The mass flow-rate (FR) of milk from the bovine teat within individual pulsation cycles was measured by continuous computation of the average optical density of the milk-air mixture within a teatcup. Measurements during milking were made on both live and excised teats using a range of teatcup liners, vacuum levels and pulsation characteristics. During a typical pulsation cycle, soon after the start of milking, milk FR from live teats reached a peak within 100 ms of the start of the flow period and fell to about 60% of the maximum value between 0 5 and 15 s of flow. If the liner was held open for longer than 1-5 s, the FR typically declined a further 10 % over the next 10-15 s. After 10-15 s little or no further change occurred in milk FR at normal milking vacuum levels. Milk flow from excised teats reached a steady value within 100 ms of the start of the flow period and showed no subsequent fall in FR.Within individual pulsation cycles the changes in FR from live teats are affected by: (i) the degree of compressive load applied by the collapsed liner to the teat in the preceding cycle; and (ii) the duration of application of the compressive load. Increasing the degree of compressive load or the length of time of loading increases the peak milk FR of the next cycle by reducing the degree of congestion in the tissues of the teat surrounding the teat canal. Muscular effects are of secondary importance. The relatively small changes in milk FR associated with muscle contractions of the teat can be seen most clearly at low milking vacuum levels (below about 30 kPa) or with an unsupported teat (e.g. using a very distensible, thin-walled liner).During the peak flow period of milking, milk flows from the teat when the liner is open and milk flow ceases when the load applied by the liner on the teat end is sufficient to compress the teat canal in the plane of liner collapse. Milk flow-rate (FR) at any time is a function of the pressure difference existing along the teat canal (intramammary pressure plus liner vacuum) and the resistance of the teat canal (Baxter et al. 1950). The resistance of the open teat canal is determined by the degree of dilation it experiences when the teat end is strained by the pressure difference which is generated across the teat wall. Teats are classified as being 'easy' or ' hard' to milk depending on the ease and degree of dilation of the teat canal when the milking vacuum is applied.Measurements of milk FR within single pulsation cycles (Thiel et al. 1966;Delwiche, Scott & Drost, 1980) and fibre-optic studies of the teat during milking (Thompson, 1978) indicate that the resistance of the teat canal to milk flow increases within the brief milk flow period of each cycle and that this change in resistance may have a physiological basis.

Half-udder comparisons were made using 72 and 59 cows in 2 experiments of 8 months duration to assess the incidence of pulsation failure, and its effects on teat condition and mastitis, as a consequence of milking with shortened teatcup liners. Diagonally opposite quarters of all cows were milked with control liners of 148 mm effective length or with the same liners shortened to an effective length of 110-130 mm by inserting a rigid sleeve in the lower barrel of the liner. Milking with shortened teatcup liners resulted in a high incidence of pulsation failure (denned for this experiment as failure of the liner to collapse fully beneath the teat in each pulsation cycle throughout milking). More than 50% of hind teats and 90% of front teats in the research herd penetrated deeply enough into the teatcup to prevent cyclic collapse of liners shortened to 130 mm. Such failure occurred with all teats in liners shortened to 125 mm for front teats or 110 mm for hind teats. The combined results for the 2 experiments showed that almost twice as many new mastitis infections occurred in quarters milked with the shortened liners (47 v. 25, P < 001). This effect was confined to front quarters for which there was a 3-fold difference in the number of new infections (32 for the short liners v. 11 for the controls, P < 0-001). In both lactations, significantly more teats had petechial haemorrhages when milked with the shortened liners and the electrical conductivity of milk from uninfected front quarters was higher. These results might reflect an interaction between pulsation failure and differences in the degree of overmilking that usually occur between front and hind quarters.

Half-udder comparisons were made using 56 cows for 2 months, in an experiment involving high bacterial challenge, to assess the combined effects of 5 min overmilking and pulsation failure (resulting from the use of shortened teatcup liners) on teat condition and mastitis. Only three new infections were confirmed in over 12500 quarter milkings in quarters milked with control liners (of 148 mm effective length) indicating little or no effect of prolonged overmilking in these quarters. A 3 -5-fold increase in the new infection rate (NIR) based on bacteriological diagnosis alone (P < 0-01), or a 9-fold increase in NIR based on bacteriological diagnosis plus raised cell count and/or iV-acetyl-/?-D-glucosaminidase (P < 0-01), occurred in quarters milked with the same liners shortened to an effective length of 120-130 mm. The mean interval to infection or teat canal colonization (41 v. 60 milkings), and for a quarter infection to be confirmed by other diagnostic tests (45 v. 79 milkings) was significantly less in quarters overmilked with short liners. The results confirm that NIR increases whenever pulsation fails. Overmilking may increase NIR when it is associated with pulsation failure.Experiments in which apparently undesirable machine conditions have been imposed, in conjunction with overmilking, have shown little or no effect of overmilking on the rate of new infections or other indicators of mastitis. Overmilking at high vacuum levels (57-70 kPa) had no effect on new infections, clinical mastitis or udder irritation in heifers (Mochrie et al. 1953;Neave et al. 1962) or in previously uninfected cows (Olney & Mitchell, 1983). Natzke (1978) and Natzke et al. (1978) compared normal milking and overmilking with and without large vacuum fluctuations and concluded that if overmilking is associated with mastitis the effects appear to be small.Milking with short teatcup liners resulted in about twice the new infection rate (NIR) of quarters milked with liners of normal length, but the experimental differences were confined mainly to front quarters (Mein et al. 1983). Although many controlled experiments have failed to show that overmilking seriously affects mastitis (see reviews by Fell, 1964;Dodd & Neave, 1968;Natzke, 1978;Schultze, 1979), it was thought that this interaction in treatment response to pulsation failure between front and hind quarters could be associated with more overmilking on the front quarters. A further, short-term experiment was conducted to assess the combined effects of overmilking and pulsation failure (resulting from the use of shortened teatcup liners) on NIR.

SummaryThe milking characteristics of conventional clusters were compared with individual teatcups or clusters fitted with valves in the short milk tubes without external air admission to the liners. Individual teatcups fitted with non-return valves had peak milk flow rates 13% higher than conventional teatcups. A daily cross-over experiment involving 36 cows compared clusters fitted with non-return valves to conventional clusters. Valved clusters milked 18% faster, showed significantly better teatcup stability and 3% higher machine milk yields, but 48% higher strip yields were recorded. Clusters with non-return valves were difficult to remove after milking because the valve closed when the vacuum supply was shut off, thereby maintaining the vacuum level under the teats. When the nominal plant vacuum level for teatcups with non-return valves was reduced by 10 kPa to compensate for the higher effective milking vacuum level, the valved teatcups had peak flow rates 20% lower than the conventional teatcups. Clusters fitted with a modified valve, which allowed some reverse flow when closed (a ‘leaky’ valve), milked significantly faster than conventional clusters, had 33% lower strip yields and were easy to remove after milking.