Static quantification measures of chemical components are commonly used to make certain assumptions about forage or feed nutritive value and quality. In order for modern nutrient requirement models to estimate intake and digestibility more accurately, kinetic measures of ruminal fiber degradation are necessary. Compared to in vivo experiments, in vitro (IV) and in situ (IS) experimental techniques are relatively simple and inexpensive methods to determine the extent and rate of ruminal fiber degradation. This paper summarizes limitations of these techniques and statistical analyses of the resulting data, highlights key updates to these techniques in the last 30 yr, and presents opportunities for further improvements to these techniques regarding ruminal fiber degradation. The principle biological component of these techniques, ruminal fluid, is still highly variable because it is influenced by ruminally fistulated animal diet type and timing of feeding, and in the case of the IV technique by collection and transport procedures. Commercialization has contributed to the standardization, mechanization, and automation of the IV true digestibility technique, for example, the well-known DaisyII Incubator. There has been limited commercialization of supplies for the IS technique and several review papers focused on standardization in the last 30 yr; however, the IS experimental technique is not standardized and there remains variation within and among laboratories. Regardless of improved precision resulting from enhancements of these techniques, the accuracy and precision of determining the indigestible fraction are fundamental to modeling digestion kinetics and the use of these estimates in more complex dynamic nutritional modeling. Opportunities for focused research and development are additional commercialization and standardization, methods to improve the precision and accuracy of indigestible fiber fraction, data science applications, and statistical analyses of results, especially for IS data. In situ data is typically fitted to one of a few first-order kinetic models, and parameters are estimated without determining if the selected model has the best fit. Animal experimentation will be fundamental to the future of ruminant nutrition and IV and IS techniques will remain vital to bring together nutritive value with forage quality. It is feasible and important to focus efforts on improving the precision and accuracy of IV and IS results.
Keywords: digestion kinetics; fiber degradation; in situ; in vitro; indigestible fiber; ruminant nutrition.
In vitro and in situ techniques are important to studying ruminant nutrition because these procedures go beyond measures of components of a feedstuff in a laboratory by fermenting a sample in ruminal fluid. The in situ procedure was first described regarding ruminant nutrition in 1938 and in vitro in 1966 and both techniques have been refined over time to improve the reliability of results. This review focused on the state of knowledge 30 yr ago and significant discoveries that have impacted these techniques in the last 30 yr and shared a vision for future opportunities to refine these methods further. Commercialization of equipment and supplies has resulted in increased standardization of these methods; however, efforts should be made to continue to improve the standardization, and reliability of the results, of these procedures. Statistical analyses and data science applications are opportunities to expand these techniques to modern nutritional models and/or forecasting animal performance. The amount and kinetics of ruminal degradation estimate that in vitro and in situ techniques provide continue to be crucial to advance the efficiency and sustainability of ruminant animal production.
© The Author(s) 2023. Published by Oxford University Press on behalf of the American Society of Animal Science.