Title:

Interactions between iodine and protein sources: Effects on milk iodine and thyroid hormones in Jersey cows

Kelp meal (KM) is a supplement made from the brown seaweed Ascophyllum nodosum known to bioaccumulate iodine (I). However, feeding KM may result in milk I above the recommend concentration of 500 µg/L. Canola meal (CM) contain goitrogenic glucosinolates (GLS) known to affect I distribution in tissues through competitive inhibition of iodide transportation by the Na-iodide symporter. Our goal was to assess the interactions between 2 sources of I and RDP on I metabolism in dairy cows. Sixteen organic-certified multiparous Jersey cows (93 ± 58 DIM) were used in a factorial replicated 4 × 4 Latin square design (21-d periods). Diets were formulated to yield similar concentrations of CP and I and consisted (DM basis) of 55% of grass-legume baleage and 45% ground corn-based concentrate. Mean dietary CP and NDF concentrations were 18.5 and 35.6%, respectively. Experimental treatments were: 10% soybean meal (SBM) + 110 mg of EDDI or 114 g of KM and 12.5% CM + 110 mg of EDDI or 114 g of KM. Kelp meal contained 775 mg/kg of I. Data analysis included the main effects of RDP and I source, and the RDP by I source interaction. Although I intake was greatest in SBM and EDDI diets, the actual difference between treatment was small. The drop in the concentration and yield of milk I was more pronounced in cows fed CM plus KM than CM plus EDDI (RDP by I source interactions). Cows fed CM showed greater concentrations of plasma TSH, total T3, and total T4 compared with those fed SBM. These responses may have been linked to GLS present in CM. Overall, milk I decreased below 500 µg/L only in cows fed CM plus KM, which was likely caused by the presence of GLS in CM. The lack of effect of CM when fed together with EDDI was unexpected and requires further investigations.

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