Natural grasslands and crop residues are the major feed resources for ruminants across Sub-Saharan Africa [1]. Although the nutritional value of feed resources is highest during the wet season, it varies widely across seasons, and often, the nutrients available are inadequate to sustain optimal animal performance [2]. Forages selected by grazing ruminants are usually different in composition and quality compared to the average available within the rangeland. Therefore, the knowledge of the nutritive value and intake of selected dietary components is important for managing different feeding systems and facilitates the choice and quantity of supplement feed required by the animals [3].

The quantity and quality of herbage selected and consumed in rangelands are influenced by several factors, which include but are not limited to the nutritional status and post-ingestive feedback of grazing herbivores, the presence of hair on and leaf size of the plant species, forage availability, as well as the botanical composition of the pasture and furthermore, differences in plant distribution, presence of chemicals as herbivory tolerance mechanism, and the number of plant species [4]. The magnitude of the influence caused by the factors that affect plant selection choices and the extent of their consumption by animals under natural conditions remains unclear [5]. The feed preference and chemical composition of the diet selected, as well as digestibility, are interrelated in regulating intake in ruminants [5,6]. These serve as important tools in evaluating animal performance in the dynamic ecosystem under which animals graze.

Diet composition (the quantitative botanical composition of what a grazing animal selects) is usually different from the average composition of the herbage present in a rangeland. The utilisation technique is one of the primitive methods used in assessing the botanical composition and intake of diet consumed by grazing herbivores [8]. This approach shows the locations grazed and the extent to which the rangeland has been utilised. This is done by estimating the biomass composition of the rangeland before and after grazing. Some of the limitations associated with this approach include the inability to measure the frequency and time of utilisation of the various plant species consumed. Furthermore, biomass regrowth after defoliation and the invasion of the rangeland by other animals, rather than those of interest, are not quantified and could negatively influence the accuracy of measurements and reduce the credibility of the results from this technique [9].

When the range is solely grazed by the animals of interest, the technique can be used to monitor longer-term (seasonal) grazing responses of herbivores as compared to shorter-term (hours to days) grazing responses. The influence of the preferred plant species in the grazed or browsed area can be estimated based on production performance indexes such as body weight changes, milk, wool, or cashmere production at the end of the seasons. Therefore, this approach can be used for seasonal formulation and allocation of supplements to grazing animals, given that the requirements of animals and the nutrient content of the plant species consumed in the range are known (Tubiello, personal communication).

Direct observation involves the visual assessment of plant species selected by the animals in comparison with the overall forage species in the grazed area at a given time. The botanical composition of diets consumed and the ingestive behaviour of ruminants can be evaluated using this method [10,11]. The principal advantage is that direct observation does not require the use of sophisticated equipment. However, the proper identification of plant species consumed remains a key drawback of this approach because animals graze continuously, and successive sample recognition becomes biased. Additionally, it is difficult to approach untamed animals close enough to observe what they are foraging accurately, and it can be hard even to locate them. In contrast, tamed animals can be used for close observation. However, only one or two animals can be observed by one observer at any given time, while diet selection is a complex concept involving constant interaction among animals [12], with the likelihood of reducing the accuracy of this technique.

The development and application of video recording tools in conjunction with global positioning system (GPS) collars and thermal imagining systems have enhanced the extent and accuracy of the direct observation method [13]. Managing stocking rate and animal distribution are crucial factors to consider when using video recording devices because the devices might not cover all the grazing areas, resulting in missing animals grazing outside the coverage area, while some animals may be difficult to distinguish from others. Remote sensing devices have been effective in assessing animal distribution in natural habitats [14]. Furthermore, bite size and time spent on foraging and rumination have been found effective in predicting the intake of grazing animals [15]. Time spent on grazing could be used as an index of species preference and/or the importance of the plant species among the forage species available to the animals. According to the equation proposed by Sanon et al. [16], the grazing time (GTi) of i^th species in the range is defined as: where RGTi is the ratio of time spent on grazing i^th species to the total time spent on all species grazed in the range, G is the proportion of the grazed species to the total biomass in the range, and T is the total time spent by animals in the range (T).

Nevertheless, the complexities in distinguishing between hedonic and active grazing make it difficult to capture and predict the actual preference of each plant species consumed in the rangeland. The experience of the observer and the number of animals grazing in the range may also hugely impact the accuracy and precision of the direct observation method. The plant stage of development and the number of plant species in the grazed area could also affect the accuracy of this method because as plant species mature, they become easier to identify [17] as opposed to the early stages of growth. Furthermore, it is easier to identify the forage species consumed when there are only a few plant species in the grazed area.

