Some studies have associated the ionophore rumensin with decreases in milkfat yield. But a recent study at Cornell University refutes that, saying that any drop in milkfat percentage is due mainly to polyunsaturated fatty acids or oil in the diet, although starch and Rumensin may have an additive effect. In a trial involving 32 primiparous and 48 multiparous cows at the Cornell University Dairy Teaching and Research Facility, researchers studied different combinations of starch, corn oil and Rumensin. Among their conclusions:
Rumensin did not induce milkfat depression in high-producing lactating cows fed either high- or low-starch diets.
Cattle consuming corn oil demonstrated increased dry matter intake and milk yield, but significantly reduced milkfat yield.
The combination of high starch, oil and Rumensin had a negative stepwise effect on milkfat percent, demonstrating an additive effect of the treatment factors.