The objective of this experiment was to measure ruminal and lactational responses of Holstein dairy cows fed diets containing 3 different starch levels: 17.7 (low; LS), 21.0 (medium; MS), or 24.6% (high; HS). Twelve multiparous cows (118 ± 5 d in milk) were assigned randomly to dietary treatment sequence in a replicated 3 × 3 Latin square design with 3-wk periods. All diets were fed as total mixed rations and contained approximately 30.2% corn silage, 18.5% grass silage, and 5.0% chopped alfalfa hay. Dietary starch content was manipulated by increasing dry ground corn inclusion (% of dry matter) from 3.4 (LS) to 10.1 (MS) and 16.9 (HS) and decreasing inclusion of beet pulp and wheat middlings from 6.7 and 13.4 (LS) to 3.4 and 10.1 (MS) or 0 and 6.8 (HS). In vitro 6-h starch digestibility of the diet increased as nonforage sources of fiber replaced corn grain (% of dry matter; 73.6, HS; 77.3, MS; 82.5, LS) resulting in rumen-fermentable starch content by 14.6, 16.2, and 18.1% for the LS, MS, and HS diets, respectively. Diets had similar neutral detergent fiber from forage and particle size distributions. Dry matter intake, solids-corrected milk yield, and efficiency of solids-corrected milk production were unaffected by diet, averaging 26.5 ± 0.8, 40.8 ± 1.6, and 1.54 ± 0.05 kg/d, respectively. Reducing dietary starch did not affect chewing time (815 ± 23 min/d), mean ruminal pH over 24h (6.06 ± 0.12), acetate-to-propionate ratio (2.4 ± 0.3), or microbial N synthesized in the rumen (585 ± 24 g/d). Total tract organic matter digestibility was higher for HS compared with MS and LS diets (69.2, 67.3, and 67.0%, respectively), but crude protein, neutral detergent fiber, and starch digestibilities were unaffected. As dietary starch content decreased, in vitro ruminal starch fermentability increased and, consequently, the range between HS and LS in rumen-fermentable starch (3.5 percentage units) was less than the range in starch content (6.9 percentage units). Under these conditions, dietary starch content had no measurable effect on ruminal fermentation or short-term lactational performance of high-producing Holstein dairy cows.
