Production responses of dairy cows when fed supplemental fat in low- and high-forage diets.

Intake of net energy for lactation (NE(L)) is often the limiting factor for milk production and is affected by stage of lactation and dietary concentrations of forage and fat. Because of the mechanisms involved, interactions are likely between those 2 diet components and stage of lactation. We conducted an experiment with 72 Holstein cows starting at 21 and ending at 126 d in milk (DIM). Cows were fed diets (dry matter basis) with 40 or 60% forage (67% corn silage, 33% alfalfa silage) each with 0 or 2.25% added saturated free fatty acids. The high- and low-forage diets contained 25 and 17% forage neutral detergent fiber and 30 and 33% total neutral detergent fiber, respectively; the low-forage diets contained several byproducts. Diets with and without fat contained approximately 5.2 and 3.2% long-chain fatty acids, respectively. Feeding fat or low-forage diets increased NE(L) intake, but no interaction was observed. The increase in NE(L) intake by cows fed low-forage diets was caused by increased dry matter intake, and the increase in NE(L) intake by cows fed fat was caused by increased energy density of the diet. Interactions between fat and forage were observed for energy utilization. When high-forage diets were supplemented with fat, the increased NE(L) intake went toward body energy reserves as measured by higher body condition scores with no change in milk yield. However, when low-forage diets were supplemented with fat, milk yield increased (2.6 kg/d) with no change in body condition. The differential partitioning of NE(L) may have been caused by nutrients other than NE(L) limiting milk production in cows fed the high-forage diets. With low-forage diets, intake of other nutrients was greater (i.e., greater dry matter intake). At 35 DIM, dietary treatments had little effect on milk fatty acids composition but in later lactation (125 DIM), feeding supplemental fatty acids or feeding low-forage diets increased long-chain fatty acids and decreased short-chain fatty acids. However, treatment did not have marked effects on concentrations of total fat or protein in milk. The amount of forage in a diet influences cow responses to supplemental fat and should be considered when diets are formulated.