| Summary: | Three experiments were conducted to determine effects of fat supplementation and ruminal starch degradability on lactation performance, and nutrient digestibilities in lactating cows. Four cannulated cows were used in a 4 x 4 Latin square design to determine effects of fat supplementation and ruminal starch degradability on ruminal fermentation and nutrient flow to the duodenum. Treatments were: (1) dry-rolled sorghum (DRS), no added fat; (2) DRS plus 2.5% fat; (3) steam-flaked sorghum (SFS), no added fat; (4) SFS plus 2.5% fat. SFS resulted in higher total VFA, propionate and butyrate, and the lowest C2:C3 ratios compared to DRS when fat was not added to the diets. Diets with SFS had higher ruminal (73.4 vs. 52.1%) and total tract (99.1 vs 95.0%) starch digestibilities then those with DRS. Duodenal flows of total CP and microbial protein were not affected by diets. Fat supplementation increased milk production, ruminal propionate and total VFA concentration and decreased C2:C3 ratio in diets containing DRS. In another, study 40 cows alloted to five dietary treatments: (1) DRS, no added fat; (2) DRS plus 2.5% fat; (3) SFS no added fat; (4) SFS plus 2.5%; (5) SFS plus 5% fat. Milk yield was increased by SFS and fat supplementation (2.5% of DM), the effects being additive. Fat supplementation at 5% did not improve lactation performance but increased body weight gain. Milk composition was not affected by treatments; however, milk protein yield was increased by SFS. Digestibilities of starch were increased (98.2 vs. 92.6%) with SFS compared to DRS. In the third trial, 48 cows were used in a 2 x 3 factorial arrangement of treatments. Diets contained DRS or SFS with 2.5% of cottonseed oil, tallow, or prilled fatty acids. Compared to DRS, SFS did not affect milk production, but increased milk protein content, body weight gains and estimated NE$\sb{\rm L}$ (22.6%) of sorghum grain, regardless of fat source. Fat sources did not affect lactation response, however prilled fatty acids tended to decrease DMI. SFS increased DM, OM, CP, and starch digestibilities, regardless of fat source.
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