F. H. Dodd

Susceptibility of the bovine udder to bacterial infection in the dry period.

The teats of 18 cows were externally exposed to infection with mastitis organisms by dipping them daily for 1 or 2 weeks in a mixed suspension of Streptococcus dysgalactiae and Str. uberis at the start, the middle or the end of the dry period. The teat sinuses of quarters which remained uninfected after external exposure at the beginning or the middle of the dry period were then infused with the same strains of streptococci. Of the 9 quarters infected following external exposure 8 occurred in animals exposed at the start and one in the middle of the dry period. Thirty-six new infections occurred after infusion of bacteria into the teat sinuses of 38 quarters. In a further trial with 10 cows, Staphylococcus aureus and Str. zooepidemicus were inoculated into the distal 3 mm of the streak canals of 5 cows immediately after drying-off and into those of 5 cows which had been dry for 28 weeks. Animals were slaughtered 48 h later and infection determined by teat puncture. Five infections occurred in cows which were at the start of the dry period and only one in the cows dry for 28 weeks. It is suggested that these differences in the rates of new infection between the early dry period and a very extended dry period are due to differences in the ease with which bacteria can penetrate the teat canal. At the later stage bacterial growth through the teat canal appeared to be inhibited.

415. Factors affecting the rate of machine milking

1. A study was made of the milking of the two hind quarters of four cows by teat cup and by teat cannula at three levels of vacuum and by teat cannula at atmospheric pressure. 2. The eight quarters milked at significantly different maximum rates by teat cup but at nearly the same rate through a teat cannula, suggesting that the teat orifice is a very important factor controlling rate of milking. 3. The maximum rate of milking by both teat cup and teat cannula increased with increasing level of vacuum from 11 to 20 in. Hg. The rate of increase in the teat-cup milking was greater than in the cannula milking, suggesting that the teat orifice was stretched open at the higher levels of vacuum. 4. The average amount of strippings increased at the higher level of vacuum in teat-cup milking, due apparently to teat-cup crawl.

623. Variations in the incidence of udder infection and mastitis with stage in lactation, age and season of the year

1. The detailed udder health records of 530 lactations have been analysed on a monthly and lactation basis, so that separate studies of the first infection rate, re-infection rate, total new infection rate and total infection and mastitis incidence for both cows and quarters could be made. These data have been used to study the relationship of stage of lactation, age and season of the year with udder infection and mastitis. 2. With advancing lactation there was a decline in the rate of first infection, reinfection, total new infection and in mastitis in infected cows and quarters. 3. In early lactation there was a marked tendency for infected cows and quarters to develop clinical symptoms. 4. During the last 4 months of lactation there was no significant rise either in infection or in mastitis. 5. There was an increase in infection and mastitis with advancing age, but infected older cows were not more liable to develop clinical symptoms than younger cows. 6. There was an indication that the longer a quarter was free from infection the greater were its chances of remaining so. 7. No major seasonal trends were found in the incidence of udder infection and mastitis, although the incidence was slightly higher in the summer months.

A method of diagnosing intramammary infection in dairy cows for large experiments

Diagnosis of microbial infections in the udders of cows in commercial dairy farms for large experiments cannot be without error. Limitations of sampling method and routine prevent collection of the necessary information for sure diagnosis. However, with an organized method of repeated bacteriological examinations using consistent and proven methods of aseptic sampling the errors were shown to be very low. A method based on bacteriological tests on aseptic milk samples was used in 32 herds (approximately 2000 cows) for a 3-year period. This is described and examined in terms of other criteria to validate its use in experimental work. With this method it was not difficult to differentiate between those quarters which regularly shed pathogens and those which did not. Other evidence indicated that it was reasonable to assume that this classification accurately distinguished between infected and uninfected quarters. The errors using this method were quite small: when measuring the state of infection of all quarters in the herds the errors did not exceed 1%. Some small modifications to the method described are suggested to improve further its diagnostic accuracy.

