Incidence of Mastitis-Causing Organisms in Angus, Brahman, and Reciprocal-Cross Cows Grazing Endophyte-Infected Tall Fescue or Common Bermudagrass

Abstract Angus (A), Brahman (B), and reciprocal cross cows (AB, BA) were sampled in 1994 (67 head) and 1995 (47 head) to evaluate the effect of breed and forage on the presence of mastitis-causing organisms (MCO). In 1994, cows were managed on either endophyte-infected tall fescue (E+) or common bermudagrass (BG) and milk samples were taken in August, September, and October. In 1995, cows were managed on E+, BG, or both forages during the appropriate seasons. Milk samples for 1995 were taken in May, July, and September. In both years, aseptic techniques were used to sample from all four quarters of each cow for diagnostic bacteriology at the Immunology and Disease Resistance Laboratory, Beltsville, MD. Any cow having at least one quarter positive for MCO on a given date was designated positive. In 1994, the percentage of cows with MCO was 52.0, 29.1, 25.4, and 40.0% for A, AB, BA, and B, respectively, averaged over month. Heterosis was estimated as −18.7% ( P P