Eighteen cows with a wide range of quarter peak flow rates (O35-2-22 kg/min) were inoculated with Streptococcus agalactiae and Str. dysgalactiae 4 mm into each streak canal every 3 d for 12 d. Thirty of the 72 quarters developed intramammary infection. Mean peak flow rate and length of streak canal of those quarters that became infected were 1-26 + 0-08 kg/min (mean + SEM) and 11-47±0-41 mm respectively, compared with 101 + 0 0 5 kg/min and 1205±035 mm for those that remained uninfected. Logistic regression analysis showed that the probability of infection increased significantly with the increase in peak flow rate (P = 0-01). The influence of streak canal length on new infection, after allowing for the effect of peak flow rate, was not significant at the 5 % level (P = 0'07), suggesting that there may be an inverse relationship between teat duct length and infection, but that it is of secondary importance to peak flow rate. If increased mass of milk distends the teat duct by raising intramammary pressure, then quarter susceptibility to infection is further compromised. These results strongly suggest that the benefits of reduced infection from mastitis control, achieved despite dramatic increases in milk flow rate and milk yield, are significantly underestimated. Some attention has been paid in the past to the relationship between teat patency, measured as ease of milking, and the incidence of intramammary disease. Little (1937) emphasized the importance of an intact teat duct sphincter as a barrier to invasion and suggested that sphincters of older cows are less efficient, rendering such cows more susceptible to infection. Dodd & Neave (1951) identified a significant relationship between fast flow and new intramammary infection. Recently Grindal & Hillerton (1991) confirmed these findings and identified the enhanced risk from mastitis of pulsationless milking and impaction of milk droplets on the teat end to fast flow rate quarters. McDonald (1971), in a histological study, showed a marked difference in the cross section between streak canals from susceptible and resistant glands. The teat canals from the susceptible quarters had a larger diameter and thinner stratum corneum than the canals from resistant quarters. Hibbitt et al. (1969) identified the antibacterial properties of keratin and emphasized its importance in the defence of the mammary gland. This supported the observations of Murphy (1959), who showed that partial removal of the keratin by a reaming process produced a temporary decrease in resistance of the canal to invading pathogens.

Measurements of teat end expansion and contraction times from step changes in pressure suggest that teat ends require a minimal duration of liner closure for response. Effect of duration of liner closure per pulsation cycle on susceptibility of the udder to infection was tested. Four durations of liner closure (liner more than half closed) were applied: zero, .17, .34, and .51 s per pulsation cycle. For the latter three treatments, duration of the liner more than half open was .66 s per pulsation cycle, resulting in pulsation rates of 72.3, 60.0, and 51.3 pulsations per min. Four groups, each of 10 British Friesian cows, were in a 4-wk experiment. High bacterial exposure of all teats was ensured by dipping them before and after each milking in a suspension of Streptococcus agalactiae and Streptococcus dysgalactiae. Numbers of quarters becoming infected per treatment group were 20, 11, 4, and 5. This result represents both a significant inverse linear and quadratic relationship between proportion of quarters becoming infected and duration of liner closure. We conclude that a sufficient duration of liner closure, i.e., one-third of a second or more, per pulsation cycle contributes to reducing risk of new mammary gland infections.

SummaryIn 3 consecutive experiments, each using 20 cows, the application of Escherichia coli to teat ends after milking led to high rates of intramammary infection. These infections were not prevented by disinfection of the teats before milking, by the installation of shields in the short milk tubes of the milking cluster or by the use of an individual quarter milking cluster. Rates of infection were significantly lower when teat contamination was applied 1 h before milking compared to contamination applied immediately after milking.These data suggest that penetration of the teat duct by the E. coli occurred in the period between contamination and milking. Seventy four percent of infections occurred in hindquarters and there were variations in the susceptibility of cows to infection.

A milking machine claw incorporating valves to prevent movement of milk between teats during milking, and its contribution to the prevention of udder infection under experimental and field conditions, is described. Under experimental conditions a suspension of Streptococcus agalactiae and Str. dysgalactiae was introduced into the milking cluster during milking; 11 of 40 quarters became infected using a conventional claw piece whereas none of 40 quarters milked with the multivalve claw developed intramammary infection. In a 12-month experiment in ten commercial herds, each split into two equal susceptibility groups, the multi-valved cluster reduced the incidence of new infection with coliforms and Str. uberis by 17% (P < 0-1). The incidence of all new infections and new clinical cases was 14 and 25% lower compared to conventional clusters but these reductions were not significant.

SummaryThree types of liner (wide bore tapered, wide bore and narrow bore) were compared in a change-over design experiment comprising three 3-d periods and using six lactating Holstein Friesian cows with a wide range in milk flow rate and teat dimensions. Vacuum levels measured within the liner showed a characteristic pattern of three distinct phases: 1, a mouthpiece chamber (MPC) vacuum that fluctuated with vacuum beneath the teat; 2, a stable MPC vacuum that coincided with the period of peak milk flow and 3, a MPC vacuum that fluctuated with pressure changes in the pulsation chamber and coincided with the period of no or reduced flow. Of the three liners examined, the Gascoigne 1030A (wide bore tapered) induced the highest MPC vacuum (29·4 ± 2·23 kPa, mean ± s.e.m), significantly higher (P < 0·01) than the Alfa-Laval 960000–01 wide bore (23·5 ± 2·32), regardless of teat size and milk flow rate. The Alfa-Laval 7275737–01 (narrow bore) gave a significantly lower value (16·5 ± 1·96) than either of the others (P < 0·001). It was not possible to link absolutely raised MPC vacuum to cow discomfort and teat trauma but circumstantial evidence encourages this view. Narrow teats in combination with slow flow rate appeared to exacerbate MPC vacuum and, because milking time for such animals was protracted, teats were exposed for longer periods.