[Effect of adrenaline and phenylephrine on milk extraction during the mechanical milking of cows]

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.