We examined the relationship between physicochemical indicators and somatic cells in the milk of dairy cows during experimentally induced mastitis and their significance as indicators for use in controlling udder health. We were concerned particularly with the effect of alveolar milk ejection on the sensitivity of these indicators. In Expt 1, Escherichia coli lipopolysaccharide (Esch. coli LPS) was injected into the left rear quarter to induce an inflammatory reaction in one quarter in each of six cows. The contralateral control quarter was injected with a solution of NaCl (9 g/l). Nine milk samples were taken from both quarters until 60 h after injection. In Expt 2, repeated milk samples were taken every 20 s from one quarter during a 120-s teat stimulation in 20 cows with different somatic cell counts (SCC). Quarters were clustered for low (<5·0 log cells/ml), mid (5·0–5·7 log cells/ml) and high (>5·7 log cells/ml) SCC of the sample taken at t=0 s. Samples were analysed for SCC, electrical conductivity (EC) and Na+ and Cl− concentrations. During the experimental inflammation SCC, EC, Na+ and Cl− peaked at 12 h from LPS administration and values in treated quarters (T) at this time were elevated to 7900, 157, 501 and 169% of the values in untreated quarters, respectively. In Expt 2, SCC, EC, Na+ and Cl− in high SCC quarters were 2520, 121, 283 and 141% of low SCC quarters at the start of stimulation (t=0 s), respectively. Highly significant (P<0·001) differences in EC, Na+ and Cl− between high and low SCC quarters disappeared owing to the onset of alveolar milk ejection 100 s after the first contact with the teat. In conclusion, SCC in cows' milk provided the strongest amplitude in the case of an intramammary inflammation. EC, Na+ or Cl− were useful tools only if the measurements were performed in cisternal milk before the start of alveolar milk ejection.

The internal arrangement of the mammary gland cavity system, cisternal and alveolar milk fractions and the characteristics of milk ejection were investigated in buffaloes. Twenty-four Murrah buffaloes in three different stages of lactation and of two age groups were used. Continuous ultrasound cross-sections during milk ejection induced by exogenous oxytocin were performed to record the latency period of milk ejection. Buffaloes had small cisterns and the cavity area in the teat and gland regions were not significantly different (P>0·05). The animals had long teat canals (3·1±0·1 cm), longer in the hind than fore quarters. Cisternal milk yield was low (0·17±0·01 kg) and cisternal fraction was only 4·9±0·1% of the total milk. The cisternal area (cm2) was 69·6±4·6, 51·61±4·8 and 26·01±4·8 while the cisternal yield (kg) was 0·32±0·05, 0·18±0·05 and 0·05±0·05 in early, mid and late lactation, respectively. A close correlation (r=0·87, P<0·05) existed between the ultrasound cisternal area and cisternal milk yield. The latency period of induced milk ejection was similar to that reported for cows (25±1 s) and was negatively correlated with milk yield (r=−0·75, P<0·05). Milk ejection occurred shortly after elevated oxytocin concentrations were present. Delayed milk ejection reported earlier in this species must therefore be due to the absence of cisternal milk and delayed oxytocin release. An increase in teat length and circumference at milk ejection was also evident in the ultrasound cross sections.

The major milk fraction stored in the bovine udder, the alveolar milk, is not available for milk removal before being actively shifted to the cisternal cavities by the milk ejection reflex (Knight et al. 1994; Pfeilsticker et al. 1996). Tactile teat stimulation, provided by the liner throughout milking, causes continuous release of oxytocin from the neuro-pituitary into the blood circulation, which induces myoepithelial contraction and shifting of alveolar milk into the cistern until the end of milking (Bruckmaier et al. 1994; Crowley & Armstrong, 1992; Gorewit et al. 1983; Lefcourt & Akers, 1983). However, milk ejection during machine milking is not complete. A residual milk fraction of 10–30% remains in the udder and is only removed after administration of supraphysiological amounts of oxytocin, usually at least 10 i.u. of oxytocin injected i.v. (Bruckmaier & Blum, 1998). In dairy practice, exogenous oxytocin is often used at high dosages to treat disturbed or incomplete milk ejection. It is reported by farmers and veterinarians that after long-term use of exogenous oxytocin animals become addicted to the treatment and withdrawal of oxytocin causes reduced milk ejection.

Occurrence of milk ejection and course of milk removal were investigated in 18 dairy cows at milking intervals of 4, 8 and 12 h in early, mid or late lactation. Milk ejection occurred fastest in early lactation at a milking interval of 12 h and was delayed at short milking intervals and in late lactation. Storage capacity of the udder was estimated and the actual milk yields of experimental milkings were calculated as a percentage of storage capacity, i.e. degree of udder filling. It was shown that the occurrence of milk ejection after the start of teat stimulation is a function of udder filling. The relationship between the degree of udder filling and the delay from the start of milking until commencement of milk ejection followed a linear regression curve. Changes in occurrence and course of milk ejection have to be considered in practical milking, mainly in late stages of lactation and after short milking intervals. In automatic milking systems where variable and sometimes extremely short milking intervals occur, the duration of pre-milking udder preparation should be adapted to the expected milk yield at each individual milking procedure.

Plasma oxytocin was measured in 11 multiparous sows for 13 h after the start of parturition to confirm the existence of early milk ejections and determine whether they were associated with pulsatile oxytocin release. The relationship between oxytocin concentrations and duration of parturition was also analysed. Oxytocin was released in pulses during and after parturition and mean concentrations reached maximum 2 to 4 h after the first birth. Discrete milk ejections occurred during parturition but only 42% of these were associated with oxytocin peaks before 4 h post partum. After this more than 80% of milk ejections occurred with oxytocin peaks. Sow grunting increased during milk ejections and peak grunt rate increased after parturition. Oxytocin peaks without milk ejections did not lead to an increased rate of grunting. Sows with prolonged parturition had lower basal and peak oxytocin concentrations. During the first 4 h only half of the piglets were born during an oxytocin peak. Discrete milk ejections with oxytocin peaks can occur before parturition is completed. However, during the first 4 h post partum, basal oxytocin concentrations may be high enough for milk ejection to occur without further oxytocin secretion. Both milk ejection and oxytocin release are necessary for the sow to increase grunt rate. Low oxytocin concentration is associated with prolonged parturition.