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