A possible improvement to the direct observation technique is the use of tamed animals by experienced observers familiar with the plant species in the particular range [18]. Furthermore, calibrating the direct observation data against other techniques (e.g., micro histological procedures, discussed later in Section 2.3) based on results obtained for the specific grazed range could enhance the accuracy of the information obtained. The adaptability of animals to tropical rangelands can advantageously be estimated from behavioural data collected using the direct observation technique of diet selection monitoring. Urination frequency, respiration rate (estimated by flank movement), and drinking frequency could be used as an index of the adaptability of different grazing animals to the harsh conditions associated with most tropical rangelands [19]. Equally, several approaches to monitoring chewing behaviour responses, such as jaw movement, bite frequency, and bite size, may enhance the accuracy of the direct observation method in predicting forage and nutrient intake [20,21]. The use of video recording devices combined with sensor technologies such as the RumiWatch system [15] and other technologies that monitor jaw movement and bite characteristics can be used to estimate intake with higher accuracy and reliability without the need for close human observation of the grazing animals [22,23].

This technique is based on the microscopic identification of plant fragments in faecal samples, stomach contents (or rumen evacuation), or oesophageal extrusa to determine diet composition and preference [24]. The stomach content method requires the slaughter of the animal, while the oesophageal extrusa and rumen evacuation methods are invasive techniques, requiring the fistulation of animals with a cannula in the oesophagus and stomach, to gain access to bolus and digesta, respectively. Generally, in microhistological analysis, the number of plant species found in the sample, measured volume, weight, and frequency of occurrence of each plant species, are used to evaluate the botanical composition of the diet consumed [25,26]. Samples collected from the rumen are contaminated with rumen contents making it difficult to analyse botanical diet composition [27].

Another important drawback of the stomach content method is the variation in the degradability of the plant species consumed. This results in a wide variation between the proportions of individual plant species in the diet consumed and their proportions in the stomach which is collected after slaughtering [28]. Furthermore, the analysis of stomach contents does not provide information about where and when the animal ingested the forage.

Wilson et al. [29] proposed the trocar method as a modification to the stomach evacuation method as a means of preventing the slaughtering of animals. In the trocar method, animals are tranquillised, cut open, and samples are taken using a trocar; the wound is then sewn up. However, an overdose of the sedative, wound infections, and illnesses are some of the complications associated with this method, which often result in the eventual death of many animals subjected to this surgical procedure. This method is not recommended for use on endangered animal species, but animals killed by wild carnivores and animals culled or slaughtered for consumption could be used to estimate the botanical composition of intake using the stomach content method. The stomach content method can also be used as a reference for calibration and validation or to improve the accuracy of other techniques, such as direct observation and marker techniques.

The faecal analysis technique has received more attention than the oesophageal and stomach evacuation methods because of the many advantages it possesses, as detailed in previous reviews [30,31,32]. For example, the analysis of faecal samples does not interfere with the natural foraging behaviour of animals, is non-invasive and applicable to both domesticated and wild ruminants. However, the size of the faecal samples that can be analysed is limited by the volume of faeces excreted and how frequently the animals defecate. Furthermore, the proportion of diet components found in faeces is often not the same as in the consumed diet as a result of the differences in the passage rate and degradation rate of the different classes of plants consumed across and within animal species. Nevertheless, the faecal analysis technique is valuable in identifying the preference and botanical composition of forage species consumed by grazing animals [24].

The DNA composition of faecal samples of ruminants could be used to estimate the botanical composition of an unknown diet consumed, such as those in a typical rangeland. This analysis is achieved using genetic sequence analysis which targets DNA segments (loci) that distinguish one plant from the other at the genus and specie levels, also called their DNA bar code [33]. The chloroplast DNA loci commonly used include the ribulose-bisphosphate carboxylase gene (rbcL), maturase K (matK), and the intron region of chloroplast tRNA gene (trnL), alone or in combination [34,35,36] to predict the taxonomic composition of feed consumed from faecal samples obtained. The technique involves total DNA extraction from faecal samples, sequencing of specific or different chloroplast DNA segments, and comparing the sequences with existing genomic databases [34].

While there has been much higher success with DNA bar code analysis of diets of carnivores, herbivore diets have specific limitations, which include the need to target specific DNA segments with maximal variability across plant species while also allowing for sufficient amplification from the DNA available. Furthermore, there is not enough reference genetic database for validating DNA sequences across the wide array of plant species that exist in natural rangelands [33,34]. In the study by Palumbo et al. [34], it was noted that the sequence abundance of the trnL gene in faeces also depends on the density of chloroplasts in the different species and on the digestibility of their plant tissues, how closely the faecal plant composition reflects the botanical composition of the diet ingested is still unclear. Therefore, while the DNA bar coding technique may be used to estimate the biodiversity of species consumed (diet composition), the quantitative estimation of each herbage consumed cannot be predicted. The cost of using this technique in practical contexts may also be a major drawback, especially in resource-limiting scenarios.

Nevertheless, this technique has been noted to be more accurate than other indirect methods of determining diet selection [35], although only limited studies have compared these methods across a wide range of ecological/animal feeding conditions in cattle [34]. Nevertheless, the DNA bar coding technique has been used as a proxy to predict diet composition in white-tailed deer, bison (Bos bison), goats (Capra hircus), western lowland gorillas (Gorilla gorilla), and colobus monkeys (colobus guereza) [33,37,38,39]. Therefore, more studies are recommended for a better understanding of the interaction between specie, feeding behaviour, and diet selection in natural and controlled environment scenarios using the DNA bar coding technique.