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Two methods of drying off cows, intermittent milking and abrupt cessation, were studied with data from 9254 quarters of cows on 36 New York dairy farms. Eighty percent of the cows were infused with nine different antibiotic preparations separately at drying off, and 20% served as controls. Cows dried off by intermittent milking has a similar number of quarters infected at drying off, had fewer spontaneous recoveries, had a higher rate of cure, and developed fewer new infections in control quarters in comparison with cows dried off by the stop method. Methods worked equally well in treated cows. However, intermittent milking resulted in fewer infections at subsequent calving than stop milking in nondry treated cows. Cows producing less than or equal to 4 kg of milk at drying off were more highly infected than higher producing cows. Hind quarters contained more infections at drying off, fewer responded to therapy, and more infections developed in the dry period. Cows with dry periods of less than or equal to 30 days had more infected quarters respond to therapy and had the fewest new infections in the dry period. The role of routine dry cow therapy in decreasing the number of infections in dairy herds by preventing new infections and removing old infections is demonstrated.

Clinical mastitis records for 6.5 yr from a large north Florida dairy and corresponding daily weather data were analyzed. Monthly incidence of clinical mastitis was expressed as percent of cow-days in milk and graphed against monthly average daily maximum temperature humidity index values and monthly total rainfall. No trends were evident with rainfall. In 3 of 6 yr, monthly incidence of clinical mastitis increased more than 50% above annual incidence, and this followed high monthly temperature-humidity values. Least squares was used to estimate regression coefficients of temperature-humidity index categories based on 999,969 Holstein records. A temperature humidity index category represented the number of days used to calculate average daily maximum temperature-humidity index value. Sources of variation in observed occurrence of clinical mastitis were cow, parity, month, year, interaction of parity by month, and continuous effects of temperature-humidity index categories 2, 6, 15, 30, 60, each to third order and 60 by parity interaction. In all temperature-humidity index categories as the temperature-humidity index value increased, occurrence of clinical mastitis increased. When values rose from 55 to 80, twice as many for 2 d cows showed signs of clinical mastitis.

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Four experiments were performed to determine whether cooling cows during final maturation of oocytes and early embryonic development or injection of vitamin E at AI prevented adverse effects of heat stress on pregnancy rates in lactating Holstein dairy cows. In Experiment 1, cows were placed in a cooling facility containing sprinklers and forced ventilation or received shade only from 2 to 3 d before until 5 to 6 d after breeding. Although cooling had no effect on detection of estrus, pregnancy rates were increased slightly for cooled cows (8 of 50 cows; 16.0%) compared with those for cows exposed to shade only (2 of 32 cows; 6.2%). In Experiments 2 through 4, cows were administered 3000 IU of vitamin E or placebo i.m. at AI during two consecutive summers and one winter in Florida. Administration of vitamin E had no consistent beneficial effect on pregnancy rates during summer or winter. Short-term cooling improved pregnancy rates slightly in heat-stressed cows, but administration of vitamin E had no beneficial effects on pregnancy rates during heat stress. Further improvements in cooling schemes during early pregnancy and delineation of antioxidant effects are necessary before such systems become practical for improvement of fertility in heat-stressed dairy cows.

This study evaluated the effect on udder health of extending the milking period with or without vacuum fluctuation. Eighty cows were allotted into treatment groups consisting of 1) Control milked with normal milking equipment and the milker removed as soon as milk flow ceases; 2) milked for 12 min with normal equipment; 3) milked 12 min with induced vacuum fluctuations; and 4) normal milking time with induced fluctuations. Groups 2 and 3 had more newly infected quarters than groups 1 and 4. However, there was no difference in the number of cows becoming infected. The increase in new infections is due to more quarters of already infected cows becoming infected rather than more new cows becoming infected. Extending the machine-on time appeared to increase the rate of cross infection presumably by lengthening the time of highest risk. Cows in groups 2 and 3 required more therapy of clinical mastitis which was a reflection of the higher new infection rate rather than of a change in trauma to the udder. Increasing the unit-on time had no apparent effect on external teat end condition.

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