view - Department of Reproduction, Obstetrics and Herd Health

CHAPTER 2
predominant lipoprotein in bovine FF. The difference in ability to transverse the bloodfollicle
barrier and the different FA profiles of the lipoproteins will have a profound
effect on the FA composition of the FF when supplementing PUFA to dairy cows.
Furthermore, it has been suggested that the ovary is capable of buffering oocytes against
fluctuations in the n-6 and n-3 content of plasma (Fouladi-Nashta et al., 2009). However,
data on transfer of n-6 and n-3 FA to the follicle, especially the very LCFA such as 20:4n-
6 or 22:6n-3 is lacking. As many mechanism by which FA affect reproduction in dairy
cows (see below) rely on an efficient transfer through the blood-follicle barrier, this
necessitates further research.
SUPPLEMENTATION OR MOBILISATION DURING NEGATIVE ENERGY BALANCE
Feeding supplemental FA to lactating dairy cows decreases the de novo synthesis
of milk FA due to the incorporation of added dietary FA into the milk. This subsequently
reduces the requirement for acetate, BHBA and NADPH, the latter being produced
through oxidation of glucose. Secondly, dietary FA can induce insulin resistance in
muscle and adipose tissue, which decreases peripheral glucose use (Chilliard and Ottou,
1995; Gaynor et al., 1996; Pires et al., 2008). The spared glucose is partitioned towards
increased lactose synthesis, hence increasing milk yield (Wu and Huber, 1994; Hammon
et al., 2008). Although highly variable, the milk yield response to supplemental FA in
TMR diets generally is curvilinear, increasing until approximately 9% of ether extract
(EE) in the total diet (Wu and Huber, 1994) or 3% of added fat (Drackley, 1999),
thereafter decreasing due to the concomitant decrease in DMI and fiber digestibility
(Palmquist and Jenkins, 1980; Coppock and Wilks, 1991; Allen, 2000). Moreover, cows
seem to respond production wise best to additional fat at the time they reach positive
energy balance (Grummer, 1995). Consequently, the effect of an increased milk yield
seems to be limited within the first 5 weeks post partum (Schingoethe and Casper, 1991;
Chilliard, 1993; Grummer et al., 1995). The response lag of a few weeks, documented for
both cows and heifers, is another drawback when supplementing FA to dairy cows in
early lactation (Grummer, 1995). In summary, these findings illustrate the limited
effectiveness of post partum dietary FA in order to ameliorate the NEBAL and
consequently have eliminated FA from most early lactation rations for dairy cows as
they may imbalance the relative proportions of glucogenic and ketogenic metabolites
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