About the connections of crude protein, metabolic energy and the daily yield of milk protein with the acid-base state of lactating cows.
An experiment was carried out with four Estonian Holstein breed cows supplied with rumen fistulas using the principle of the 4×4 Latin square. The animal researchers investigated the intake of silage when 25, 40, 55 or 70% of the test cows’ metabolic energy was covered with concentrated feed and Lucerne silage was fed ad libitum.
We investigated the influence of the rations in the same experiment on the blood’s acid-base state of lactating cows acid-base state (ABS). In the rations when the importance of the concentrated feed grew from 25% to 70% (R 25%… R 70%), the average amount of crude protein grew from 1490 g to 1725 g and the average metabolic energy from 81.7 to 118.5 MJ (P<0.05). At the same time the average ratio between crude protein and metabolic energy decreased from 18.2 to 14.5 (P<0.05).
Metabolic load is also reflected by the excreted acid and base residues of metabolism. In the case of the rations R 40%, R 55% and R 70% the excretion of bases remained in the limits of 1100–1200 mmol/8h. At the same time, the excretion of acids began to increase and in the case of the last ratio – R70% – exceeded the initial value and the ratio of bases and acids in urine was the lowest. When consuming R 55%, in urine the content of bases was the lowest and in blood the content of the carbonic acid’s anion (HCO3) was the highest, i. e. in the kidneys intensive regeneration of the carbon acid’s anion was taking place.
When feeding according to the fourth – R 70% – ration, the blood’s bicarbonate buffer system is not sufficient to neutralize the acid metabolic residues and the blood’s pH begins to fall and reaches μ–1σ level. The excretion of acids with urine is the highest at the same time: Polynomial regressions were found between the acids content in urine and the blood’s pH, between the content of carbon acid’s anion and the excess of buffer bases. From this prognosis model it became evident that the blood’s pH values varied in narrow limits (μ±1σ) which speaks about the strong regulation of the given indicator. In the case of the first three rations the average acid contents in urine in different rations reached 71–86 mmol/l and the growth of the blood’s pH, HCO3 and BE was taking place parallelly, which means that in the ABS an alkaline shift appeared – in blood the concentration of H+ ions decreased and the content of carbon acid’s anion and buffer base excess increased.
As the significant changes in the partial pressure of carbon dioxide in blood were not seen, the above-mentioned alkaline shift is of metabolic appearance mechanism. In feeding according to the fourth ration (R 70%), the acids content in urine grew more radically and the average reached 115 mmol/l. The individual values of carbon acid’s anion in blood fell lower the μ–1σ level. It is possible to conclude that here the compensated alkalosis ends and in feeding the fourth ration metabolic acidosis begins to appear. The daily production of milk protein, being a factor characterizing the metabolic load of lactating cows, also correlated with the increase renal excretion of acid metabolic resides. When the daily yield production of milk protein increased, the ratio of acids and bases excreted by urine decreased significantly.
As the blood’s ABS indicators varied during the experiment in the limits of μ±1σ, it is more correct to use this interval for the indicators of the reference interval of ABS than the classical μ±2σ.
Keywords: crude protein, metabolic energy, ratio between protein and energy ,the blood’s acid-base state, pH, pCO2, HCO3-, the blood’s buffer bases excess, reference interval, milk protein, the content of acids in urine and excretion, the bases of urine and excretion, metabolic alkalosis, metabolic acidosis.