AMINO ACID COMPOSITION OF FIRST-CALF COWS' MILK PROTEIN
The purpose of research is to study the amino acid composition of the milk protein of first-calf heifers of different genotypes in the conditions of Western Kazakhstan. The study was conducted in LLP AF Akas of the West Kazakhstan Region of the Republic of Kazakhstan. From the number of first-calf heifers, according to the principle of groups-analogues, taking into account the origin, live weight, physiological state, 5 groups of animals were formed 12 animals in each. I group - black-and-white, II group - Holstein of German selection, III group - Holstein of Dutch selection, IV group - 1/2 Holstein of German selection × 1/2 black-and-white, V group - 1/2 Holstein of Dutch selection × 1/2 black-and-white. In first-calf heifers of all genotypes, such non-essential amino acids as glutamic acid, proline, aspartic acid and serine differed in high content in milk. The concentration of glycine, alanine and tyrosine was significantly lower. At the same time, first-calf heifers of the Black-and-White breed of group I were inferior to their peers of groups II-IV in terms of the content of alanine in milk by 1.28-0.40 g/kg (25.45-36.36 %), glycine by 0.18-0.30 g/l (25.35-42.25 %), serine - by 0.12-0.26 g/l (5.82-12.62 %), terosine - by 0.14-0.19 g/l (10.53-14.29 %), but exceeded them in the concentration of glutamic acid by 0.06-0.24 g/l (0.81-3.33 %) and proline by 0.08-0.27 g/l (2.86-10.34 %). The milk protein of first-calf heifers of the Black-and-White breed of the I group differed in the minimum value, the maximum - in the crossbreeds of the IV and V groups, the purebred Holsteins of the II and III groups occupied an intermediate position. Thus, animals of the Black-and-White breed of group I were inferior to Holstein first-calf cows of German selection of group II in terms of amino acid index by 1.15 %, Holsteins of Dutch selection of group III - by 2.30 %, crossbreeds of IV and V groups, respectively - by 5.75 and 5.75 %. Crossbreeds of groups IV and V exceeded the value of the amino acid index of first-calf heifers of Holstein breed of German selection of group II by 4.55 %, Holstein of Dutch selection of group III - by 3.37 %, which is due to the manifestations of the effect of heterosis on the analyzed trait in crossbred animals. Judging by the amino acid composition, milk proteins of cows of all genotypes were distinguished by high nutritional and biological value. Crossing cows of black-and-white breed with Holsteins of foreign selection contributed to the improvement of these traits, as evidenced by both the content of essential amino acids in the protein and the value of the amino acid index.