphologically and how they function physiologically, and marked variation is common even among closely related animals in needs for nutrients and abilities to cope with toxins. Transient food aversions compound the inefficiency of single–food diets – whether in confinement, on pastures, or on rangelands – by depressing intake among individual animals, even if they are suited “on average” to that nutrient or toxin profile. Thus, feeding and grazing practices that allow producers to capitalize on the individuality of animals are likely to improve performance of the herd by enabling the uniqueness of individuals to become manifest. Finally, past experiences play a crucial role in an animal's propensity to learn to eat different foods. When herbivores are allowed to eat only the most preferred plants, they are not likely to learn to mix foods high in nutrients with foods that contain toxins. Conversely, herbivores encouraged to eat all plants in an area are more likely to learn to eat mixes of plants that mitigate toxicity. Experienced animals that have learned to eat a variety of foods that differ in nutrients and toxins do so even when nutritious alternatives are available, whereas naive animals familiar only with the nutritious alternatives eat only that subset of familiar foods. Different systems of management alter how animals forage. Continuous grazing at low stock densities encourages selectivity and reduces diet and habitat breadth, whereas short-duration grazing at high stock densities increases diet and habitat breadth. Thus, what was traditionally considered proper grazing management – rotational grazing at low stock densities – may have trained generations of livestock to “eat the best and leave the rest” thus inadvertently accelerating a decline in biodiversity and an increase in the abundance of less desirable plant species.

Plant secondary metabolites (PSM) are widely known for their antinutritional properties; their excessive consumption can detrimentally affect herbivore health and, in some cases, survival. However, a growing body of evidence is suggesting that some classes of PSM may also have some positive effects on herbivores. Such positive effects include the antiparasitic properties of certain PSM and their beneficial consequences on the herbivore's fitness. One of the better-exploited classes of PSM that have been reported with anthelmintic properties is that of the condensed tannins. The consumption of moderate concentrations of condensed tannins has resulted in reduced level of parasitism in sheep and other ruminants. In this paper we discuss the conditions that should be met in order for the positive anthelmintic properties of PSM, and condensed tannins in particular, to be beneficial for parasitised sheep. We also examine whether PSM could have a role in controlling parasitism in ruminants and point out the future research needed to achieve an efficient use of PSM for parasite control.

Infections of the gastrointestinal tract with nematodes remains one of the main pathological constraints associated with outdoor breeding of goats both in temperate and tropical areas. The usual mode of control of these parasitic infections is based on the repeated use of chemical anthelmintics, but the prevalence and diffusion of anthelmintic resistances in nematode populations is particularly high in goats, and the sustainability of the system is now questioned. Therefore, there is a deep interest to seek and explore alternative solutions to chemotherapy. Among those, tannins and tanniferous plants represent one of the most promising options. However, information on the effects of these polyphenolic compounds on nematode infection in goats is far less abundant than in sheep. This review presents data obtained from both outdoor and indoor studies aiming at examining the relationship between the consumption of tannins or tanniferous plants, particularly those composing the vegetation of rangeland and the resistance and resilience of goats to trichostrongyle infections. Results from in vitro studies with extracts from different plants which are grazed or browsed by goats are also discussed in relation with the possible mechanisms of action of tannin in goats. These results are compared to those acquired in sheep and are discussed in relation with the physiological adaptation of goats to tannin consumption. Further research perspectives and possible applications for the control of nematode infections in goats are also briefly evoked.

The paper reviews the scientific information concerning the utilisation of tannin-rich forages by livestock and examines methods to improve it. Findings on the impact of tannins on diet selection, forage intake and livestock performance are presented. The paper elaborates on the application of polyethylene glycol (PEG) in mitigating the adverse effects of tannins and the potential improvement in livestock production. The use of PEG as part of a regular management scheme, including self-regulation of its consumption by the animals, is discussed based on the findings of controlled and field experiments. Topics for future research on relationships between livestock-forage-PEGenvironment-economy are also defined

During grazing, animals interact with many compounds in the diet. Plant secondary metabolites (PSM) such as tannins, saponins, cyanogenic glycosides, mimosine. could have wide ranging effects –beneficial or harmful, depending on the nature, quantity in the plant, and amount and rate of ingestion of the PSM. A wide range of methods is available to quantify these PSM. The methods presented for tannins are based on the oxidation-reduction, metal complexing, polyethylene glycol binding and protein precipitation/binding principles of tannins. A tannin bioassay based on in vitro gas production using a medium containing rumen microbes, and incubation of the plant sample with and without polyethylene glycol (a tannin-inactivating agent) is also discussed. Saponins have haemolitic activity, and a qualitative and a quantitative assay are based on this property are described, in addition to a spectrophotometric assay. Cyanogenic glycosides are measured using picric acid methods with and without distillation. The methods described for mimosine determination are based on its reaction with ferric chloride and diazotised p-nitroaniline. Alkaloids are a chemically heterogeneous group of compounds. Therefore, it is difficult to assay alkaloids using a single method. An approach, for qualitative assessment of alkaloids in feeds and forages, based on the use of various spray reagents on the developed TLC plate or a paper chromatographic paper is presented.

