A high percentage of heifers calve with intramammary
infections. One of the measures available to control intramammary
infections is treatment with
antibiotics before calving. In this study, the effects of prepartum treatment of nonlactating heifers with a 600-mg
cloxacillin dry cow treatment on the prevalence of culture-positive milk samples at calving and 10 to 14 d in milk (DIM), the incidence of clinical
mastitis, somatic cell count (SCC), and milk production during first lactation were quantified. A total of 184 heifers on 13 dairy farms were treated with
antibiotics 8 to 10 wk before the expected calving date. Another 185 heifers served as untreated controls.
Coagulase-negative staphylococci were the most frequently isolated group of bacteria in the treatment and control groups at calving (32 and 42%), and
at 10 to 14 DIM (15 and 19%), respectively. The prevalence of minor pathogens at calving was lower in the treatment group compared with the control group (34 and 43%, respectively). Staphylococcus aureus was the most frequently isolated major pathogen in the treated and control heifers at calving (4 and 5%) and
at 10 to 14 DIM (2 and 3%), respectively. The prevalence of major pathogens was lower in the treated heifers
at 10 to 14 DIM compared with the control group (4 and 6%, respectively). Cumulative incidence risk of clinical
mastitis during the lactation was 9 and 18% in the treatment and control groups, respectively. Treatment with
cloxacillin 8 to 10 wk before calving resulted in a lower prevalence of culture-positive milk samples at calving and lower quarter milk SCC in early lactation [30,000 +/- 4,600 (standard deviation) cells/mL in treated heifers versus 40,000 +/- 4,600 cells/mL in control heifers], and was associated with lower average test-day SCC (55,000 +/- 1,400 cells/mL in treated heifers versus 71,000 +/- 1,500 cells/mL in control heifers) and lower incidence of clinical
mastitis throughout lactation. The improved udder health resulted in a higher average test-day milk production in the first lactation (24.5 +/- 3.2 kg in treated heifers versus 23.6 +/- 3.1 kg in control heifers). Dairy farms with heifer
mastitis problems need to analyze their
mastitis management. Prepartum treatment of heifers with dry cow
antibiotics may be helpful by decreasing the prevalence of
mastitis-causing pathogens at calving and
at 10 to 14 DIM.