Choices between red clover and fescue in the diet can be reliably estimated in heifers post-weaning using n-alkanes

Measuring diet choice in grazing animals is challenging, complicating the assessment of feed efficiency in pasture-based systems. Furthermore, animals may modify their intake of a forage species depending on its nutritive value and on their own physiological status. Various fecal markers have been used to estimate feed intake in grazing animals. However, plant-wax markers such as n-alkanes (ALK) and long-chain alcohols may provide reliable estimates of both dietary choices and intakes. Still, their use in beef cattle has been relatively limited. The present study was designed to test the reliability of the ALK technique to estimate diet choices in beef heifers. Twenty-two Angus-cross heifers were evaluated at both post-weaning and yearling age. At each age, they were offered both red clover and fescue hay as cubes. Following 3-week acclimation periods, daily intake of each forage species was assessed daily for 10 days. During the final 5 days, fecal grab samples were collected twice daily. The ALK fecal concentrations were adjusted using recovery fractions compiled from literature. Diet composition was estimated using two statistical methods. Post-weaning, dietary choices were reliably estimated, with low residual error, regardless of the statistical approach adopted. The regression of observed on estimated red clover proportion ranged from 0.85±0.08 to 1.01±0.09 for fecal samples collected in the p.m. and for daily proportions once averaged, respectively. However, at yearling age, the estimates were less reliable. There was a tendency to overestimate the red clover proportion in diets of heifers preferring fescue, and vice versa. This was due to greater variability in ALK fecal concentrations in the yearling heifers. Overall, the ALK technique provided a reliable tool for estimating diet choice in animals fed a simple forage diet. Although further refinements in the application of this methodology are needed, plant-wax markers provide opportunities for evaluating diet composition in grazing systems in cattle.

n-Alkane and long-chain alcohol recovery in moose (Alces alces), a browsing herbivore

Habitat management for herbivores often depends on an understanding of the food habits of animals. Plant cuticular waxes containing nearly indigestible complex mixture of n-alkanes and long-chain alcohols (LCOHs) have recently shown promise for diet analyses, but the accuracy of the technique depends strongly on the efficiency of recovery of the markers in feces. Fecal recovery of n-alkanes and LCOHs from 10 browse stems or leaves and two ensiled grass hays fed to moose (Alces alces (Linnaeus, 1758)) during in vivo digestion trials was investigated. n-Alkanes and LCOHs were extracted using a single-step accelerated solvent extraction technique and the recovery of these cuticular components was calculated from the feces of the animals. n-Alkane recoveries from feces averaged 0.82, ranging from a low of 0.58 (haylage) to a high of 0.95 (browse stems). LCOH recoveries averaged 0.92 across all forages, ranging from 0.80 (haylage) to a high of 1.13 (browse stems). n-Alkane and LCOH fecal recovery increased with increasing chain length, similar to findings in other studies. Although fecal recovery of n-alkanes and LCOHs were variable, we conclude that they are inversely related to forage digestibility, are consistent within forage classes, and are therefore predictable markers for use in assessing herbivore diets.

The use of n–alkanes and other plant–wax compounds as markers for studying the feeding and nutrition of large mammalian herbivores

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, C31and C33alkanes 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 C33and dosed C32alkanes 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 asn-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, fromAgrostis capillarisherbage, were slightly lower (60-65%) than the respectiven-alkanes (C30and 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, liken-alkanes, increase progressively with chain length from about 60% to 90%. It has been shown experimentally that the use of alcohols, together withn-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. Liken-alkanes and long-chain fatty alcohols, the faecal recoveries of plant-wax fatty acids in sheep increase with carbon chain length. Comparisons withn-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.

