The herd experienced a low incidence of new intramammary infections and clinical mastitis during lactation. The positive aspects of mammary health during lactation may be attributed to a low level of exposure to pathogens between milkings and an excellent milking hygiene regime. Cows on intensive grazing programs are generally exposed to fewer environmental mastitis pathogens than conventionally housed cows. The limited use of water and practice of predipping teats in a germicide were means of decreasing pathogen loads at the teat end during milking. Another factor that may account for the low incidence of mastitis was the relatively low milk production per cow in the herd. The risk of mastitis in a herd increases as milk production increases.
In contrast, mammary health at calving was a concern. Both percentage of quarters infected and incidence of clinical mastitis at calving were greater than those anticipated for cows under conventional management practices. Extended dry periods (often 120 days) and calving cows in manure-pack bedded box stalls probably contributed to the high incidence of mastitis at calving. The relative risk of mastitis at calving increases as dry periods extend past 60 days. Manure-packs should be avoided as calving areas because they generally contain extremely high counts of mastitis pathogens.
Teats were prepared for milking by predipping with an iodophor teat skin sanitizer, allowing the germicide to remain on teats at least 30 seconds, and completely drying predip from teats with individual paper towels. Water was used to wash teats only when mammary glands were excessively dirty. In the rare instances when water was used to prepare teats for milking, teats were washed, dried, Redipped, and dried. All teats were dipped in post-milking teat sanitizer after milking machine removal.
At drying-off day, all lactating quarters of each cow were infused with commercially available antibiotics approved by the Food and Drug Administration for use in nonlactating cows. Cows were dried off by abrupt cessation of milking. Subsequently, at the next lactation, most cows calved in a box stall bedded with a manure-pack, because the calving time preceded the grazing season.
The bacteriological status of mammary glands was determined by collecting duplicate quarter foremilk samples within seven days after calving, at drying off, semi-monthly during lactation, and from all quarters of cows showing clinical signs of mastitis.
Percentage of quarters infected with major pathogens at calving averaged 9% of quarters. The predominant major pathogens isolated from infected glands at calving were environmental streptococci and coliforms. The predominant minor pathogens isolated at calving time were coagulase-negative staphylococci. The percentage of quarters infected at calving with the minor pathogens, coagulase-negative staphylococci, was greater in first-lactation cows (averaged 23% of quarters) than multiparous cows (11% of quarters). Percent quarters infected with all pathogens did not differ from July to December. No contagious mastitis pathogens were isolated from the herd.
The incidence of clinical mastitis was highest within seven days after calving (66% of clinical cases) compared with other stages of lactation. Environmental streptococci were the bacteria most frequently isolated from clinical quarters. Incidence of clinical quarters was greater in multiparous cows than in first-lactation cows, which is contrary to the infection rates reported above.
Herd geometric mean somatic cell counts did not exceed 260,000/ml in
any year of the project. Somatic cell counts increased throughout
lactation, corresponding with a decrease in milk production. Somatic
cell counts did not differ among parity groups.