This study was carried out in order to determine the influence of nitrogen excess in rumen and deficit of digestible organic matter in a feed ration for dairy cows on hypomagnesaemia at the beginning of green forage feeding. The balance of nitrogen and energy metabolism in rumen was determined using the rules of evaluation of the nutritional value of fodder (PDI system) according to the Institut National de la Recherche Agronomique (INRA), taking into consideration a potential protein synthesis from nitrogen by microorganisms in rumen (MPRN) and microbial protein synthesized from digestible organic matter (MP(RE)). It was found that an increase in the amount of rumen degradable protein (RDP) and MPRN-MPRE balance leads to a significant increase of urea nitrogen in serum (p < 0.05). An increase of total MPRN-MPRE balance in rumen results in a decrease of Mg concentration in serum. It has been demonstrated that in cows fed green forage at the end of spring (2nd half of May to 1st half of June) when the MPRN-MPRE balance is not higher than 187 g, adding 7 g or 25 g of MgO/cow/day to the same extent successfully prevents hypomagnesaemia.