N-acetyl-beta-D-glucosaminidase activities, milk somatic cell counts, and blood leukocyte and erythrocyte counts in cows after heat-induced stress or after intravenous administration of adrenocorticotropic hormone.

Six midlactation, nonpregnant cows were subjected to heat-induced stress or to repeated injections of adrenocorticotropic hormone (ACTH). The heat stress (experiment 1) consisted of subjecting the cows to 7 days of fluctuating temperatures (21 C during the day, 32 C during the night). The ACTH-induced stress (experiment 2) was accomplished by giving each of the cows 100 IU of ACTH twice daily for 4 consecutive days. The cows' milk somatic cell counts (MSCC), milk N-acetyl-beta-D-glucosaminidase (NAGase) activity, quarter milk production (ie, milk production per mammary quarter), blood leukocyte counts, and plasma concentrations of NAGase were monitored in both experiments. Although heat stress did not result in significant differences in NAGase, heat stress did significantly (P less than 0.05) decrease milk production. The ACTH administrations resulted in increased milk NAGase activity, MSCC, and blood leukocyte counts, and in decreased plasma NAGase activity and quarter milk production. Therefore, milk NAGase activity and MSCC were not affected by short-term heat stress, but were increased by ACTH-induced stress. Blood erythrocyte concentrations were not affected by heat stress or by ACTH-induced stress.

Variation in N-acetyl-beta-D-glucosaminidase activity and somatic cell count among various milk fractions.

Six Holstein cows with uninfected quarters were quarter machine milked for four consecutive morning milkings. Foremilk, bucket, and stripping fractions were collected for all milkings. For the first three milkings, an additional milk sample was collected 1 h after milking. After the final milking, oxytocin (20 IU) was injected in the tail vein 20 min after milking, and residual milk was collected. All samples were assayed for NAGase activity and somatic cell concentration. Mean NAGase activity for foremilk, bucket, and stripping samples were 1.65, 1.55, and 1.84 loge nmol/min/ml. Hour after milking and residual samples averaged 2.07 and 1.78. Cell counts (loge thousand cells/ml) for foremilk, bucket, and strippings averaged 2.52, 2.91, and 4.30. Hour after and residual samples averaged 4.56 and 5.53. Results suggest that among uninfected cows, foremilk approximates bucket levels of NAGase and cell count.