Highly yielding cows, as compared to low productivity ones, use their body energy reserves more intensively for milk secretion and have more lasting negative energy balance. This can result in premature deterioration of their reproductive function, health disorder and productivity lost if the condition is not overcome. Metabolism and rumen digestion condition in highly productive cows depends on quantity and quality of food consumed during this period, its physical form, regular and simultaneous consuming the nutrients and bioactive substances.

There is lack of micronutrients in highly productive cows feed ration in the Forest-Steppe zone of Ukraine. Thus, sulphate and chloride salts are as used to compensate the lack. The salts, due to their low absorption, form insoluble complexes in the intestine, which results in low minerals assimilation of trace elements which makes only 12–23.5 %, thereby increasing their content in faces, urine and causes environmental pollution.

In this regard, of particular interest are microelements of organic origin and, particularly chelate complexes – compounds of metal with lysine and methionine amino acids. Microelements of these compounds are absorbed well in the gut and are deposited in tissues with no dissociation, they can be easily included into metabolic active forms.

Therefore, studying mixed ligand complexes of cobalt, which is a part of vitamin B₁₂ and is an activator of many enzymes and hormones as well as determining its optimal dose for high-yielding cows is rather important, especially in the first 100 days of lactation.

The research aimed at studying the effect of cobalt mixed ligand complexes in combination with zinc sulphate, copper and sodium selenite on rumen metabolism nature in high-yielding cows, which allows to determine the optimal level of this element in the first 100 days of lactation.

Research farming experiment on the effect of different mixed ligand complexes of cobalt was conducted on Ukrainian black-and-white dairy cows.

Five groups of cows, each amounted 10 animals, were formed on the analogue principle to do the experiment.

Feeding cows in experimental research and preparatory period was done on the same ration. The difference in feeding was that within 80 days of the research period, of the control group cows were fed with the preparation period premix, which was composed of zinc sulphate, copper, cobalt and sodium selenite and experimental groups cows were fed cobalt mixed ligand complexes instead of cobalt sulphate.

Using Cobalt microelement in various doses in the rations affected the metabolic processes in the high-yielding cows rumen.

The obtained results on pH level show that in  research groups high-yielding cows receiving mixed ligand complexes of cobalt, despite the high level of concentrate feed in the structure of their rations had maximum pH level within 7.16–7.21 and the difference was statistically significant (p<0.01) as compared with the animals in the control group. pH level in 1st control and 2nd experimental groups cows the ration of which contained cobalt sulphate was below the optimum made 6.84–6.95. This is because most organic compounds of Cobalt rumen micro-organisms are used by the microorganisms faster for the synthesis of vitamin B₁₂ and the ration nutrients are used better by the animal body in the rumen metabolism.

Use mixed ligand complexes of cobalt in cows rations of the research groups resulted in higher specific activity of microorganisms. Providing animals with mineral components and calorious components of micro-organisms volatile fatty acids caused the conditions for more complete content scar intake of nitrogen metabolism metabolites.

Кey words: high-yielding cows, premix, microelements, chelates, Cuprum, Zink, Cobalt sulfuric acid salts Cobalt mixed ligand complex, rumen metabolism.