We investigate the effects of the substitution of grazing time by grass silage and two levels of supplements with concentrates, on two milk cows during spring grazing, on the production and chemical com position of milk, intake of nutrients, nitrogen balance, the model of ruminal fermentation and profitability. Twenty dairy cows were divided into 4 groups of 5 each, in the following treatments: short‑time grazing [12 hours of grazing, PP + (SG)] + 2 kg of concentrate + silage grass (boles); short‑time grazing [12 hours of grazing, (SG)] + 4 kg of concentrate + silage grass (boles); ful‑­time grazing [19 hours of grazing, (FG)] + 2 kg of concentrate + 2 kg of dry matter silage maize; and [19 hours of grazing, (FG)] + 4 Kg of concentrate + 2 kg of dry matter silage maize. The results showed a lower milk production (P < 0,001) when grazing time is reduced and is replaced by grass silage, but the percentage of fat, the protein, the urea in the milk and non‑fat solids (P < 0,001) increase with no differences in the chemical composition of milk depending on the quantity of concentrate given. The concentrations of far, protein and non‑fat solids were higher with 4 kg in full‑time grazing (P < 0,05).The highest consumption of dry matter took place in the short‑time grazing (P < 0,001). The efficiency of the metabolic energy taken with respect to the production of gross energy in milk as well as in the nitrogen was higher in full-time grazing (P < 0,001) and lower with 4 kg concentrate (P < 0,001). The concentrations of N‑Ammoniacal, Propionic, Butyric were higher in full‑time grazing (P < 0,001), with no important differences in N‑ammoniacal with 2 or 4 kg of concentrare. The cost of the production for the liter to be become the highest when limit the wheather for the grazing for silage grass; the same betewen doge to concentrate a both sistems is more chepear than with 4 kg.