Unsaturated fatty acids (UFA) cannot be synthesized by mammalian cells due to a lack of
desaturase enzymes. Combined with their limited supply to the small intestines, UFA have
been proposed as nutraceuticals to ameliorate dairy cow fertility. However, field studies
based on a large number of animals are lacking on this subject. Therefore the aim of the
present study was to analyze a large dataset containing individual cow fertility records
from dairy herds and link fertility key-performance-indicators like conception rate to first
insemination (CRFI), days in milk to first insemination (DIMFI) and days in milk to conception
(DIMCONC), to the level of UFA in bulk tank samples, the latter being a proxy for
the dietary fatty acid profile on these herds. Within the two year study period, information
from 15,055 lactations and 35,433 bulk tank milk samples was collected on 90 herds.
The multilevel logistic regression model used, revealed a decreased CRFI on herds with a
higher bulk tank UFA level. The decrease in CRFI was larger for higher producing herds.
Increased bulk tank UFA was furthermore associated with higher DIMFI which, together
with the lower CRFI, subsequently increased DIMCONC. Interestingly, higher variability in
UFA, expressed by an increased coefficient of variation, was associated with an increased
CRFI and decreased DIMFI and DIMCONC. In conclusion, the present study demonstrates
that increasing the UFA content of milk should not be a goal as such when supplementing
UFA to dairy cows as higher bulk tank UFA are associated with worsened fertility results.