Practical Steps to Improve Diet Digestibility with guests: Dr. Jeff Firkins, The Ohio State University; Dr. Bill Weiss, Emeritus, The Ohio State University; Dr. Kirby Krogstad, The Ohio State University

This episode was recorded in Fort Wayne, Indiana, during the 2025 Tri-State Dairy Conference.

Dr. Firkins’ presentation covers key aspects of how to improve digestibility of different diet components: fiber, starch, fat and total diet. (4:31)

The negative relationship between starch in the diet and fiber digestion has been well known for more than 50 years. A recent meta-analysis showed the depression in fiber digestibility starts at very low starch concentrations. Dr. Weiss points out this can’t be due to low rumen pH at that starch level. Dr. Firkins agrees pH is probably only about half of the relationship and the other half is factors called the carbohydrate effect. He goes on to say adequate ammonia and amino acids are necessary for the fibrolytic bacteria in the rumen to maximize fiber digestibility, and urea alone is not adequate. The panel agrees there is little knowledge about what exactly the amino acid requirements of fibrolytic bacteria are or should be. (6:05)

Dr. Firkins shares some of his findings regarding how sugars impact fiber digestibility. Dr. Krogstad notes grain particle size can have key impacts on fiber digestibility and the panel discusses some of the challenges in nailing down optimal particle size. (13:39)

Dr. Weiss indicates diets should be formulated  for rumen degradable starch - but how do we get that number? Dr. Firkins explains some of the difficulty in making accurate, cohesive predictions. The panel discusses some field measures that may be helpful. (18:24)

Ruminants have lower fat digestibility than monogastrics. The panel explores biological factors that might be limiting fat digestibility. Dr. Firkins believes palmitic and oleic acids probably help the microbes and that is why we see improved diet digestibility when those fatty acids are supplemented. He encourages further exploration into the reason behind this and the mechanism by which it occurs. Dr. Krogstad mentions a Utah State study that also saw improved diet digestibility and also evaluated microbial fractions and phospholipids. (22:44)

The panel explores the relationship between rumen-degradable protein and fiber digestibility. What happens when RDP is too high or too low? What is the optimal level of RDP to maximize digestibility? How does the composition of the base diet influence how much RDP you might need? What role do peptides play in the rumen? (29:00)

Panelists share their take-home thoughts. (41:52)

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