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CHAPTER 6
records. The UFA content of selected herds was 30.8% and aligns with literature on milk
UFA (Jensen, 2002; Lock and Bauman, 2004). From our data it cannot be concluded
whether a higher UFA content originates from an increase in mono unsaturated FA
(MUFA), poly unsaturated FA (PUFA) or both, which mainly depends of the FA profile of
dietary lipids (Chilliard and Ferlay, 2004). However, next to PUFA supplementation,
increased UFA in bulk tank milk samples can be explained through other pathways. The
highest proportion of UFA in MF is accounted for by 18:1 cis-9 (Jensen, 2002). An
increased concentration in 18:1 cis-9 in MF can originate from both an increased supply
of 18:1 cis-9 or its precursor 18:0 to the mammary gland (Glasser et al., 2008). An
increase in 18:1 cis-9 has been associated with increased body fat mobilisation during
ketosis (Van Haelst et al., 2008). However, a contribution of FA from lipomobilisation in
cows in positive EBAL to milk FA cannot be excluded, especially for 18:0 and 18:1 cis-9
(Chilliard et al., 1991). Furthermore, the induction of a MFD has been associated with an
increase in trans-FA and proportional decrease in saturated FA increasing the milk UFA
(Griinari et al., 1998; Bauman and Griinari, 2003). An increased rumen unsaturated FA
load (RUFAL) has been determined as one of the risk factors for a lower FAT content
when accompanied by an altered rumen fermentation (Relling and Reynolds, 2007;
Harvatine et al., 2009). The prerequisite of both risk factors is mirrored in the significant
but small proportion of the variation in FAT explained by UFA in our study (10.2%).
Researchers have focussed on many aspects of dairy cow fertility but the
complete biological window in which UFA attenuate fertility is yet to be revealed.
Though small differences were found (IQR max 3 d) which might not be of real biological
relevance, we did demonstrate a positive association between DIMFI and milk UFA.
However, earlier resumption of postpartum cyclicity has not been observed when
feeding FA to dairy cows (Juchem et al., 2010; Silvestre et al., 2011). Interestingly, high
UFA was associated with the highest DIMFI especially when accompanied by a low level
of variation, which has to our most recent knowledge not been described. Feeding n-6
UFA to dairy cows has been shown to stimulate PGF2α metabolism improving uterine
health (Petit et al., 2004; Robinson et al., 2002). In contrast, feeding n-3 UFA reduced
endometrial secretion of PGF2α, thereby inducing antiluteolytic effects on the corpus
luteum (Staples et al., 1998). These subtle differences between n-6 and n-3 FA could
explain differences in DIMFI but cannot be attributed to our study from the parameter
milk UFA which does not distinct between FA.
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