## 1 Introduction  
Among the agricultural residues, wheat straw is the second largest biomass feedstock in the world, however, most of it has not been properly used and even been burned for a long time [1]. The burning of wheat straw leads to not only large amounts of resource waste but also severe environment pollution including carbon dioxide (CO\({}_{2}\)), carbon monoxide (CO), methane (CH\({}_{4}\)), N\({}_{2}\)O, and fine particulate matter (PM\({}_{2.5}\)) [2]. Therefore, finding an effective way to dispose wheat straw is of great significance concerning over the adverse impact of burning straw. Additionally, developments in plant breeding and agronomic techniques in the production of whole-crop wheat, together with its lower production cost sometimes, have resulted in the increased use of this crop as feed for ruminants [3]. The whole-crop wheat, which consists of wheat straw and wheat grain, must be more nutritious than wheat straw for ruminants. Several studies have shown that fermented whole-crop wheat can increase dry matter intake and improve rumen fermentation such as rumen concentration of ammonium nitrogen NH\({}_{3}\)-N, total volatile fatty acid (VFA) concentration, and the efficiency of microbial N synthesis [4; 5]. However, there is little published information about the comparative analysis of wheat hay and wheat silage in fermentation characteristic using in vitro rumen cultures. On the other hand, an environmental issue exists in the beef production systems, that is the production of enteric methane. Methane, which could be generated during the process of feed fermentation in the gastrointestinal tract of ruminants, is a forceful greenhouse gas, globally totals about 80 megatons annually [6]. Besides, rumen methane production represents 2 to 12% feed energy loss, depending on types of diets [7]. It has been reported that increasing the starch content of diet can benefit methane mitigation, which can fight greenhouse warming [8]. It was revealed that the increased ratios of grain to straw plus chaff of whole-crop wheat silage in the finishing diets of steers can reduce the enteric methane emission [9]. Therefore, the increased starch content of whole-crop wheat relative to wheat straw may create potential benefits not only for diminishing greenhouse gases, but also reducing feed energy loss. The rumen is a complex microbial ecosystem and is inhabited by diverse microbiota including archaea, bacteria, fungi and protozoa, which play an important role in digesting and degrading feeds to microbial proteins and volatile fatty acids (VFAs) [10]. The study of ruminal microecology is complicated, owing to the daily variation in the environment, microenvironments within the rumen, the effects of animal physiology on the rumen, and the substrate degradation capability of ruminal microorganisms [11]. Different forages have a great impact on rumen metabolism, thus affecting the rumen microbiota [12]. For example, using 165 rRNA gene-cloning library technology, [PERSON] et al. [13] reported that whole-crop wheat hay can be used to replace Levmus chinensis in the finishing diet of dairy bulls without adverse effect on the ruminal bacteria composition. Therefore, necessary study should be done to discover the microbiota variation of whole-crop wheat hay and silage in different harvest stage and time when fermenting in the rumen liquid. The previous study found that the nutritional value of the whole-crop wheat varied from different growth period [14]. Harvesting wheat crops in the afternoon can improve the ensiling characteristics compared to that in the morning, because treated with sunlight illumination, the wheat has a greater ruminal digestion and utilization of NH\({}_{3}\)-N by microbes [15]. To facilitate the research of the rumen microbial ecology, an in vitro system as a model was used to evaluate the ruminal microenvironment.