Lactational responses of dairy cows fed unsaturated dietary fat and receiving bovine somatotropin.

Feeding unsaturated dietary fat to lactating dairy cows receiving bST may effectively alter the fatty acid composition of milk fat. This was tested using 16 Holstein cows assigned to one of four treatments during midlactation. Treatments were control, control diet with 15.5 mg of bST/d per cow, dietary fat from sunflower seeds and bST, or dietary fat from safflower seeds and bST. Diets were formulated to contain 19% CP and contained 25% corn silage, 25% alfalfa hay, and 50% concentrate mix on a DM basis. Milk yield was not significantly higher when bST was administered and increased with added fat diets (29.5, 32.7, 40.0, and 34.1 kg/d for the control, control with bST, sunflower seed with bST, and safflower seed with bST treatments, respectively). Percentage of milk fat was similar for all treatments. Concentrations of long-chain and unsaturated fatty acids in milk were increased slightly by bST and substantially with added fat. Milk protein percentages were not influenced by bST but were reduced by approximately .2 unit with added fat. Added unsaturated dietary fat coupled with bST increased milk yield and produced a greater concentration of unsaturated fatty acids in milk.