Estimation of herbage intake and digestibility of grazing sheep in Zhenglan Banner of Inner Mongolia by using n-alkanes

To reveal the seasonal dynamics of herbage intake, diet composition and digestibility and clarify the relationship of those with herbage nutrient and botanical composition of grazing sheep in Zhenglan Banner of Inner Mongolia, the n-alkane technique was used to test in sheep grazed during June, August and December. The results showed that the sheep mainly ate Fringed sagebrush, Stipa krylovii and Carex in proportions of 33.5, 17.9 and 21.2%, respectively, in spring. In summer, the sheep consumed cleistogenes, Potentilla tanacetifolia, Thyme, etc; the intake of Fringed sagebrush, Carex and Stipa declined. In winter, Fringed sagebrush accounted for 50.1% of herbage intake, and the intakes of Cleistogenes and Stipa krylovii increased to 15.3 and 18.4%, respectively. Herbage intake by the sheep in spring was 1.8 kg DM/d, and digestibility was 71.4%. Herbage intake and digestibility decreased slightly to 1.7 kg DM/d and 68.4% during the summer, respectively and decreased significantly to 1.2 kg DM/d and 36.4% in winter. There were significant correlations between diet composition and CP content in winter, diet composition and botanical composition in summer. A highly positive correlation between herbage intake and digestibility was observed in grazing sheep.

Plant and soil intake by organic broilers reared in tree- or grass-covered plots as determined by means of n-alkanes and of acid-insoluble ash

Free-range birds such as organic broilers may ingest soil and plants during exploration. The estimation of such intakes is of great interest to quantify possible nutritional supplies and also to evaluate the risk of exposure to parasites or to environmental contaminants. Marker-based techniques are now available and would allow to quantify plant and, especially, soil intake in free-range birds, and this quantification was the aim of this study. Methodologically, the proportion of plants in diet intake was determined first using a method based on n-alkanes. Subsequently, the fraction of soil in the total intake was estimated with a second marker, acid-insoluble ash. This approach was carried out to estimate ingested amounts of plants and soil for five successive flocks of organic broilers, exploring grass-covered yards or those under trees, at two time points for each yard: 51 and 64 days of age. Each factor combination (yard type×period=flock number×age) was repeated on two different yards of 750 broilers each. The birds' plant intake varied widely, especially on grass-covered yards. The proportion of plant intake was significantly higher on grass-covered plots than under trees and was also affected, but to a lesser extent, by age or flock number. The ingestion of plants would generally not exceed 11 g of DM daily, except two extreme outliers of nearly 30 g. The daily plant intake under trees tended to be lower and never exceeded 7 g of DM. The amount of ingested plants increased significantly for spring flocks. It increased slightly but significantly with age. The proportion of ingested soil was significantly higher under trees than on grass-covered yards. Dry soil intake was generally low with not more than 3 g per day. Only in adverse conditions - that is, older birds exploring yards under trees in winter - soil intake reached the extreme value of nearly 5 g. Broilers on yards under trees ingested significantly more soil than on grass-covered yards with least square means of, respectively, 2.1 and 1.1 g dry soil per day. These quantifications would allow us to evaluate the impact of plant and soil intake in the management of free-range broilers, especially for the management in organic farming systems. Nevertheless, under the two rearing conditions tested in the current study, the quite low proportions of soil intakes would represent a low risk for the safety of the produced food, unless the birds explore yards on heavily contaminated soil.

Grazing behaviour, physical activity and metabolic profile of two Holstein strains in an organic grazing system

The challenge for sustainable organic dairy farming is identification of cows that are well adapted to forage-based production systems. Therefore, the aim of this study was to compare the grazing behaviour, physical activity and metabolic profile of two different Holstein strains kept in an organic grazing system without concentrate supplementation. Twelve Swiss (HCH ; 566 kg body weight (BW) and 12 New Zealand Holstein-Friesian (HNZ ; 530 kg BW) cows in mid-lactation were kept in a rotational grazing system. After an adaptation period, the milk yield, nutrient intake, physical activity and grazing behaviour were recorded for each cow for 7 days. On three consecutive days, blood was sampled at 07:00, 12:00 and 17:00 h from each cow by jugular vein puncture. Data were analysed using linear mixed models. No differences were found in milk yield, but milk fat (3.69 vs. 4.05%, P = 0.05) and milk protein percentage (2.92 vs. 3.20%, P < 0.01) were lower in HCH than in HNZ cows. Herbage intake did not differ between strains, but organic matter digestibility was greater (P = 0.01) in HCH compared to HNZ cows. The HCH cows spent less (P = 0.04) time ruminating (439 vs. 469 min/day) and had a lower (P = 0.02) number of ruminating boli when compared to the HNZ cows. The time spent eating and physical activity did not differ between strains. Concentrations of IGF-1 and T3 were lower (P ≤ 0.05) in HCH than HNZ cows. In conclusion, HCH cows were not able to increase dry matter intake in order to express their full genetic potential for milk production when kept in an organic grazing system without concentrate supplementation. On the other hand, HNZ cows seem to compensate for the reduced nutrient availability better than HCH cows but could not use that advantage for increased production efficiency.

