Effect of cereal straw and alfalfa hay diet on amino acid profile of gastrointestinal digesta in lactating dairy cows

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The MP absorbed in the small intestines of ruminants is a mixture of digested rumen microbial crude protein (MCP), rumen un‐degraded protein (RUP) and endogenous protein (Clark, Klusmeyer, & Cameron, 1992). An improved representation of the biology processes of milk protein synthesis should include individual amino acid (AA), rather than an aggregated representation provided by MP (Arriola Apelo, Knapp, & Hanigan, 2014). The MCP is a high‐quality protein, and its AA content is relatively constant (Korhonen, Ahvenjärvi, Vanhatalo, & Huhtanen, 2002; Schingoethe, 1994; Schwab, 1994). However, some variations exist in AA composition of bacterial protein (Rodríguez‐Prado, Calsamiglia, & Ferret, 2004; Yang, Beauchemin, & Rode, 2001; Zebeli, Tafaj, Weber, Steingass, & Drochner, 2008), depending on the quality of the feedstuffs, content of crude fibre, N sources or forage‐to‐concentrate ratio (F:C). Up to 66% of the protein in synthesized MCP was derived from peptides or AA, and the rest of the protein was from ammonia N (Russell, Sniffen, & Van Soest, 1983). Therefore, AA utilization by rumen micro‐organisms is important to clarify the role of ruminal MCP synthesis. In addition, it is necessary to achieve an ideal protein at the duodenal level, which influences the pool of AA available for absorption (Lapierre et al., 2006; NRC, 2001).
Alfalfa hay is always used with Chinese wild rye hay (Leymus chinensis) as forage source for high‐yielding lactating dairy cows in China. Corn stover (CS) consisting of the leaves and stalks of maize (Zea mays ssp. mays L.) and rice straw (RS) are abundant, with approximately 200 million metric tonnes of CS and similar amounts of RS produced annually in China (Pang et al., 2008), but they have not been widely used as forage sources for lactating dairy cows due to the unclear information about nutrients degradation and metabolism such as AA utilization. In a previous study by Wang et al. (2014), a significant decrease in milk performance, nitrogen (N) efficiency, MCP and RUP was observed when cows were fed CS or RS as a forage source relative to alfalfa hay. It has been reported that the key factor that restricting the animal performance is less of dietary energy supply and glucose anabolism (Wang et al., 2014; Wang, Zhao, Zhang, & Liu, 2016). However, the mechanism underlying this apparent phenomenon, from the aspect of AA metabolism, needs to be clarified as we hypothesis that AA profiles of ruminal microbes, absorbable free AA profiles across the gastrointestinal tract, especially for small intestinal contents, may be also important limiting factors in restricting the AA absorption and subsequent milk protein synthesis except for the digestive AA flow and AA uptake in mammary gland that had been identified as decreased value in cows fed cereal straw diets (Wang, Sun, Xu, Zhu, & Liu, 2016). In addition, there is a lack of accurate predictability in the quantity and ratio of the needs and delivery of AA in ruminal MCP and MP, especially for in vivo data pertaining to AA profiles of gastrointestinal digesta in lactating dairy cows, which can help improve accuracy in predicting milk protein synthesis. Therefore, the present study was conducted to determine the effects of low quality forages (com stover and RS) replacing a high‐quality forage (alfalfa hay) on the AA profile of runimal MCP and gastrointestinal digesta including free absorbed AA and hydrolysed AA of dairy cows.
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