In situ digestibility tests (feed to be evaluated is placed in Dacron bags inside of a fistulated cow) are excellent and the gold standard, but the cost can vary from $150 to $250 a sample. Thus, cost versus benefits must be considered along with herd size (a 400-cow herd can recover the cost faster than a herd of 50). We recommend in vitro testing (bags in rumen fluid outside of the cow) because it is cheaper ($25 to $30 per sample) and provides useful results.
Mike Hutjens, Extension Dairy Specialist, University of Illinois - Illini DairyNET
Both in situ (Dacron bags placed inside the cow's rumen) and in vitro (fermentation in tubes in the lab) have issues regarding standardization of results. The outcome can be affected by what the donor cow is being fed or the degree of processing of the feed prior to the procedure. However, both procedures can be useful to rank differences in feeds, particularly when there are major changes in new batches of feeds. The degree of change can help a nutritionist decide how best to adjust a ration. The in situ procedure probably more closely resembles the rumen conditions of cows on farms, but soluble feeds can wash through the 50-micron pore size. The Dairy NRC protein system was developed based on in situ protein degradability, and the NRC provides a list of recommended practices to standardize conditions among labs.
Jeff Firkins, Dairy Nutritionist, Ohio State University
