Supplemental Tables for JDS.2024-25731

The aim of our study was to assess the effects of low or high-starch diets with or without palmitic acid (C16:0) supplementation on the yield of milk, milk components, and energy partitioning of primiparous and multiparous dairy cows between mid and late-lactation. Thirty-two Holstein cows, 12 primiparous ([mean ± SD] 163 ± 33 d in milk) and 20 multiparous ([mean ± SD] 179 ± 37 d in milk), were used in a split-plot Latin square design. Parity was considered the main plot, and within each plot, treatments were then randomly assigned in a replicated 4 × 4 Latin square with 21 d periods and balanced for carryover effects. Treatments were in a 2 × 2 factorial arrangement of dietary starch level and fatty acid (FA) supplementation as main factors. The starch factor comprised either low (LS; 16% of diet dry matter [DM]) or high (HS; 32% of diet DM) dietary starch. Dietary starch was altered by varying the proportion of ground corn, whole cottonseed, and soyhulls, with LS and HS diets containing ~15 and 18% forage neutral detergent fiber (fNDF) and ~ 37 and 25% neutral detergent fiber (NDF), respectively. The FA supplementation factor consisted of either control with no FA supplementation (CON) or a C16:0-enriched supplement at 1.5% of diet DM replacing soyhulls (PALM). Compared with LS, HS increased plasma insulin (0.20 μg/L) and energy partitioning to body reserves (2.04 percentage-units), tended to increase body condition score (BCS) and body weight (BW) change, and decreased dry matter intake (DMI; 1.10 kg/d), milk fat yield (0.04 kg/d) and milk urea nitrogen (MUN; 2.10 mg/dL). Compared with CON, PALM increased energy output to milk (1.10 Mcal/d), the yields of milk fat (0.10 kg/d), 3.5% fat-corrected milk (FCM; 1.90 kg/d) and energy corrected milk (ECM; 1.60 kg/d), and had no effect on BW-related variables. We observed a three-way interaction between parity, starch, and FA supplementation affecting multiparous but not primiparous cows, where PALM increased plasma insulin concentration (0.14 μg/L) in LS but had the opposite effect in HS by tending to decrease insulin. Our results demonstrate important interactions between parity, starch levels, and FA supplementation. Overall, feeding a low-starch diet to mid and late-lactation dairy cows helped prevent excessive body condition and weight gain while increasing milk fat yield compared to a high-starch diet. Multiparous cows may benefit the most from a low-starch diet as multiparous cows seemed more prone to accumulating body reserves. Feeding a C16:0-enriched supplement increased milk fat yield and ECM without increasing weight gain, regardless of starch level or parity.