The association between occurrence and severity of subclinical and clinical mastitis on pregnancies per artificial insemination at first service of Holstein cows.

The objective of this prospective study was to determine associations between occurrence and severity of clinical (CM) and subclinical mastitis (SM) during a defined breeding risk period (BRP, 3d before to 32d after artificial insemination) on pregnancies per artificial insemination at first service (P/AI1). Dairy cows (n=3,144) from 4 Wisconsin herds were categorized based on the occurrence of one or more CM or SM events during and before the BRP: (1) healthy, (2) mastitis before BRP, (3) SM during BRP, (4) chronic SM, (5) CM during BRP, or (6) chronic CM. Clinical mastitis cases were categorized based on etiology (gram-negative, gram-positive, and no growth) and severity (mild, moderate, or severe). Compared with healthy cows, the odds of pregnancy were 0.56, 0.67, and 0.75 for cows experiencing chronic CM, CM, or SM during the BRP, respectively. The occurrence of chronic SM was not associated with reduced probability of P/AI1. Compared with healthy cows, the odds of pregnancy were 0.71 and 0.54 for cows experiencing mild or moderate-severe cases of CM during the BRP, respectively. The odds of pregnancy for cows experiencing CM caused by gram-negative or gram-positive bacteria during the BRP were 0.47 and 0.59, respectively. The occurrence of CM that resulted in no growth of bacteria in cultured milk samples was not associated with reductions in P/AI1. Regardless of etiology, microbiologically positive cases of CM with moderate or severe symptoms were associated with substantial reductions in P/AI1. Etiology, severity, and timing of CM were associated with decreases in the probability of pregnancy at first artificial insemination. Severity of the case was more important than etiology; however, regardless of severity, microbiologically negative cases were not associated with reduced probability of pregnancy.