Influence of the estrous cycle on selected biochemical and cytologic characteristics of milk of cows with subclinical mastitis.

The purpose in this experiment was to determine the relationship between subclinical mastitis and the estrous cycle. Subclinical mastitis was defined as a quarter in which milk had a somatic cell concentration greater than 500,000/ml and was bacteriologic culture-positive on 2 consecutive examinations. Normal quarters were defined as quarters from which milk had a somatic cell concentration less than 500,000/ml and was bacteriologic culture-negative on 2 consecutive examinations. Selected biochemical and cytologic characteristics of milk from 10 cows with subclinical mastitis were measured at 5-day intervals during the estrous cycle. Milk and blood progesterone concentrations were determined. Analysis of variance of dependent variables was performed, using a model with the following factors: cows, treatments, days of estrous cycle, and interaction of treatment with day of the estrous cycle. Treatment consisted of comparison of normal and subclinically mastitic quarters. Clinical signs of mastitis tended to occur more often during estrus (days 0 and 20) than during diestrus (day 10). Day of the estrous cycle was a significant source of variation for bovine serum albumin concentrations in milk. Somatic cell concentrations, electrical conductivity, milk bacterial concentrations, and log10 milk bacterial concentrations did not vary significantly during days of the estrous cycle. A significant treatment-by-day interaction occurred for milk progesterone concentrations; blood progesterone concentrations were not similarly affected. Milk progesterone concentrations for mastitic and normal quarters were similar at estrus (days 0 and 20) and metestrus (day 5). Milk progesterone concentrations for mastitic quarters were significantly decreased on day 15 of the estrous cycle, compared with values from normal quarters.