Subclinical mastitis of beef cows.

Two beef cow herds were investigated to determine the prevalence of subclinical mastitis and to determine whether it contributed to decreased weaning weight of calves nursed by affected cows. Quarter samples obtained about 30 days after calving were evaluated by the California mastitis test and by direct microscopic cell count. In 1 herd, a 92-cow confinement operation, 17 quarters of 12 cows were infected with Staphylococcus aureus, Streptococcus sp other than agalactiae, or Klebsiella sp. Mean 205-day adjusted weights were 203.9 kg and 218.2 kg for calves nursed by infected vs noninfected cows. In the other herd, a 75-cow range-pasture operation, 8 quarters of 8 cows were infected, all with S aureus. Mean 205-day adjusted weights were 162.5 kg and 174.6 kg for calves nursed by infected vs noninfected cows.