Rumen-Inert Fat Is Energy with a Catch

Rumen-inert fat can add energy while limiting the rumen disruption associated with feeding unprotected unsaturated fat. But "inert" means little effect on fermentation at moderate feeding rates, not complete protection. Its value depends on what the cow digests and whether the response pays.

Common options are saturated fatty acid supplements, which may be prilled (small solid beads) or flaked, and calcium salts of fatty acids. Prilled or flaked describes physical form, which does not by itself tell you the fatty acid profile or digestibility. Products differ in palmitic, stearic and oleic acids. In laboratory tests, calcium salts became less stable as pH fell, more so with more unsaturated fatty acids. Some unsaturated fatty acids in a calcium salt can still be modified in the rumen.

Fat is energy dense, but energy on a specification is not energy available to the cow. NASEM 2021 uses different fatty acid digestibility coefficients for different feed and supplement classes. Its default of 73% true digestibility applies to fatty acids from most feeds, not automatically to every commercial fat supplement. Compare products on evidence for their composition and form, not kilograms of fat.

Digestibility may help explain differences between products. In one trial of two products, estimated apparent fatty acid digestibility was about 77% for a palmitic-rich product and 55% for a palmitic-stearic blend. The palmitic-rich product supported more milk fat and energy-corrected milk than the blend, yet the average milk fat yield of the two supplemented treatments did not differ from the unsupplemented control.

More milk energy does not necessarily mean better body reserves. In a trial starting supplementation soon after calving, multiparous cows fed a palmitic-rich supplement made more milk fat but had lower body weight (about 21 kg) and body condition score (0.09 points) than cows fed no supplemental fat, a gap that opened after the second week. That does not show all supplemental fat worsens the fresh-cow energy gap. It shows why milk response and body condition should be judged together, especially soon after calving.

Intake and milk components can respond differently. In a meta-analysis of 33 publications, calcium salts of palm fatty acids fed at up to 3% of diet dry matter, compared with diets without supplemental fat, reduced intake by about 0.56 kg/d and milk protein concentration by 0.05 percentage points, with protein yield unchanged. Milk yield rose about 1.5 kg/d, milk fat yield about 0.04 kg/d and fiber digestibility about 1.6 percentage points. A lower protein percentage alone does not show a loss of protein output. These are averages, not a guaranteed farm result.

Educational only: not a diagnosis, prescription, or ration formula. Defer clinical and ration decisions to the herd veterinarian and nutritionist.