Pattern of γ-glutamyl transferase activity in cow milk throughout lactation and relationships with metabolic conditions and milk composition

The main objective of this experiment\r\nwas to study the γ-glutamyl transferase (GGT) activity\r\nin milk during lactation and its relationship with\r\nmetabolic status of dairy cows, milk yield, milk composition,\r\nand cheesemaking properties. The study was\r\nperformed in a tied stall barn and involved 20 lactations\r\nfrom 12 healthy multiparous Italian Friesian dairy\r\ncows. During lactation starting at d 10, milk samples\r\nwere collected weekly and analyzed for composition,\r\nsomatic cells count, titratable acidity, and milk coagulation\r\nproperties. The GGT activity was measured in\r\ndefatted samples. Blood samples were collected weekly\r\nto assess biochemical indicators related to energy, protein,\r\nand mineral metabolism, markers of inlammation\r\nand some enzyme activities. The lactations of each cow\r\nwere retrospectively categorized into 2 groups according\r\nto their milk GGT activity value through lactation.\r\nA median value of GGT activity in the milk of all lactations\r\nwas calculated (3,045 U/L), and 10 lactations\r\nwith lower GGT activity were classiied as low while\r\n10 lactations with greater GGT activity were classiied\r\nas high. The average value of milk GGT activity during\r\nlactation was 3,863 and 3,024 U/L for high and low,\r\nrespectively. The GGT activity decreased in early lactation\r\nand reached minimum values in the second month\r\n(3,289 and 2,355 U/L for high and low, respectively).\r\nThereafter GGT activity increased progressively, reaching\r\nvalues in late lactation of 4,511 and 3,540 U/L in\r\nhigh and low, respectively. On average, milk yield was\r\n40.81 and 42.76 kg/d in high and low, respectively, and\r\na negative partial correlation with milk GGT activity\r\nwas observed. A greater milk protein concentration was\r\nobserved in high (3.39%) compared with low (3.18%),\r\nand a positive partial correlation with milk GGT activity\r\nwas observed. Greater titratable acidity in high than that\r\nin low (3.75 vs. 3.45 degrees Soxhlet-Henkel/50 mL,\r\nrespectively) was also observed. Plasma glucose was\r\ngreater in cows of high than in low group, while plasma\r\nurea was lower in the high than in the low group. No\r\nrelationship between plasma GGT and milk GGT activity\r\nwas observed. Our results show an important effect\r\nof lactation stage on milk GGT activity. The individual\r\neffect observed from consecutive lactations and the relationship\r\nbetween milk GGT activity and milk protein\r\nconcentration in healthy cows could open prospects for\r\nGGT as a future tool in improving milk protein content.