Stable isotope labeled n-alkanes to assess digesta passage kinetics through the digestive tract of ruminants

We describe the use of carbon stable isotope (¹³C) labeled n-alkanes as a potential internal tracer to assess passage kinetics of ingested nutrients in ruminants. Plant cuticular n-alkanes originating from intrinsically ¹³C labeled ryegrass plants were pulse dosed intraruminally in four rumen-cannulated lactating dairy cows receiving four contrasting ryegrass silage treatments that differed in nitrogen fertilization level (45 or 90 kg nitrogen ha⁻¹) and maturity (early or late). Passage kinetics through the gastrointestinal tract were derived from the δ¹³C (i.e. the ratio ¹³C:¹²C) in apparently undigested fecal material. Isotopic enrichment was observed in a wide range of long-chain n-alkanes (C₂₇–C₃₆) and passage kinetics were determined for the most abundant C₂₉, C₃₁ and C₃₃n-alkanes, for which a sufficiently high response signal was detected by combustion isotope ratio mass spectrometry. Basal diet treatment and carbon chain length of n-alkanes did not affect fractional passage rates from the rumen (K₁) among individual n-alkanes (3.71–3.95%/h). Peak concentration time and transit time showed a quantitatively small, significant (p≤0.002) increase with carbon chain length. K₁ estimates were comparable to those of the ¹³C labeled digestible dry matter fraction (3.38%/h; r = 0.61 to 0.71; p≤0.012). A literature review has shown that n-alkanes are not fermented by microorganisms in the rumen and affirms no preferential depletion of ¹³C versus ¹²C. Our results suggest that ¹³C labeled n-alkanes can be used as nutrient passage tracers and support the reliability of the δ¹³C signature of digestible feed nutrients as a tool to measure nutrient-specific passage kinetics.

The trials and tribulations of estimating the pasture intake of grazing animals

The present paper reviews estimation methods for measuring pasture intake of individual grazing animals, in particular, the use of indigestible plant markers. Natural alkanes and alcohols function essentially as an internal marker and thus accommodate differences in forage digestibility among individuals and those arising from interactions between supplement and forage. An estimate of diet composition partitions total intake into its component plant species. Estimates of diet composition require correction for incomplete faecal marker recovery, where relative recoveries (i.e. the recovery of the alkanes relative to each other) suffice. If estimates of whole-diet digestibility are also wanted, actual faecal alkane recoveries must be used. Using known labelled supplement intakes as a means of estimating the intake of all other diet components avoids the need to dose animals separately with synthetic alkanes. The results, problems and possible workarounds for a commercial system under development to estimate individual animal pasture intakes using known labelled-supplement intakes are outlined. A prototype bin system was trialled and, despite many initial technological problems, it showed enough promise for Sapien Technology to continue to develop the system with Proway Livestock.

