Use of cobalt-acetate to estimate the endogenous synthesis of milk cis-9 trans-11 18:2 in dairy ewes fed linseed oil

Abstract

Administration of cobalt-acetate may represent a low cost alternative to other more expensive and involved methods used to estimate the endogenous synthesis of milk c9t11-18:2 (a conjugated linoleic acid isomer; Shingfield et al 2008; Frutos et al 2014). Cobalt inhibits the enzyme stearoyl-CoA desaturase (SCD), which is responsible for the conversion of t11-18:1 to c9t11-18:2 in body tissues (Palmquist et al 2005). Using Co-acetate or sterculic acid, 9-desaturation was estimated to account, respectively, for 51 or 74% of the c9t11-18:2 found in ewe milk (Bichi et al 2012; Frutos et al 2014), these inconsistent results being probably due to differences in diet composition rather than in methodological approaches. Thus, although t11-18:1 is produced as a major intermediate with diets rich in either 18:2n-6 or 18:3n-3, the latter minimizes the amount of milk c9t11-18:2 coming from ruminal origin (Loor et al 2005), which may affect the relative contribution of 9-desaturation to milk c9t11-18:2 secretion. The aim of this study was therefore to examine, through oral administration of Co-acetate, the endogenous synthesis of milk c9t11-18:2 in sheep receiving a diet enriched in 18:3n-3. Twelve Assaf ewes fed a TMR (forage:concentrate ratio 50:50) supplemented with 2% linseed oil were allocated to 2 experimental groups and received an oral drench supplying either 0 (Control) or 9 mg of Co (as Co-acetate)/kg of liveweight.day. Treatments were administered in 3 equal doses at 8 h intervals, for 6 days. On days 0 (i.e., before Co-administration) and 6, milk production was recorded and samples were collected for analysis of fat content and fatty acid (FA) composition (Frutos et al 2014). Administration of cobalt had no effect on milk yield or milk fat content (P > 0.10) but decreased (P < 0.01) milk 9-desaturation ratios, consistent with an inhibition of SCD (Table 1). Changes in the content of c9-10:1, c9-12:1 and c9-14:1 to Co were used as an indication of incomplete inhibition and allowed to estimate that 92% of milk c9t11-18:2 was endogenously synthesized. Cobalt had negligible effects on other milk FA. ...