Effects of Adding Fat and Lasalocid to Diets of Dairy Cows

Holsteins (n = 12) and Jerseys (n = 12) averaging 90 d postpartum were fed 70 d in a 2 × 2 × 2 factorial arrangement of breeds and diets containing 3.0 or 6.8% ether extract with or without 36.7 mg/kg lasalocid in the diet DM. For many variables, the breeds responded differently. Holsteins usually performed better on high fat diets without lasalocid and Jerseys responded better to lasalocid. Influence of increased dietary fat generally was small except when fat acted synergistically with lasalocid. Dry matter intake was depressed substantially only when lasalocid was added to the higher fat diet. This detrimental effect precipitated a reduction in milk production. Dietary lasalocid reduced 4% FCM yield, milk fat percent, and milk fat yield without any lasalocid × fat interactions on these variables; however these reductions were much larger with Holsteins than Jerseys. Feeding lasalocid decreased ruminal acetate and increased ruminal propionate, but the changes were smaller with Jerseys. Higher fat diets increased serum cholesterol, but increases were less with Jerseys than with Holsteins. After 56 d of feeding, increased dietary fat and lasalocid had little influence on digestibility of feed nutrients. Adaptation to lasalocid feeding appeared to be gradual and prolonged, extending beyond 35 d. Results suggest no economically important beneficial synergistic effects from feeding added fat plus lasalocid to lactating dairy cows.