Short-term milk production responses differ when balancing dietary 18-carbon fatty acids using high-oleic soybeans or a commercially available stearic acid-enriched supplement.

Recent evidence suggests that dietary 16- and 18-carbon fatty acids (FA) play an important role in influencing milk fat synthesis and nutrient partitioning. However, limited research has directly compared the effects of oleic- versus stearic-based supplemental sources under controlled conditions. We evaluated the effects of supplying 18-carbon FA primarily as oleic acid (cis-9 C18:1) from high-oleic soybeans versus stearic acid (C18:0) from a commercially available C18:0-enriched supplement, while balancing for similar 16-carbon FA intake, on short-term production responses in mid-lactation Holstein cows. Thirty-two cows were assigned to a 2-period crossover design with two 14-d periods, with sampling during the final 4 d of each period. Treatments were diets containing either 8% of diet DM as roasted, ground high-oleic soybeans (HOS) or 2% of diet DM as a C18:0-enriched supplement (SAT). High-oleic soybeans replaced soybean-based bypass protein to maintain similar overall dietary nutrient composition. Because inclusion of the C18:0-enriched supplement also increased dietary palmitic acid (C16:0), a C16:0-enriched supplement was added to the HOS diet to balance C16:0 supply and better isolate the effects of C18:0 versus cis-9 C18:1. The statistical model included cow within square as a random effect, and treatment, period, and square as fixed effects. Although diets were formulated to provide similar amounts of total and 16- and 18-carbon FA, analyzed FA profiles revealed that the C18:0-enriched supplement in SAT delivered a lower 18-carbon FA supply than its formulation values suggested. Compared with SAT, HOS reduced DMI (31.2 vs. 32.0 kg/d) but increased milk yield (50.8 vs. 49.0 kg/d), milk fat (2.05 vs. 1.97 kg/d), milk protein (1.59 vs. 1.52 kg/d), and lactose (2.49 vs. 2.39 kg/d). Accordingly, HOS increased 3.5% FCM by 2.3 kg/d, and ECM increased by 2.4 kg/d. A decrease in mixed FA yield was observed with HOS (721 vs. 778 g/d), along with an increase in preformed FA yield (717 vs. 614 g/d), consistent with the greater milk fat output. Body weight gain was also higher for HOS (0.78 vs. -0.01 kg/d), although the short evaluation period limits conclusions regarding longer-term effects. Collectively, these results indicate that feeding high-oleic soybeans, which mainly provide 18-carbon FA as cis-9 C18:1, led to better short-term milk production and component yields than using a commercially available C18:0-enriched supplement. These findings provide further evidence that individual 18-carbon FA exert different metabolic and production effects and should not be treated as interchangeable in diet formulation. More broadly, they highlight the value of accurate nutrient composition data in feed libraries and demonstrate that high-oleic soybeans may offer a practical on-farm alternative to certain commercially available fat supplements.