Using n-alkanes to estimate diet composition of herbivores: a novel mathematical approach

N-alkanes are long-chain saturated hydrocarbons occurring in plant cuticles that can be used as chemical markers for estimating the diet composition of herbivores. An important constraint of using n-alkanes to estimate diet composition with currently employed mathematical procedures is that the number of markers must be equal or larger than the number of diet components. This is a considerable limitation when dealing with free-ranging herbivores feeding on complex plant communities. We present a novel approach for the estimation of diet composition using n-alkanes which applies equally to cases where the number of markers is lower, equal or greater than the number of plant species in the diet. The model uses linear programming to estimate the minimum and maximum proportions of each plant in the diet, and avoids the need for grouping species in order to reduce the number of estimated dietary components. We illustrate the model with two data sets of n-alkane content of plants and faeces obtained from a sheep grazing experiment conducted in Australia and a red deer study in Portugal. Our results are consistent with previous studies on those data sets and provide additional information on the proportions of individual species in the diet. Results show that sheep included in the diet high proportions of white clover (from 0.25 to 0.37), and relatively high proportions of grasses (e.g. brome from 0.14 to 0.26) but tended to avoid Lotus spp. (always less than 0.04 of the diet). For red deer we found high proportions of legumes (e.g. Trifolium angustifolium and Vicia sativa reaching maximum proportions of 0.42 and 0.30 of the diet, respectively) with grasses being less important and Cistus ladanifer, a browse, also having relevance (from 0.21 to 0.42 of the diet).

Net transfer of nutrients to the duodenum and disappearance ofn-alkanes in the reticulo-rumen and the hindgut of sheep fed grass/legume combinations

An experiment was carried out to examine the effect of increasing the proportion of Wimmera ryegrass hay in a lucerne hay-based diet on net transfer of nutrients to the intestine, and on the disappearance ofn-alkanes in the reticulo-rumen and the hindgut of sheep. Following a latin square design, four adult ewes were fed 1:0, 0·33:0·67, 0·67:0·33 and 0:1 proportions of legume and grass. Increasing the proportion of ryegrass in the diet linearly decreased the intake of DM (P= 0·017), organic matter (P= 0·021) and N (P= 0·001). However, neutral-detergent fibre intake was not affected (P= 0·148), nor was its digestibility coefficient (P>0·10). Diet had no effect on duodenal flows of nutrients (P>0·10), although the proportion of N intake (NI) recovered at the duodenum as non-NH3N (NAN) increased linearly withLolium rigidumin the diet (P= 0·002). Full recovery of NI as NAN was achieved at NH3concentrations in the rumen below 110 g/l. Microbial N contribution to NAN varied in a quadratic manner (P< 0·05) with the proportion of grass in the diet, although efficiency of microbial synthesis was not affected (P>0·10). Duodenal recovery of consumedn-alkanes was not affected by diet and was complete for those present in higher concentrations in the forages. Isolated rumen bacteria contained significant amounts ofn-alkanes, contributing to the duodenal flow of these hydrocarbons in variable proportions depending on the diet consumed.

Heterologous expression of two Medicago truncatula putative ERF transcription factor genes, WXP1 and WXP2, in Arabidopsis led to increased leaf wax accumulation and improved drought tolerance, but differential response in freezing tolerance

Cuticular waxes are the major components of plant cuticle and play an important role in protecting aerial organs from damage caused by biotic and abiotic stresses. Here we report the functional characterization of two putative ERF transcription factor genes WXP1 and its paralog WXP2 from Medicago truncatula. Transgenic expression of WXP1 and WXP2 in Arabidopsis (ecotype Columbia) led to significantly increased cuticular wax deposition on leaves of 4-week-old and 6-week-old transgenic plants, assessed based on fresh weight or based on surface area. Differences in the accumulation of various wax components as well as their chain length distributions were found in the WXP1 and WXP2 plants. The major wax component in Arabidopsis, n-alkanes, increased substantially in both WXP1 and WXP2 transgenics, however, another wax component, primary alcohols, increased in WXP1 plants but decreased in WXP2 plants. Cuticle properties of the transgenic leaves were analyzed by chlorophyll leaching assay; while the WXP1 plants had no change, the WXP2 plants showed more chlorophyll leaching. Analysis of fresh weight loss from detached leaves revealed that the transgenic leaves tend to retain more water than the control. Both WXP1 and WXP2 transgenic plants showed significantly enhanced whole plant drought tolerance. Analysis of freezing tolerance at the whole plant level and measurement of electrolyte leakage from detached leaves revealed that the WXP1 plants had increased freezing tolerance while the WXP2 plants were more sensitive to low temperature when compared to the control. Transgenic expression of WXP1 had no obvious effects on plant growth and development, however, the expression of WXP2 led to slower plant growth. These results indicate that WXP1 is a useful candidate gene for improving plant drought and freezing tolerance by genetic transformation.

