The dairy cow's teat is the first line of defence against mastitis pathogens. The milking process may affect the teat's condition, increasing the risk of mastitis. It is well-proven that teat-ends with severe erosions or broken skin will have an increased risk of mastitis. However, more common changes in teat condition because of milking have not been related to udder health problems. The focus of this thesis was on the relationship between teat-end condition, machine milking and occurrence of mastitis. In this thesis, two types of changes of teat-end condition were distinguished: callosity rings around the orifice and machine-induced teat swelling. \ud A classification system of the callosity rings around the orifice was defined: the teat-end callosity (TEC) classification system. In this system, a distinction is made between roughness of the callosity ring (TECR) and thickness of the callosity ring (TECT). The developed TEC classification system was used in a 1½ year longitudinal field study on 15 farms to examine the relationship between TEC and the incidence of clinical mastitis. Teats with a thin and smooth TEC ring showed the lowest incidence risk of clinical mastitis. \ud To evaluate TEC in the field, a simplified 4-category scoring system is suggested and used in an observational study on 200 dairy farms. Variation in %ROUGH between farms is explained by cow factors such as teat-end shape and machine-on time and milking machine factors such as the liner and the vacuum. \ud In order to measure machine-induced teat swelling, a methodology, using ultrasound, has been developed. Using this method, the changes of teat tissue in relation to machine milking and the recovery time of teat tissue after milking were evaluated. 8 h after milking, the teat-end width and the teat-canal length still differed from before milking. The teat-wall thickness and the teat-cistern width were recovered after 6 and 8 h. \ud The overall conclusion of this thesis is that a healthy teat of a dairy cow has a good balance between the physiological reaction to machine milking and maintaining its first line of defence mechanism against invading mastitis pathogens. Increasing rates of IMI were related to one or more of the following: a high degree of machine-induced swelling, a high level of TECT, a high level of TECR and the absence of TEC. Pathways through which these machine-induced changes lowered the resistance of the teat to bacterial invasion are the openness of the teat canal, harbouring of pathogens in TEC, and significantly increased or decreased level of keratin regeneration rate. Part of the impaired reaction of the teat to machine milking may lay in the peak milkflow rate. Suggestions are made to adjust the characteristics of machine milking to the milk flow profile of an individual cow. This can minimise machine-induced teat condition problems. \ud Teat condition changes can be used as an early warning signal for enhanced risk of clinical mastitis. Classification of teat condition is an essential tool in milking machine research and a useful monitoring tool of the quality of milking in the field. Protocols for systematic evaluation of teat condition are available
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