TWO trials were conducted to investigate the effect of pre-milking teat dipping (PMTD) on mastitis caused by environmentally associated pathogens. The first trial showed considerable variation in effect between herds, so a second, larger trial was conducted. In this second trial a comparison of the rate of clinical mastitis was made between nine matched pairs of dairy herds over 24 weeks of the winter housed period. All herds were near the national average incidence of mastitis before the trial. One member of each pair used their normal method of udder preparation throughout the trial and disinfected all teats after milking with an iodophor disinfectant. In the other nine herds the preparation of all teats, at all milkings, included dipping in a 025 % available iodine disinfectant, which was left on the teat for 30 s. Every teat was then wiped with a paper towel before cluster attachment. There was no difference in the overall rate of mastitis or the incidence of mastitis caused by any particular type or group of pathogens between the trial groups of herds. Both groups showed a reduction in mastitis compared with the previous winter. Although there were apparent benefits in some pairs of herds there was no overall benefit. In comparison with the previous winter the control herds reported a greater reduction in mastitis than the PMTD herds. The effect of trial supervision on normal practice gave a benefit which overwhelmed any effect of PMTD. There appeared to be no effect of PMTD on the total bacterial count, cell count or iodine content of bulk tank milk. There appears to be no justification for wholesale use of PMTD although most farms and risk groups could benefit from better attention to conventional mastitis control.The application of mastitis control techniques has reduced the incidence of clinical mastitis by some 75% in England and Wales over the last 20-25 years (Booth, 1988). Much of the effect has come from adoption of the five-point control plan and improvements in milking technology (Bramley & Dodd, 1984). This overall reduction in mastitis has been due to a significant reduction in the incidence of infections caused by Staphylococcus aureus, Streptococcus agalactiae and Streptococcus dysgalactiae, the principal causes of contagious mastitis. It is now generally accepted that the major problems remaining are in control of infections caused by

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In a trial conducted in the south of England in January to February 1989, blood samples were obtained from nine dairy herds with more than 30 cases of clinical mastitis/100 cows and from nine herds with less than 30 cases/100 cows during the previous 12 months. Whole blood glutathione peroxidase (GSHPx) activity and plasma vitamin E concentration were determined for 12 cows in each herd. The mean (+/-sd) values for the herds with the lower incidence of mastitis were 7.57 +/- 1.86 micrograms/ml plasma vitamin E and 23.8 +/- 22.8 U/ml rbc GSHPx activity, compared with 7.74 +/- 1.69 micrograms/ml plasma vitamin E and 20.61 +/- 8.8 U/ml rbc GSHPx activity for the herds with the higher incidence of the disease. These values indicate that the vitamin E levels were generally adequate but that some animals and herds had low GSHPx activities, suggesting that their diets may have contained inadequate selenium. The activities of GSHPx and the vitamin E levels in plasma were not significantly different in the two groups of herds, and no relationship was found between the two nutrients and the incidence of clinical mastitis. However, there was a significant negative correlation between the activity of GSHPx and the bulk milk cell counts in the herds with a low incidence of mastitis suggesting that there was an association between the incidence of subclinical mastitis or inflammation and the selenium status of these herds.

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