Protocol for the analysis of n-alkanes and other plant-wax compounds and for their use as markers for quantifying the nutrient supply of large mammalian herbivores

Plant-wax markers can be used for estimating forage intake, diet composition and supplement intake in grazing livestock, wild ruminants and other mammals. We describe protocols for using the saturated hydrocarbons (alkanes) of plant wax as markers for estimating fecal output, intake and digestibility. Procedures for investigating digestion kinetics are also discussed. Alkanes can also be used to estimate diet composition and the procedures required to do this are also described, including the special case where supplementary feed is treated as a component of the diet composition estimate. The long-chain alcohols (LCOHs) and very long-chain fatty acids (VLCFAs) of plant wax show particular promise for discriminating a greater number of species in the diet. The use of all these plant-wax markers in nutrition studies depends on having quantitative, repeatable and mutually compatible assay procedures for alkanes, LCOHs and VLCFAs; we present protocols for these assays in detail. Analysis of a single sample of feces or plant material for all these plant-wax markers can be completed within 2 days; however, it is possible to process up to 50 samples (analyzed in duplicate) per week.

Using n-alkane markers to estimate forage intake of mule deer

Dry matter intake (DMI) of free-ranging mule deer ( Odocoileus hemionus (Rafinesque, 1817)) in the aspen parkland of east-central Alberta was estimated using the double n-alkane ratio and bite count methods. Eleven female mule deer were given an intraruminal capsule containing synthetic n-alkanes and released into 4–8 ha paddocks. Known concentration of even-chained carbon (C32) was released from the capsules and recovered 7–10 days later from freshly voided faeces. Odd carbon chains of adjacent length (C31:C33) were extracted from vegetation samples gathered during behavioural observations of four tame deer. Calculations from the pairings (C31:C32; C33:C32) provided estimates of DMI that ranged from 1.29 to 2.73 kg/day. DMI was highest during autumn, possibly to prepare for increased energy demands for the upcoming winter. No differences were found in seasonal or annual comparisons of bite rates, but bite sizes differed in all comparisons, suggesting bite size was a critical factor affecting consumption rates for mule deer on northern ranges. Maximum bite sizes rose from 49 ± 5 mg/bite in July to 213 ± 58 mg/bite in October. Maximum consumption rates were in October of both years (3.6–4.4 g/min) and occasionally approached theoretical maxima (14.3–15.6 g/min).

Determining diet composition on complex swards using n-alkanes and long-chain fatty alcohols

We conducted an experiment to quantify the accuracy of methods based on n-alkanes and long-chain fatty alcohols for determining the diet composition of animals grazing complex swards. We cut forage from two indigenous vegetation communities, a Molinia caerulea-dominated grassland and a Calluna vulgaris-dominated dwarf-shrub community, and offered it to mature ewes in different ratios in a zero-grazing experiment. Nine dietary categories were identified within the forage offered: Molinia caerulea, Festuca spp., Juncus effusus, Carex spp., Calluna vulgaris, Erica tetralix, Vaccinium myrtillus, and dead grass. Samples of each of these categories together with fecal samples from each individual animal were analyzed for n-alkane and long-chain fatty alcohol concentrations. We analyzed the data using optimization software to minimize the sum of squares differences in the proportional profiles of n-alkanes and fatty alcohols in the diet and feces. Different combinations of n-alkane and fatty alcohols were investigated to assess which gave the most accurate measures of diet composition from the fecal profile. The most accurate estimates were obtained using combinations of the n-alkanes C25, C29, C31, and C33 and the long-chain fatty alcohols 1-C24-ol, 1-C28-ol, and 1-C30-ol, and these gave values for Lin's concordance correlation coefficient between estimated and actual values of >0.98. Our results demonstrate that n-alkanes and long-chain fatty alcohols can be used to estimate several components within the diet of animals grazing complex swards. Diet composition information obtained using this methodology has wide-ranging applications in terms of the assessment of the impact of grazing animals on particular ecosystems or the quantification of nutrient supply to the animal from different selection choices.

