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 recommended concentration of 500 µg/L. Canola meal (CM) contain goitrogenic glucosinolates (GLS) that 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 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% baleage and 45% concentrate. Dietary CP and NDF concentrations averaged 18.5 and 35.6%, respectively. Experimental treatments were: 10% soybean meal (SBM) + 110 mg/d of ethylenediamine dihydroiodide (EDDI) or 114 g/d of KM and 12.5% CM + 110 mg/d of EDDI or 114 g/d of KM. Kelp contained 775 mg/kg of I. Data analysis included the main effects of RDP and I source and interaction. Although I intake was greatest in SBM and EDDI diets, the actual difference between treatments 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 x I interaction). Cows fed CM had greater concentrations of plasma thyroid stimulating hormone (TSH), triiodothyronine (T₃), and thyroxine (T₄) than those fed SBM possibly in response to GLS found in CM. Milk I decreased below 500 µg/L only in CM plus KM diet, which was also linked to CM-GLS. The fact that milk I did not drop below 500 500 µg/L with feeding CM plus EDDI was unexpected and requires further investigations. Table 1 (Abstr. M166). Iodine metabolism in Jersey cows

Item	CM			SBM		SEM	P-value
EDDI	KM	EDDI	KM	RDP	I	RDP × I
I intake mg/d	96.3	93.3		95.7	92.8	0.30	<0.001	<0.001	0.83
Milk I, ng/ml	786	471		799	652	56.53	0.009	<0.001	0.02
Milk I, mg/d	16.7	10.2		17.8	15.1	0.58	<0.001	<0.001	<0.001
Plasma TSH, µIU/mL	0.393	0.388		0.396	0.396	0.003	0.01	0.25	0.19
Plasma total T3, ng/mL	1.07	0.98		1.18	1.13	0.09	0.05	0.30	0.81
Plasma total T4, ng/mL	40.5	37.9		46.0	42.7	3.10	0.05	0.26	0.88