Milk yield and reproductive performance of brucellosis-vaccinated but seropositive Holstein cows.

The aim of this research was to study if seropositivity for brucellosis in vaccinated cows against this disease hampers reproductive performance and milk production in high-yielding Holstein cows. For this purpose 1,026 healthy cows and 372 cows seropositive for brucellosis were enrolled in this study. Cows positive to card test and subsequently to the rivanol test were further subjected to the radial immunodiffusion (RID) test. It was found that only 11% of the presumably infected cows by brucellosis screening tests were really infected with this disease. The reproductive performance of the group of cows with 11% Brucella-infected animals was not impaired; overall pregnancy rate did not differ between seropositive and healthy cows (30.9 vs. 29.6%). The abortion rates were similar between seropositive cows (5.3%) and seronegative animals (6.9%). Cows in the herd with 11% Brucella-infected animals produced significantly more milk than unaffected cows over a 305-day lactation (10,684 ± 1,720 vs. 10,345 ± 1,736; mean ± SD; P < 0.05). It was concluded that in dairy herds vaccinated against brucellosis with both 19 and RB51 strains, supplemental tests such as RID need to be conducted on all reactors in order to maintain diagnostic accuracy. These results also indicate that 11% animal prevalence of brucellosis did not exert a detrimental effect on 305-day milk yield and reproductive performance in high milk-yielding Holstein cows.