Parasite-induced anorexia in a free-ranging mammalian herbivore: an experimental test using Soay sheep

Theory suggests that gastrointestinal parasites can influence herbivore population dynamics by increasing host mortality rates. In addition, parasites may have a non-lethal range of both physiological and behavioural effects on their hosts. Two potential behavioural effects are parasite-induced anorexia and the alteration of diet selection patterns — both of which may influence plant communities without necessarily causing herbivore mortality. We report here the results of an experiment carried out in August–September 2001 to examine herbivore response to parasitism using feral Soay sheep ( Ovis aries L., 1758) living on Hirta, St. Kilda (Scotland, UK), as a model system. The aim of the experiment was to determine whether the sheep exhibit significant parasite-induced anorexia and whether diet selectivity was also altered in response to parasitism. The n-alkane technique was used to estimate forage intake rates and diet composition. Overall, intake rate increased with body mass and the mean (±SE) over both sexes was 737 ± 40 g DM / day (range 458–1241 g DM / day). However, we found no effect of parasite burden upon intake rate or selection patterns. The results of a power analysis imply that any reduction in intake rate was less than 30%, which is less than the 30%–60% reduction in intake rate recorded for domestic sheep under moderate parasite burdens. This suggests that Soay sheep are more resilient to parasitism than domestic breeds. Despite evidence to the contrary from simple farm-based studies, the lack of any effect on selection patterns in this case suggests that parasites do not cause significant alterations to the selection patterns of herbivores in complex non-agricultural environments.

Quantifying species composition in root mixtures using two methods: near-infrared reflectance spectroscopy and plant wax markers

Understanding of plant interactions is greatly limited by our ability to identify and quantify roots belonging to different species. We proposed and compared two methods for estimating the root biomass proportion of each species in artificial mixtures: near-infrared reflectance spectroscopy (NIRS) and plant wax markers. Two sets of artificial root mixtures composed of two or three herbaceous species were prepared. The proportion of root material of each species in mixtures was estimated from NIRS spectral data (i) and the concentration patterns of n-alkanes (ii), n-alcohols (iii), and n-alkanes +n-alcohols combined (iv). For each data set, calibration equations were developed using multivariate statistical models. The botanical composition of root mixtures was predicted well for all the species considered. The accuracy varied slightly among methods: alkanes < alcohols = alkanes + alcohols < NIRS. Correlation coefficients between predicted and actual root proportions ranged from 0.89 to 0.99 for alkanes + alcohols predictions and from 0.97 to 0.99 for NIRS predictions. These two methods provide promising potential for understanding allocation patterns and competitive interactions.

Comparison of Techniques for Estimating Herbage Intake of Grazing Dairy Cows

For estimating herbage intake during grazing, the traditional sward cutting technique was compared in grazing experiments in 2002 and 2003 with the recently developed n-alkanes technique and with the net energy method. The first method estimates herbage intake by the difference between the herbage mass before and after grazing and the regrowth between the 2 points in time. The second technique estimates herbage intake by the ratio of a dosed even-chain synthetic n-alkane (C32) and a naturally occurring odd-chain n-alkane (C31 or C33) in the herbage and feces. The third technique calculated the intake from the animal's energy requirements for milk production and maintenance. The sward cutting technique estimated herbage intake with the highest coefficient of variation and had different results in the 2 experimental years. The n-alkanes method yielded less variable results, whereas the net energy method gave the least variable results. In 2002, the estimates of the alkane ratio C32:C33 were best related with estimations of the net energy method. In 2003, the estimates of the alkane ratio C32:C31 were best related. The estimate based on the alkane ratio C32:C33 had a lower coefficient of variation than the one based on the alkane ratio C32:C31. Therefore, the C32:C33 alkane method was considered to be a better direct estimator for herbage intake by grazing lactating dairy cows.

