Effect of different levels of supplied cobalt on the fatty acid composition of bovine milk.

In previous studies, administration of high amounts of Co decreased the proportion of MUFA in bovine milk. The present study was conducted to examine the amount of Co needed to obtain this effect. High-yielding dairy cows (n 4), equipped with ruminal cannulas, were used in a 4 × 4 Latin square design study. The basal diet consisted of concentrate mixture (9 kg/d) without added Co and grass silage (ad libitum). The following four levels of Co were administrated as cobalt acetate dissolved in distilled water: no Co (treatment 1, T1); 4·0 mg Co/d (T2); 380 mg Co/d (T3); 5300 mg Co/d (T4). Each period lasted for 18 d, including 11 d of treatment. During the treatment periods, the solutions were continuously infused into the rumen. Milk yield and milk concentration of fat, fatty acids (FA), protein, lactose, Co, Zn, Fe and Cu were determined. Blood plasma was analysed with respect to FA, Co, Zn, Fe and Cu. Feed intake and total tract digestibility of feed components were also determined. There was a linear effect of increasing the level of Co on milk FA composition. The effects of Co on FA composition in blood were insignificant compared with the effects on milk. In milk fat, the concentration of cis-9-18 : 1 was reduced by as much as 38 % on T4 compared with T1. Feed intake and milk yield were negatively affected by increasing the Co level.