Effects of including a palmitic acid-enriched supplement in low- and high-starch diets on milk production and energy partitioning of primiparous and multiparous dairy cows between mid and late lactation.

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 (163 ± 33 DIM; mean ± SD) and 20 multiparous (179 ± 37 DIM; mean ± SD), 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 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 NDF (fNDF) and ∼37% and 25% 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 BCS and BW change, and decreased DMI (1.10 kg/d), milk fat yield (0.04 kg/d), and MUN (2.10 mg/dL). Compared with CON, PALM increased energy output to milk (1.10 Mcal/d) and the yields of milk fat (0.10 kg/d), 3.5% FCM (1.90 kg/d), and ECM (1.60 kg/d), and had no effect on BW-related variables. We observed a 3-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 with a high-starch diet. Multiparous cows may benefit the most from a low-starch diet because 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.