Plant-wax alkanes as seasonal markers of red deer dietary components

n-Alkanes are long-chain hydrocarbons commonly occurring in plant cuticles that can be recovered in herbivore faeces. Differences among plant species in their content of cuticular wax n-alkanes can be exploited to estimate diet composition of herbivores. n-Alkanes have been used mainly in domesticated herbivores feeding on relatively simple diets over short-term periods. Extending the method to wild herbivores feeding on seasonal complex diets is possible provided that n-alkanes act as effective dietary component markers in different seasons. The n-alkane content of browse species and herbage layer and of faeces of red deer (Cervus elaphus L., 1758) males and females in a region with a Mediterranean climate is described. Information on the n-alkane content of plant species was related to that of faeces to estimate the diet composition of red deer. Plant species had distinct n-alkane contents, some of which varied seasonally. The n-alkane content of faeces also varied seasonally and between red deer sexes. Both red deer males and females had relatively high proportion of browse in their diets during summer and of herbage layer in spring, as shown by other studies in Mediterranean environments.

Validation of the n-alkane technique for estimating diet composition, digestibility and dry matter intake in the brushtail possum (Trichosurus vulpecula)

Few validated studies have tested the n-alkane method for diet estimation in non-ruminants and the technique has rarely been validated for estimating diet composition for more than two dietary components. The arboreal marsupial Trichosurus vulpecula was fed leaf and fruit diets with up to five component species in two trials. In Trial 1, alkane recovery was estimated, and in Trial 2 these estimates were used to predict digestibility, diet composition and dry matter intake. Alkane recovery increased non-linearly (convex up) with increasing n-alkane chain length. Recovery was linearly and negatively correlated with diet digestibility and appeared to explain a progressive bias with observed digestibility in the alkane estimates of digestibility (slope = 0.37). Diet composition was successfully estimated for up to five leaf dietary components without correction for recovery. Correcting for recovery gave less reliable diet-composition predictions, indicating that the recovery estimates could not be extrapolated to the second trial. Dry matter intake appeared to be relatively robust to variation in alkane recovery between individuals (mean bias =1.6%). If recovery is shown to vary with digestibility in other taxa, calibration of the alkane technique will require diet- and taxon-specific calibration trials to give reliable estimates of diet composition and dry matter digestibility.

Current issues and uncertainties in the measurement and modelling of air-vegetation exchange and within-plant processing of POPs

Air-vegetation exchange of POPs is an important process controlling the entry of POPs into terrestrial food chains, and may also have a significant effect on the global movement of these compounds. Many factors affect the air-vegetation transfer including: the physicochemical properties of the compounds of interest; environmental factors such as temperature, wind speed, humidity and light conditions; and plant characteristics such as functional type, leaf surface area, cuticular structure, and leaf longevity. The purpose of this review is to quantify the effects these differences might have on air/plant exchange of POPs, and to point out the major gaps in the knowledge of this subject that require further research. Uptake mechanisms are complicated, with the role of each factor in controlling partitioning, fate and behaviour process still not fully understood. Consequently, current models of air-vegetation exchange do not incorporate variability in these factors, with the exception of temperature. These models instead rely on using average values for a number of environmental factors (e.g. plant lipid content, surface area), ignoring the large variations in these values. The available models suggest that boundary layer conductance is of key importance in the uptake of POPs, although large uncertainties in the cuticular pathway prevents confirmation of this with any degree of certainty, and experimental data seems to show plant-side resistance to be important. Models are usually based on the assumption that POP uptake occurs through the lipophilic cuticle which covers aerial surfaces of plants. However, some authors have recently attached greater importance to the stomatal route of entry into the leaf for gas phase compounds. There is a need for greater mechanistic understanding of air-plant exchange and the 'scaling' of factors affecting it. The review also suggests a number of key variables that researchers should measure in their experiments to allow comparisons to be made between studies in order to improve our understanding of what causes any differences in measured data between sites.