Count the Unsaturated Fat, Not Just the Fat

The ration sheet says the fat is fine, yet the tank test can say otherwise. Crude fat on a feed tag is a rough count, and it cannot tell unsaturated fat from saturated. What the rumen meets each day is the unsaturated C18 fat from every ingredient together: oleic, linoleic and linolenic acids, summed as the rumen unsaturated fatty acid load, or RUFAL. It was proposed as a better warning than the percentage of added fat, and a higher load raises the odds of the shift described in how milk fat depression starts in the rumen.

NASEM 2021 works from total fatty acids, not crude fat, which is the ether-soluble part of a feed and a poor index of true fatty acid content in many feeds. The gap is uneven. In a large laboratory dataset, total fatty acids rose by about 0.57 per unit of ether extract in forages, against about 0.9 in oilseeds and by-product feeds.

The load hides in several places. Distillers grains carry corn oil, mostly linoleic acid, and the amount varies: 20 samples ranged from 10 to 18% total fatty acids in dry matter, and reduced-fat products exist. Oilseeds such as soybean, cottonseed, canola and flax differ in profile and in how available their oil is. Forages and grains add a background share. Added oils count, and so does oleic acid: at 5% of diet dry matter, a high-oleic safflower oil still lowered milk fat percentage in a 24-cow trial.

The load can stack with starch, and how fast starch arrives matters too. In a distillers diet, added oil or added starch each lowered the concentration of fatty acids the udder makes itself by 13% (four cows), and both together by 27%. In 20 cows, milk fat fell from 3.3% to about 3.0% with either one and 2.7% with both, though fat yield was down about 0.25 kg/day on all three and fell no further with both.

A sizable load can be tolerated in some rations. In diets near 28% NDF, distillers grains or added corn oil left fat yield unchanged, although milk fat percentage fell with distillers because milk volume rose. Soybean oil at 2% of dry matter also left fat yield unchanged while percentage dipped and udder-made fatty acids fell. Neither trial was designed to test fiber, so they show tolerance in that ration, not why.

No validated ceiling exists. In commercial herd records, a higher load went with lower milk fat in a continuous way, and the data did not allow a threshold to be set. NASEM 2021 repeats an older general advice against more than 7% total fatty acids in dry matter. That advice was framed around feed intake, not milk fat. The extension rule of thumb for unsaturated fat is a suggestion, not a validated level. Pooled supplement trials varied widely: oilseeds lowered milk fat percentage on average but raised fat yield.

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