Effects of Precision Dietary Protein Reduction on Lactational Performance, Milk Fatty Acid Profile, and Antioxidant Status in Mid-Lactation Dairy Cows.

This study evaluated the effects of precision dietary crude protein (CP) reduction through corn dust substitution for soybean meal on lactational performance, milk fatty acid composition, and antioxidant status in mid-lactation dairy cows. Six Holstein Friesian cows were assigned to a replicated 3 × 3 Latin square design with three 28-day periods and fed total mixed rations containing 17.40% (TMR17), 16.29% (TMR16), or 15.18% CP (TMR15). Forage was maintained at 40% of dietary dry matter. Dry matter intake and plasma metabolites were unaffected by dietary treatment (p > 0.05). However, apparent digestibility of dry matter, organic matter, and crude protein declined significantly (p < 0.05), while acid detergent fiber digestibility showed a linear downward tendency (p = 0.06). Actual milk yield, lactose yield, and solids-not-fat (SNF) yield were unaffected by treatments (p > 0.05), but milk protein percentage decreased linearly (p = 0.01) as dietary CP declined. Body weight change also tended to decline (p = 0.09), suggestive of possible increased body reserve mobilization, although this was not confirmed with body condition score or direct metabolic indicators. Milk urea nitrogen decreased linearly (p = 0.05), which suggested improved nitrogen utilization. Plasma superoxide dismutase (SOD) activity, GSH-Px activity, and total antioxidant capacity were not significantly modified (p > 0.05); however, plasma DPPH radical scavenging capacity increased linearly with higher protein levels (p = 0.01; Trt: p = 0.02). Conversely, TMR15 improved milk lipid quality by significantly increasing monounsaturated fatty acids (p = 0.03; Trt: p = 0.05), particularly oleic acid (p = 0.05), whereas total saturated fatty acids remained unchanged (p = 0.55). Overall, reducing dietary CP improved MUFA proportions and nitrogen efficiency but decreased milk protein percentage and plasma DPPH antioxidant protection, highlighting a trade-off between production performance, animal health, and milk quality.