Abstract
During diet-induced milk fat depression (MFD), the short and medium-chain fatty acids (SMCFA), which are synthesized de novo in the mammary gland, are reduced to a much greater extent than the long-chain fatty acids (LCFA) that originate from the circulation. Our hypothesis was that increased availability of SMCFA might rescue conjugated linoleic acid (CLA)-induced MFD in lactating dairy cows. To test that hypothesis, 4 rumen-fistulated lactating Holstein cows (128 ± 23 d in milk) were used in a 4 × 4 Latin square design with 3-wk experimental periods. Treatments were applied during the last 2 wk of each period and included 3× daily abomasal infusion of a total of (1) 230 g/d of LCFA (blend of 59% cocoa butter, 36% olive oil, and 5% palm oil); (2) 420 g/d of butterfat (BF); (3) 230 g/d of LCFA with 27 g/d of CLA (LC-CLA), containing 10 g/d of trans-10,cis-12 CLA; and (4) 420 g/d of butterfat with 27 g/d of CLA (BF-CLA). Butterfat provided 50% of C16 (115 g/d) and similar amounts of C18 FA as found in LCFA, such that the difference between the BF and LCFA treatments was 190 g/d of SMCFA. No treatment effects were observed for DMI or milk yield. Milk fat content was reduced by 41 and 32%, whereas milk fat yield was reduced by 41 and 38% with LC-CLA and BF-CLA, respectively, compared with their respective controls. Abomasal infusion of CLA reduced de novo synthesized fatty acid (DNFA; SMCFA and 50% C16:0) concentration, whereas DNFA tended to be greater with BF infusion. An interaction was observed between SMCFA and CLA as the increased availability of SMCFA reduced stearoyl-CoA-desaturase-1 gene expression, whereas it tended to reduce lipoprotein lipase (LPL), 1-acylglycerol-3-phosphate O-acyltransferase 6 (AGPAT-6), sterol regulatory element-binding protein cleavage-activating protein (SCAP), and peroxisome proliferator-activated receptor γ (PPAR-γ) gene expression in the presence of CLA. The mRNA expression of genes involved in de novo fatty acid synthesis [acetyl-coenzyme A carboxylase α (ACACA) and fatty acid synthase (FASN)], fatty acid uptake (LPL), and triglyceride synthesis [AGPAT-6 and diacylglycerol O-acyltransferase 1 (DGAT-1)] along with protein abundance of the ACC and FASN were reduced with CLA. However, the increased availability of SMCFA had no effect on lipogenic gene expression except for LPL, whose expression was increased with BF infusion. The nutritional manipulation by increasing the intestinal availability of SMCFA was not sufficient to rescue CLA-induced MFD.