ON THE MILK EJECTION MECHANISM IN COWS UPON AN INCREASE IN ONE-TIME MILK YIELD

Abstract

Sympathetic nervous system regulates milk flow through the udder duct system. However, the mechanisms of contraction of the alveoli upon accumulation of different amounts of milk in the udder have not been studied. In the paper, it is found for the first time that the excretion of an increased amount of milk is accompanied by a change in the parameters of the udder blood supply caused by an increase in the contractile response of the alveolar complex. Thus, the goal of the paper is to investigate the contractile reaction of the alveolar complex and lactation in cows with an increased single-milking yield. Nine black-and-white dairy cows of the 2-5th calving were tested, 7 times each, in the first half of lactation (All-Russian Scientific Research Institute of Physiology, Biochemistry and Nutrition of Farm Animals, Kaluga Province). A serial ADU-1 milking machine and a counter sensor (Latvia) were used to record milking parameters. The single-milking yield ranged from 3.4 to 6.7 kg in the control period and was 24.5 % higher (p < 0.001) in the test period. The volumetric blood flow velocity (VBFV) in the udder was assessed by the electromagnetic flowmetry method. A blood flow sensor (Nihon Kohden, Japan) was fixed on the external pudendal artery of the udder. The average VBFV values were recorded for 3 minutes prior to milking (initial period) and in milking. The points characterizing udder blood supply were marked on the VBFV curve, i.e., the beginning of udder irritation, a sharp increase in the blood supply, its maximum and the baseline restoration. The time intervals until a sharp increase in VBFV and an increased blood supply period were calculated. The average and maximum VBFV values and an increase in volumetric blood flow velocity during milking in relation to the initial indicators were determined. The parameters of the udder blood supply were used to assess the contractile response of the alveolar complex. The latent period until milk secretion was determined based on the time from the beginning of udder irritation to a sharp increase in udder blood supply. It was established that the single-milking yield affects the parameters of milking and the udder blood supply. The increased single-milking yield led to a shorter period for removal of the first portion of milk (p < 0.001), an increase in the average intensity (p < 0.001) and maximum intensity (p < 0.001) of milk removal as well as an increase in the machine-milking period (p <0.001). The increased milk secretion was also accompanied by an increase in the udder blood supply. I.e., the period before the udder VBFV increased sharply shortened (p < 0.05), while the average (p < 0.05) and maximum (p < 0.05) VBFV values during milking, as well as period of intensive blood supply to the udder (p < 0.001) increased. The increased blood flow during the test period resulted from an increased contractile response of the myoepithelium and alveoli. The intensive contraction of the alveoli, with the increased contraction amplitude and duration, leads to a shorter latent period. The milk let-down and secretion are under regulation of the sympathetic nervous system. Probably, the excitation of the udder sympathetic nervous system depends on the amount of accumulated milk. The udder tone during milk ejection determines both the alveoli contraction and the rate of milk movement through the milk ducts and its removal through the nipple. The increased udder tone when low filling with milk resulted in a delayed let-down, delayed first portion of milk, low contractile response of the alveoli, and low milk removal. With an increase in the amount of milk in the udder, its tone decreases, resulting in a shorter latent period and faster removal of the first portion of milk, strengthening the contractile response of the alveoli and increasing the intensity of milk removal. Therefore, the latent period of secretion of the first portion of milk can be proposed to characterize the tone of the udder sympathetic nervous system.