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Dairy cows develop frequently negative energy balance around parturition and in early lactation, resulting in excessive mobilization of body fat and subsequently in increased risk of ketosis and other diseases. Dietary conjugated linoleic acid (CLA) supplements are used in dairy cows mainly for their depressing effect on milk fat content, but are also proposed to have antioxidative properties. As negative energy balance is associated with oxidative stress, which is also assumed to contribute to disease development, the present study was conducted to examine effects of CLA on oxidative and antioxidative status of lactating dairy cows. German Holstein cows (primiparous n = 13, multiparous n = 32) were divided into 3 dietary treatment groups receiving 100 g/d of control fat supplement, containing 87% stearic acid (CON; n = 14), 50 g/d of control fat supplement and 50 g/d of CLA supplement (CLA 50; n = 15), or 100 g/d of CLA supplement (CLA 100; n = 16). The CLA supplement was lipid-encapsulated and contained 12% of trans-10, cis-12 CLA and cis-9, trans-11 CLA each. Supplementation took place between d 1 and 182 postpartum; d 182 until 252 postpartum served as a depletion period. Blood was sampled at d -21, 1, 21, 70, 105, 140, 182, 224, and 252 relative to calving. The antioxidative status was determined using the ferric-reducing ability of plasma, α-tocopherol, α-tocopherol-to-cholesterol mass ratio, and retinol. For determination of oxidative status concentrations of hydroperoxides, thiobarbituric acidreactive substances (TBARS), N'-formylkynurenine, and bityrosine were measured. Mixed models of fixed and random effects with repeated measures were used to evaluate period 1 (d -21 to 140) and 2 (d 182-252) separately. Cows showed increased oxidative stress and lipid peroxidation during the periparturient period in terms of increased serum concentrations of hydroperoxides and TBARS, which decreased throughout lactation. During period 1, the supplemented cows had lower TBARS concentrations, which was not detectable in period 2. The other determined parameters were not affected by CLA supplementation. The obtained results show that dietary CLA supplementation in the chosen dosage, formulation, and application period had a marginal antioxidative effect in terms of lipid peroxidation in lactating dairy cows. [ABSTRACT FROM AUTHOR]

The objective of the present study was to evaluate if grass silages with high contents of free amino acids (FAA) lead to enhanced amino acid catabolism compared with grass silages with low contents of FAA when fermented in a rumen simulation system (Rusitec). Eight grass silages with contents of FAA lower than 300 mmol/kg dry matter (DM; GS-LOW) and eight grass silages with contents higher than 300 mmol/kg DM (GS-HIGH) were tested in vitro for their effects on selected variables of nitrogen metabolism. Ammonia concentration, production of the branched-chain volatile fatty acids (BCVFA) isobutyric and isovaleric acid, concentration of FAA in fermenter fluid, and the content of bacterial protein in fermenter fluid were measured daily from days 1 to 28 (ammonia, BCVFA), 3 and 6 to 28 (bacterial protein), and days 7, 8, 11 to 13, 16 to 21, and 26 to 28 (FAA), respectively. Subsequent to an initial 9-day adaptation period during which hay (10.5 g DM) and concentrate (3.0 g DM) were added to fermenters, grass silages (10.5 g DM) were individually incubated in the fermenters for a period of 10 days (experimental period). The experimental period was followed by the recovery period during which the additions of hay and concentrate were equal to those of the adaptation period. Addition of GS-HIGH increased the concentration of ammonia (P<0.05) and production of isobutyric (P<0.05) and isovaleric (P<0.05) acid in contrast to GS-LOW. The concentration of FAA, as well as the content of bacterial protein in fermenter fluid, showed no differences after addition of GS-LOW or GS-HIGH (P>0.05). The calculated efficiency of nitrogen fixation in bacterial protein was expressed as units of bacterial protein per g nitrogen input with the silages and was lower or tended to be lower, respectively, with GS-HIGH on 5 days of the experimental period (P<0.1). Increased concentration of ammonia and production of BCVFA indicate that amino acid catabolism was increased by additions of GS-HIGH compared to GS-LOW whereas efficiency of nitrogen fixation in bacterial protein seemed to be decreased. The observed effects may be due to higher metabolic activity of amino acid-fermenting bacteria. [ABSTRACT FROM AUTHOR]