This study aimed to investigate the interaction between dietary starch and fatty acid (FA) supplementation on production responses in multiparous Holstein cows. Sixty multiparous Holstein cows were used in a randomized complete block design and assigned to one of 4 treatments in a 2 × 2 factorial arrangement. Treatment diets were fed from 1 to 23 d postpartum (fresh period; FR): (1) a diet containing 22% DM starch and no supplemental FA (LS); (2) a diet containing 28% DM starch and no supplemental FA (HS); (3) diet containing 22% DM starch and a calcium salt containing 70% C16:0 and 20% cis-9 C18:1 (LS+FA); and (4) a diet containing 28% DM starch and a calcium salt containing 70% C16:0 and 20% cis-9 C18:1 (HS+FA). The starch concentration of the treatment diets was altered by partially replacing ground corn with soyhulls and soybean meal. The FA supplement was added at 2.6% of diet DM to deliver approximately 2.0% of FA and replaced soyhulls in the FA-supplemented diets. During the carryover period (CO; 24–44 d postpartum), all cows were offered a common diet (27.8% starch and 3.05% FA of DM) to evaluate the carryover effects of the treatment diets. During the FR period, dietary starch and FA supplementation interacted with time to impact DMI, plasma BHB, and tended to interact to impact ECM yield and milk lactose content. The starch × FA interaction affected the yields of milk, milk fat, milk lactose, 3.5% fat-corrected milk (FCM), ECM, and tended to affect milk protein yield and FCM/DMI. Based on pairwise comparisons among treatment combinations, HS increased milk and lactose yields compared with the other treatments. The HS and LS+FA diets increased milk fat yield compared with LS, and HS increased 3.5% FCM and ECM yields compared with LS, whereas LS+FA tended to increase 3.5% FCM yield compared with LS. Overall, increasing dietary starch increased milk and lactose yields, tended to increase DMI, and reduced plasma BHB concentration. Supplemental FA increased FCM/DMI, and plasma NEFA and BHB concentrations; reduced DMI, BW, plasma insulin and glucose concentrations; and tended to increase ECM/DMI and BCS loss. During the CO, increasing dietary starch during FR increased milk lactose yield, tended to increase milk yield, and milk lactose content, while FA supplementation reduced BW, tended to reduce milk lactose content, and tended to increase BCS loss. In conclusion, the effect of FA supplementation on milk production of early-lactation cows depended on dietary starch level. Additionally, feeding high-starch diets during the fresh period improved milk yield, with effects that were partially carried over into established lactation, indicating that early fresh-period diet can influence performance in the weeks thereafter.

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Lock, A., J. Parales-Girón, A. Benoit. 2025. Effects of Dietary Starch and Fatty Acid Supplementation on Milk Production and Metabolic Responses During the Immediate Postpartum in Dairy Cows. Journal of Dairy Science. DOI: 10.3168/jds.2025-27561