Forty-eight multiparous cows were used in a randomized complete block design experiment with a 2×2 factorial arrangement of treatments to determine the interaction between a highly saturated free FA supplement (SFFA) and dietary forage NDF (fNDF) content on energy balance and metabolic responses in postpartum cows. Treatment diets were offered from 1 to 29 d postpartum and contained 20 or 26% fNDF and 0 or 2% SFFA (Energy Booster 100; 96.1% FA: 46.2% C18:0, and 37.0% C16:0). Overall, low fNDF versus high fNDF and 2% SFFA versus 0% SFFA increased digestible energy intake (DEI; 67.5 vs. 62.2 Mcal/d and 68.1 vs. 61.6 Mcal/d, respectively). The low fNDF diet with SFFA increased energy balance compared with the other treatments early during the treatment period, but treatment differences diminished over time. Overall, low fNDF versus high fNDF diets and 2% SFFA versus 0% SFFA improved energy balance (-13.0 vs. -16.3 Mcal/d and -12.0 vs. -17.3, respectively) decreasing efficiency of utilization of DEI for milk (milk NEL/DEI; 0.575 vs. 0.634 and 0.565 vs. 0.643). Low fNDF diets increased plasma insulin (308 vs. 137µg/mL) and glucose concentrations (50.5 vs. 45.7mg/dL) and decreased plasma nonesterified FA (606 vs. 917µEq/L) and β-hydroxybutyrate (9.29 vs. 16.5mg/dL) concentrations and liver triglyceride content. Compared with 0% SFFA, 2% SFFA decreased plasma nonesterified FA concentration during the first week postpartum (706 vs. 943µEq/L) and tended to decrease plasma nonesterified FA overall throughout the treatment period, but did not affect liver triglyceride content. During a glucose tolerance test, 2% SFFA increased plasma insulin concentration more in the low fNDF diet (84.5 vs. 44.6µIU/mL) than in the high fNDF diet (40.4 vs. 38.0µIU/mL). After glucose infusion, 2% SFFA increased insulin area under the curve by 64% when included in the low fNDF diet, but only by 5.2% when included in the high fNDF diet. Even though 2% SFFA did not affect weekly plasma insulin concentration, it increased plasma insulin baseline concentration before the tolerance tests. Supplementation of 2% SFFA and low fNDF diets increased DEI and improved energy balance, but decreased apparent efficiency of utilization of DEI for milk production. Fat supplementation affected energy partitioning, increasing energy balance and decreasing body condition score loss, especially in the lower fNDF diet. The decrease in body condition score loss observed was likely related to an increase in plasma insulin concentration. Feeding SFFA in a low fNDF diet during the first 29 d postpartum might have primed the cows to limit fat mobilization at the expense of milk.
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