Feeding Ascophyllum nodosum meal or ethylenediamine dihydroiodide to transition dairy cows: Effects on milk yield and plasma concentrations of thyroid hormones and markers of metabolic status.

Ascophyllum nodosum (ASCO) is a brown seaweed rich in I and bioactive compounds. Previous research demonstrated that feeding incremental amounts of ASCO meal linearly decreased the plasma concentrations of nonesterified fatty acids (NEFA) and cortisol in lactating dairy cows, suggesting that ASCO may improve cows' metabolic status during the transition period. We aimed to investigate the effects of supplementing transition dairy cows with ASCO meal or ethylenediamine dihydroiodide (EDDI) on plasma concentrations of thyroid hormones and markers of metabolic status, DMI, and milk yield and composition. Forty pregnant Holstein cows averaging (mean ± SD) 3.2 ± 1.1 lactations were blocked by expected calving date and assigned to 1 of 4 treatments 28 d before parturition through 28 DIM as follows: 1) control (CON): 11 mg/d of EDDI to meet the NRC (2001) recommended I concentration of 0.5 mg of I/kg of DMI (actual I = 0.70 mg/kg of DMI), 2) low seaweed (LSW): CON plus 57 g/d of ASCO meal (3.13 mg of I/kg of DMI), 3) high seaweed (HSW): CON plus 113 g/d of ASCO meal (5.52 mg of I/kg of DMI), and 4) high EDDI (HEDDI): CON plus 124.8 mg/d of EDDI to match the I concentration of the HSW diet (5.55 mg of I/kg of DMI). All cows were fed the same pre- and postpartum basal TMR with forage:concentrate ratios of 72:28 and 56:44, respectively. The I sources were top-dressed into the TMR, manually mixed to ensure uniformity, and offered once daily during the morning feeding. Blood samples were collected once weekly prepartum and on d 0, 1, 7, 14, 21, and 28 postpartum. Preplanned, nonorthogonal contrasts were used to compare treatment effects using ANOVA, with df partitioned into linear and quadratic effects of ASCO meal supplementation and I source (HSW vs. HEDDI diet). Prepartum BW, BCS, and plasma concentrations of BHB, glucose, insulin, and thyroid hormones did not change linearly or quadratically in response to ASCO meal supplementation or I source. Whereas plasma concentration of NEFA linearly increased in cows fed graded levels of ASCO meal, it was not affected by I source. Dry matter intake, yields of milk and milk fat and lactose, and milk fat and true protein concentrations were not affected linearly or quadratically by ASCO meal. However, milk lactose concentration and true protein yield tended to respond quadratically. Cows fed HSW tended to have lower DMI than those fed HEDDI. Contrarily, milk fat concentration tended to increase and that of MUN increased in HSW versus HEDDI cows. Total I intake and milk I concentration and yield linearly increased with ASCO meal supplementation but did not differ between I source. Postpartum plasma concentrations of NEFA, BHB, glucose, insulin, and thyroid hormones were not impacted linearly or quadratically by ASCO meal. In contrast, HSW cows had greater plasma free triiodothyronine and lower total thyroxin concentrations than HEDDI cows. Overall, ASCO meal linearly increased prepartum concentration of plasma NEFA without improving metabolic health status or milk yield during the transition period.