Ruminal infusions of cobalt EDTA modify milk fatty acid composition via decreases in fatty acid desaturation and altered gene expression in the mammary gland of lactating cows

Background: Intravenous or ruminal infusion of cobalt ethylenediaminetetraacetic acid (Co-EDTA) or Co-acetate alters milk fat composition in cattle but the mechanisms involved are not known. 
Objective: The present study evaluated the effect of ruminal Co-EDTA infusion on milk fatty acid (FA) composition, mammary lipid metabolism, and mammary lipogenic gene expression. 
Methods: Four cows in mid-lactation and fitted with rumen cannulae were used in a 4 x 4 Latin square with 28 d periods. Co-EDTA was administered in the rumen to supply 0, 1.5, 3.0 or 4.5 g of Co/d over an 18-d interval with a 10 d washout between experimental periods. Milk production was recorded daily and milk FA composition determined on alternate days. Mammary tissue was biopsied on d16 and arterio-venous differences of circulating lipid fractions and FA uptake across the mammary gland measured on d18.
Results: Co-EDTA had no effect on intake, rumen volatile FA or milk production, but caused dose-dependent changes in milk FA composition. Alterations in milk fat composition were evident within 3 d of infusion and characterized by linear or quadratic decreases (P < 0.05) in FAs containing a cis-9 double bond, an increase in 4:0 and 16:0, and linear decreases in milk 8:0, 10:0, 12:0, and 14:0 concentrations. Co-EDTA progressively decreased (P < 0.05) the stearoyl-CoA desaturase (SCD) catalyzed desaturation of FAs in the mammary gland by up to 72%, but had no effect on mammary SCD1 mRNA or protein abundance. Changes in milk FA composition were accompanied by altered expression of specific genes involved in de novo FA and triacylglycerol synthesis. 
Conclusion: Ruminal infusion of Co-EDTA alters milk FA composition in cattle via a mechanism that involves decreases in the desaturation of FAs synthesised de novo or extracted from blood and alterations in mammary lipogenic gene expression, without affecting milk fat yield.