shown to give different results, highlighting the importance
of choosing the right measurement unit (Chauvin
et al., 2008). For the aim of the current study (i.e., the
selection of farm factors that are associated with the
use of antibiotics, NDDi) was considered most appropriate.
When the antibiotic treatments themselves are
studied in more depth, including the characterization of
different classes of antibiotics and their trends in time,
other measurement units may be chosen. The NDDi of
antibiotics is expressed per average pig present on the
farm. This theoretical pig, representing an average pig
(having an average BW) that is present on the farm the
entire year, receives antibiotics for the specific number
of days. For the sow farms, average daily dosages were
expressed per average piglet present on the farms since
birth, considering the orally given antibiotics only. As
oral administration to sows is negligible and the piglets
are sometimes also treated with injectable antibiotics,
this number may have given an underestimation to the
true antibiotic treatment of piglets. Obviously, the average
piglet present is theoretical as piglets will not
stay on the farm the entire year. The average number
of dosages for the true period a piglet is present on
the farm can be calculated from this average NDD2.
For example, an average of 130 daily dosages per average
piglet (overall average NDD2 found in this study)
implies that a piglet that is present on the farm for 60
d receives, on average, antibiotics on 21.4 d (35.6% of
the period).