[Effect of adrenaline and phenylephrine on milk extraction during the mechanical milking of cows]. / Effets de i'adrénaline et de la phényléphrine sur l'extraction du lait au cours de la traite mécanique de la vache.

During mechanical milking in the morning between days 60 and 130 of lactation, 6 or 10 French Friesian Pie-Noire (FFPN) cows were given an intrajugular injection of 0.3 mg of adrenaline or 5 mg of phenylephrine (table 1). The treatments, each given 7 times to each cow, were alternated every other day with a control injection of 0.9% NaCl. The adrenaline caused: - a non-significant decrease in total milk yield (-4.9%: 11.57 l vs 12.17 l with NaCl) and in the volume of machine-stripped milk (0.360 l vs 0.430 l with NaCl); - an increase (+8.7%) in milk flow (1.165 l/min vs 1.072 l/min with NaCl; table 4). Phenylephrine caused a significant 10% reduction in total milk yield (13.35 l vs 14.84 l with NaCl) but had no effect on the volume of machine-stripped milk (0.235 l vs 0.295 l with NaCl) or on milk flow (0.961 l/min vs 0.962 l/min with NaCl; table 3). In agreement with our previous results (Bernabé and Peeters, 1980; Bernabé and Ricordel, 1985), beta-receptor (adrenaline) stimulation appeared to increase milk flow by relaxing the teat muscles. However, when contact was avoided between the teats and the milking machine, the stimulation of teat alpha-receptors (phenylephrine) did not inhibit milk flow. It is supposed that during milking the machine caused the teat tissue to contract enough to mask the effect of the alpha-mimetic treatment on milk flow.