Dietary starch levels modulate production performance and whole-body nutrient metabolism in dairy cows

The objective of this study was to evaluate the effects of increasing dietary starch concentrations on production performance and whole-body nutrient metabolism in lactating dairy cows. Twelve multiparous Holstein cows at mean (±SD) 89 ± 13 DIM were assigned to a replicated 3 × 3 Latin square design comprising three 21-d periods. Each period consisted of 14-d dietary adaptation followed by 7 d of data and sample collection. Treatments consisted of low-starch (LS; 9.66% of DM), medium-starch (MS; 20.6% of DM), and high-starch (HS; 29.1% of DM) diets. Crude protein concentrations were 15.9%, 14.7%, and 14.7%, and NDF concentrations were 47.9%, 38.3%, and 36.6% for the LS, MS, and HS diets, respectively. Dry matter intake and milk yield and composition were measured throughout the sampling week. Gas exchange including CH₄, O₂, and CO₂ were measured using a head chamber (GreenFeed system) on the last day of the sampling period for the calculation of respiratory quotient, net carbohydrate oxidation (COX_net), net fat oxidation (FOX_net), and metabolic heat production (mHP). Least squares means and SEM are presented in the sequence of LS, MS, and HS. Increasing dietary starch concentration linearly increased net energy intake (38.9 vs. 40.3 vs. 40.7 ± 0.87 Mcal/d) and milk yield (33.5 vs. 36.6 vs. 38.9 ± 1.24 kg/d), and tended to linearly increase ECM yield (39.1 vs. 43.5 vs. 43.1 ± 1.50 kg/d), without a concurrent increase in DMI or BW loss, resulting a tendency for improved feed efficiency (ECM/DMI; 1.62 vs. 1.77 vs. 1.78 ± 0.06 kg/kg) in dairy cows. Yield of milk true protein increased linearly (1.17 vs. 1.37 vs. 1.34 ± 0.06 kg/d), whereas MUN concentrations decreased linearly (18.1 vs. 12.9 vs. 10.4 ± 0.56 mg/dL) with increasing dietary starch. Respiratory quotient tended to respond quadratically, remaining similar in LS and MS but higher in HS-fed cows (0.90 vs. 0.90 vs. 0.94 ± 0.01), indicating a shift in substrate oxidation toward greater carbohydrate utilization. Correspondingly, increasing dietary starch concentration linearly increased COX_net, both in absolute (5,432 vs. 5,848 vs. 7,276 ± 366 g/d) and metabolic BW (mBW) adjusted terms (39.0 vs. 41.6 vs. 52.0 ± 2.6 g/kg mBW). In contrast, FOX_net responded quadratically, peaking in MS both in absolute (742 vs. 906 vs. 450 ± 150 g/d) and mBW-adjusted terms (5.28 vs. 6.42 vs. 3.29 ± 1.07 g/kg mBW). Consistently, mHP increased linearly with starch concentration, both in absolute (34.8 vs. 37.4 vs. 38.1 ± 0.73 Mcal/d) and mBW-adjusted terms (0.25 vs. 0.26 vs. 0.27 ± 0.01 Mcal/kg mBW), indicating greater energy expenditure. Overall, these findings indicate that increasing dietary starch shifted whole-body substrate oxidation toward carbohydrates and improved energy utilization and feed efficiency in mid-lactation dairy cows.