A non-return was defined as the absence of another artificial insemination (AI) after the first one while the cow was still present.
Three different statuses were defined based on both individual and herd test results: positive cow, negative cow in a negative herd and negative cow in a positive herd.
237,612 AI from 72,135 cows in 1,470 herds were studied by logistic regression after
adjusting on known factors influencing reproduction.
Non-return rate was higher for infected cows compared to negative cows from negative herds (OR of 1.14, or +3.2 point of % of nonreturn rate).
This increase was higher for parity 1 cows (OR of 1.20, or +4.4 point of % of non-return rate) than for other parities.
The effects were lower when comparing positive cows to negative cows in the positive herds.
Looking at these observations, the hypothesis of Mapeffect based on the relation between Map-infection, production and reproduction is formulated.
Due to the lack of protein absorption in the intestine, the milk production is reduced.
In the early stages of the infection, this could lead to a lower negative energy
balance that could be associated with improved fertility.