Many browse plants that grow in northeastern Mexico are consumed by domestic and wild range small ruminants. However, their nutritional value is determined by their chemical composition and digestibility. Chemical composition depends on the nature of the plant, but climatic conditions determine leaf maturity. Thus, studies to estimate the seasonality of the nutrient profiles of forages from shrubs are necessary. This paper has the objective to discuss the nutrient profiles and the organic matter digestion from the forage of 32 browse plants (13 legumes and 19 non legumes), that grow in northeastern México, and which are consumed by range small ruminants. Browses such as Acacia berlandieri (L.) Wild Acacia rigidula Benth., Ziziphus obtusifolia T and G., Desmanthus virgathus L. and Cercidium macrum I.M. Johnst. are the most abundant in the range, and also are the most selected by ruminants. The seasonal crude protein (CP) content in browse species varied within a range of 4.0 to 22.0%. The cell wall (from 24 to 53%) and its components: cellulose (from 5 to 31%) and hemicellulose (from 2 to 28%) also varied, and in most evaluated shrubs were lower than Medicago sativa hay. Seasonal lignin content also varied among plants species (from 1 to 24%; annual mean), and was higher than Medicago sativa hay. Condensed tannins were low and seasonally variable in most plants (from 0 to 23%). In general, during winter OMD (ranging from 30 to 68%) was higher than in other seasons. Lignin and condensed tannins negatively affected OMD (r = -0.40; P<0.01 and r = -0.53; P<0.001, respectively). The most selected shrubs were those with the highest tannin content. All plants were low in P and Na relative to the needs of adult goats, sheep and white-tailed deer. It is concluded that the chemical composition and OMD of browse plants were very similar among seasons. Because of their high CP, low cell wall, high mineral content and OMD in all plants, these could be considered as good protein supplements for range small grazing ruminants. However, they must be supplemented with P and Na throughout the year.

The objective was to highlight the role of supplementary feeding on resilience and resistance of browsing small ruminants against natural gastrointestinal nematode (GIN) infections. GIN infections have a negative impact on productivity of browsing Criollo kids especially during the wet season of Yucátan. Epidemiological studies have demonstrated the threat of GIN infectivity in the native vegetation available for browsing goats. Control of GIN in browsing ruminants is threatened by the emergence of anthelmintic resistant nematodes strains in the local flocks. Browsing trials with kids and lambs have shown that supplementary feeding (100–108g/day) improved resilience and resistance of Criollo kids against GIN. Supplements used successfully include sorghum:soybean meal (74:26 respectively), maize:soybean meal (74:26) and maize only (108g/day). The later result showed that animals might benefit from supplementation with rumen fermentable energy that could improve utilization of available browsing legumes by the goats. Wet season trials showed that, in spite of supplementation, infected animals have lower productivity compared to non-infected animals. However, the combined use of supplementation and copper oxide wire particles or tactic anthelmintic treatment, can improve growth rate of infected kids to maximum production levels. Short-term supplementation strategies have shown to be less effective than long-term strategies in achieving better resilience against GIN in kids. Lambs are more resistant but less resilient against GIN infections than kids. Improved resistance of lambs seemed to require more nutrients than in goats. In conclusion, supplementary feeding can play an important role in improving both resilience and resistance of growing small ruminants in the tropics but it can be further improved through the complementary use of other strategies leading to a more sustainable integrated parasite control strategy.

Data on the chemical composition and gas production from some indigenous fodder plants from Yucatán and Quintana Roo are shown in order to demonstrate the potential of these species as supplements for livestock However, contents of polyphenols but above all, their reactivity, present a serious constraint on their use in large proportions. Gas production techniques seem to be a good assay for estimating the reactivity of phenolic compounds since the closed environment used in the technique might exacerbate the negative effects of the reactive fraction of polyphenols on microbial population and enzymes.

In this paper the diversity of the Leguminosae family in the Yucatán Peninsula is presented, explaining it by subfamilies, genus and species and in each of the three states of the Peninsula, as well as the distribution of the family in the different vegetation types and associations and in the secondary vegetation derived from both of them. The forage species that can be used combined with graminae to feed cattle like goats and sheep are also presented.