506. The importance of machine milking rate in dairy cow management and breeding

The discoveries of the last decade have considerably improved our understanding of the neural and hormonal mechanisms controlling milk ejection. This, in turn, has stimulated research into the many practical problems concerned with hand and machine milking. The discovery that milk ejection is a transitory mechanism encouraged the study of the rate of machine milking of dairy cows since it was inferred that less milk is obtained when milking is slow. This research work has shown that there are very wide differences in the machine milking rate of cows in the same herd, even when they are under identical management (l, 2, 3). However, the milking rate of a particular cow is relatively stable over long periods (2, 3, 4) and cannot be altered even by severe changes in management (5, 6,7). The explanation of this appears to be that milking rate is mainly controlled by the structure of the teat orifice (8). The results given in this paper carry this study a stage further. They show the effect of inherent milking rate on milk yield, milk composition and the rate of decline of milk yield. In addition, some preliminary observations on the inheritance of milking rate are given.

Observations on Corynebacterium bovis infection of the bovine mammary gland. I. Natural infection.

Data from experiments in 55 commercial herds have been examined to study the patterns of Corynebacterium bovis infection. Such infections are most common in herds that are not subjected to regular control methods but their commercial significance appears to be small. In herds using teat disinfection and dry cow therapy levels of infection with this organism are low. There is no evidence from these data to suggest that C. bovis infections protect the udder against invasion by a major pathogen. On the contrary these infections do not persist when major pathogens invade.

703. Cineradiographic observations on machine milking

Machine milking has been studied in three cows by cineradiography at 50 frames/sec. The findings of Pier et al.(5) have been confirmed. The effects of variation in the pulsation rate, ratio and liner tension were investigated and the results compared with those obtained by other methods. The proportion of each milking cycle during which milk flows has been determined.

663. The relationship between milk secretion and the rate of milking by machine

Lactation has two phases: the continuous process of secretion and the mechanism of ejection which occurs only during milking. Because ejection is a transitory phase it is to be expected that within certain limits milk yield will be inversely related to the duration of milking, which was, in fact, demonstrated many years ago with hand-milked cows(1). With machine milking the situation is different because the forces which the machine applies to the udders of all cows within a herd are the same, and thus the differences in milking rate that occur are due to differences between the cows. Under these conditions, if milking rate can be shown to affect milk yield it is a demonstration of how the physiological factors controlling milking rate can modify the expression of the inheritance of milk secretion.

Consequences of diagnostic errors in mastitis therapy trials

The effect of errors that occur in the diagnosis of intramammary infectious mastitis on the precision of experiments measuring the efficacy of mastitis therapy has been investigated. Diagnostic errors within the range found by experienced workers can create large biases in the apparent cure rate of therapy particularly at cure rates of less than 0.5. Using confirmed methods of diagnosis rather than single samples and reducing the probabilities of false positive and false negative diagnoses to 0.01 and 0.05 respectively, the biases in the apparent cure rates are reduced to acceptable levels. A method is given for calculating the rates of occurrence of false positive and false negative diagnoses from the results of trials using confirmed diagnoses. These errors cannot be calculated from therapy trial data when diagnosis is based on single milk samples. Because the bias in the measurements of the cure rate is greatest at the lowest levels of elimination, estimates of spontaneous recovery in untreated quarters have the greatest error. For this reason experiments incorporating an untreated control group of infected quarters usually reduce the precision of the therapy trials. An experiment in which the efficacy of a test product is measured relative to a reference product has advantages. It minimizes the difficulties arising from scale of measurement, diagnostic errors and herd differences in response rate, and makes possible comparisons between trials. Further investigations are required on the importance of spontaneous recovery, particularly for studies of Escherichia coli therapy and dry period therapy. The results of this investigation have relevance to all types of mastitis investigation that measure the change in mastitis status of udder quarters, i.e. new infection rates.

649. The effect of design of teat-cup liners on mastitis incidence

1. An experiment was carried out to study the effect on udder health and milk yield when half-udders of dairy cows were milked with either a moulded or an extruded teat-cup liner. 2. There was a higher incidence of mastitis in quarters that were milked with moulded than with extruded liners, but the incidence of new infection was similar for each group of quarters. 3. The types of liners used had no measurable effect on teat orifice erosion or leucocyte content of the milk, nor was the decline in milk yield with advance in lactation significantly different for the groups of quarters. We wish to thank Dr A. T. R. Mattick and Mr A. S. Foot for advice and encouragement, and Miss H. R. Chapman for assistance in the cleaning of teat-cup liners. We are indebted to Miss M. S. Hawkins and Messrs T. M. Higgs and D. S. Simpkin for technical assistance.