Lactational changes in oxytocin release, intramammary pressure and milking characteristics in dairy cows

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Abstract
Summary Two experiments were conducted to investigate possible changes of milking-related oxytocin release (Expt 1) and of intramammary pressure and milking characteristics (Expt 2) throughout entire lactations in German Braunvieh dairy cows. Mean oxytocin concentrations after stimulation at onset of milking increased from 18·3 ± 15·9 to 30·7 ± 24·1 pg/ml in Expt 1 and decreased from 23·9 ± 17·6 to 15·4 ± 9·1 pg/ml in Expt 2, respectively, but remained above the level necessary to elicit complete milk ejection in both trials. Premilking baseline intramammary pressure had its maximum in early lactation until about month 4 and then decreased to ∼50% of its initial level. Ejection pressure followed a similar pattern, but dropped only to ∼75% of its maximum. This was due to the constant elevation of pressure increase, reaching its highest level in late lactation. Time from commencement of stimulation until maximum pressure exceeded 1 min in almost all instances even in early lactation and increased throughout lactation. Despite the normal decrease of milk yield average milk flow fell only slightly while maximum flow rate remained almost constant. Pressure increase, milk yield and milk flow were not different after 1 min and after extended stimulation. Thus there were no indications of a decreasing sensitivity of the milk ejection reflex during lactation, and milking characteristics were positively affected by intense teat stimulation. Suggestions for practical dairying are made.