4.1 | Diet characterization
The determination of diet
composition of large grazing herbivores has been a difficult problem due
to methodological limitations and habitat complexity. In comparison with
traditional diet analysis of herbivore fecal samples, metabarcoding
approaches provide a fast and high-resolution identification to species
level (Vesterinen et al., 2016). Both frequency of occurrence (FOO) and
relative read abundance (RAA) were applied to obtain semi-quantitative
diet data. Overall, our survey of sheep feces revealed 56 plant taxa (at
genus level or species level) (Figure 2). Both approaches have shown
accurate estimates when the number of food taxa in samples is small
(Mcinnes et al., 2017). However, there are taxa biases in recovery of
sequences. FOO tends to overestimate the importance of food groups with
small amounts. RRA can reflect more accurate relative proportions of the
population-level diet.
Metabarcoding dietary analysis allows quantification of variation in
animal diets due to changes in usability and taxa composition (Table 3
and Figure 3). The herbage taxa, such as Lespedeza sp .,Caragana korshinskii , Oxytropis sp. , Salsola sp. ,Bassia sp. , Setaria italica , and Cenchrus sp. , in
the diets of Tan sheep as characterized by metabarcoding data were
broadly consistent with our knowledge base for the diets of Tan sheep in
this area. The high diversity of food taxa observed in sheep diets can,
to a large extent, be attributed to the capability of metabarcoding to
quickly detect plant items derived from food residues that are often
hard to identify by other conventional techniques. Sequencing data based
on RRA metrics demonstrated that lambs presented variant diets as a
response to grazing type or grazing period, with approximatively all
food taxa assigned to the species level.
The selection of dietary components is affected by many factors such as
habitat characteristics and availability of plant resources (biomass,
nutrients, or palatability), but traditional techniques can only
determine at most up to 8–12 and thus might not provide a comprehensive
evaluation of diet composition in the natural grassland (Zhang, Jin,
Badgery, & Tana, 2017). The observations herein showed that there were
significant differences in herbage composition among the grazing
patterns that resulted from the distinction of feeding
regimes. The energy and albumen
contents of concentrated pelleted feed in the time-limited grazing group
were greater than in the forage roughage in the grazing group, and this
influenced the plant groups consumed by sheep in the concentrate
dominant group. Compared to the time-limited grazing group (grazing for
4 h), sheep grazing for 12 h were able to acquire more diverse plant
species rather than relying primarily on Lespedeza sp. , and this
can be attributed to their having sufficient time for choice. Regarding
to the supplementary feeding group, Lespedeza sp. seemed to
prevail due to its ubiquity as well as satisfactory palatability. In
comparison, the grazing time in the time-limited grazing group was less
than in the all-day continuous grazing group, resulting in less
selectivity for plant species and higher transformation efficiency for
forage.
It has been reported that the feedback for feeding behavior of sheep can
be associated with the sensory quality traits of herbage taxa (Provenza,
1995). In the present study, the findings on the diet composition for
the two periods showed a significant difference. The diet intake of
sheep in the early grazing mainly consisted of Lespedeza sp. , but
this changed to Chenopodium sp. , Artemisia, sp. , andLespedeza sp. in the late
grazing group. Tan sheep have been shown to forage largely onLespedeza sp. , a common plant item in local desert steppe.
Furthermore, metabarcoding diet
analysis also demonstrated that Tan sheep indeed consume medicinal
plants such as Phellodendron amurense , Arnebia euchroma ,Ixeris tamagawaensis , and Convolvulus sp. , consistent with
other studies.