Substitution of starch by soluble fiber and Saccharomyces cerevisiae dose response on nutrient digestion and blood metabolites for precision-fed dairy heifers.

The objective of this experiment was to determine the effects of 2 levels of dietary starch and the dose at which the effects of yeast culture (YC) derived from Saccharomyces cerevisiae (Yea-Sacc¹⁰²⁶, Alltech Inc., Nicholasville, KY) were maximized based on nutrient total-tract digestibility (AD), N utilization, and blood metabolites of precision-fed dairy heifers. A split-plot design with starch level as the whole plot and YC dose as subplot was administered in a 4-period (21 d), 4 × 4 Latin square. Eight Holstein heifers (432.49 ± 6.81 kg of body weight) were allocated to 2 starch treatments (28% starch, high starch, HS, or 17% starch, low starch, LS) and to a sequence of YC doses (0, 10, 30, and 50 g/d). Dry matter (DM) and neutral detergent fiber (NDF) AD were not different between HS and LS; however, HS decreased acid detergent fiber (ADF) and increased hemicellulose AD. Digestibility of DM and organic matter (OM) increased quadratically in response to increasing YC dose. Hemicellulose, NDF, and ADF AD increased or tended to increase quadratically with increasing YC dose. No significant effects were noted on fecal or urine output between dietary starch concentrations; YC decreased wet and dry fecal output corresponding to the effect in DM and OM. Apparent N digestibility was greater in the LS group. As YC dose increased, fecal N output decreased quadratically and was lowest at 30 g/d YC. A corresponding quadratic increase was observed for N retention expressed as a percentage of N digested: N output in urine tended to increase with increasing YC dose, resulting in no differences in retained N (g/d). Dietary starch concentration did not affect blood glucose, triglyceride, creatinine, or lactate concentration. However, HS increased plasma urea N concentration. Glucose concentration tended to increase quadratically with daily YC dose in both starch treatments, with the greatest response at 30 g/d. For triglycerides, dietary starch concentration and YC dose interacted, decreasing quadratically in the LS group and increasing in the HS group (lowest and highest value for 10 g/d respectively). We observed a significant time effect for all blood metabolites measured. We conclude that starch level did not affect DM AD, but influenced ADF and hemicellulose AD. Yeast culture had the greatest effect on DM, NDF, ADF, and hemicellulose AD when added at 30 g/d. Addition of YC influenced glucose and triglyceride concentrations differently according to the dietary starch concentration of the diet.