The objective of this study was to investigate the relationship between the concentrations of the different ketone bodies in milk and blood and to evaluate these concentrations for the detection of subclinical ketosis. A total of 60 multiparous cows were used. Concentrations of acetone, acetoacetate, and beta-hydroxybutyrate were analyzed quantitatively in blood and milk, and the Ketolac strip test was used for semiquantitative determination of beta-hydroxybutyrate in milk. Cows were defined subclinically ketotic when their concentration of blood beta-hydroxybutyrate was over 1200 micromol/L. High correlation coefficients were observed between blood acetone and blood acetoacetate, and between blood and milk acetone. On the contrary, concentrations of milk and blood beta-hydroxybutyrate were poorly correlated with the other concentrations of ketone bodies. The Ketolac strip test overestimated the concentrations of beta-hydroxybutyrate in milk. For the detection of subclinical ketosis, the best sensitivity-specificity combination was obtained with the determination of acetoacetate in blood or milk, with threshold concentrations of 125 and 50 micromol/L, respectively. Determination of beta-hydroxybutyrate in the milk via an enzymatic analysis or via the Ketolac strip test provided valuable results, with threshold concentrations of 70 and 100 micromol/L, respectively. The simplicity of use of the Ketolac strip test makes it a valuable way to investigate subclinical ketosis.

The objectives of this study were to evaluate the capacity of 2 dietary feed additives, sodium bicarbonate and live yeast Saccharomyces cerevisiae (strain Sc 47), in optimizing ruminal pH in dairy cows and to determine their modes of action. Three early lactating Holstein cows, fitted with ruminal cannulas, were allocated in a 3 x 3 Latin square design. They were given a total mixed ration as control diet (CD) at a daily feeding rate of 28.0 kg of dry matter (DM)/cow supplemented with 150 g/d of sodium bicarbonate (SBD) or 5 g/d of live yeast (YD) during a 21-d experimental period (14 d of diet adaptation, 4 consecutive days of measurement and sampling and 3 d of transition). The pH and redox potential (E(h)) were measured from 1 h before feeding to 8 h after feeding at 1-h intervals, and samples of ruminal fluid were taken at 0, 2, 4, 6, and 8 h after feeding for the determination of volatile fatty acids and lactate concentrations. Total tract apparent digestibility of the diet was also determined. Ruminal pH fluctuated between 6.53 at feeding and 5.57 at 5 h postfeeding. Mean pH was greater with SBD (6.21) and YD (6.14) compared with CD (5.94), showing that both additives had a pH stabilization effect. The E(h) varied from -88 mV at 1 h before feeding to -165 mV at 1 h after feeding. Mean E(h) and Clark's Exponent (rH) were lower with YD (-149 mV and 7.31, respectively) than with SBD (-137 mV and 7.85, respectively) and CD (-115 mV and 8.05, respectively), indicating that the yeast strengthened the reducing power of the milieu. Total volatile fatty acids were greater in SBD (95.3 mM) and YD (99.4 mM) compared with CD (85.3 mM). Acetate concentration was greater in SBD (60.8 mM) and YD (59.1 mM) compared with CD (53.2 mM). Propionate concentration was greater in YD (25.8 mM) than in SBD (20.0 mM) and CD (18.0 mM). Butyrate remained constant between diets. Mean total lactate concentrations were 16.5, 12.2, and 5.4 mM for CD, SBD, and YD, respectively, with a 67% decrease with YD. Total tract organic matter digestibility was greater for YD (66.6%) compared with SBD (61.7%) and CD (62.2%). The neutral detergent fiber digestibility was greater with YD (41.6%) compared with SBD (34.3%) and CD (29.6%), whereas acid detergent fiber digestibility was greatest in YD (32.3%), intermediate in SBD (24.4%), and lowest in CD (18.1%). By inducing a lower ruminal E(h) and rH, live yeast prevented accumulation of lactate and allowed better fiber digestion, whereas sodium bicarbonate seemed to act only as an exogenous buffer.

The effect of dietary lipids on the fatty acid (FA) profile of cows' milk fat is mainly dependent on digestive processes and mammary gland uptake and metabolism of FA. The objective of this study was to determine the separate effects of high arterial concentrations of 16:0, 18:0 and cis-18:1(n-9) on uptake, synthesis and 18:0 desaturation rate in the mammary gland of lactating dairy cows, via arterio-venous differences and mammary gland balance of FA. In a 4 x 4 Latin square, four lactating Holstein cows with cannula in the proximal duodenum were infused duodenally with a mixture providing daily 0 (C treatment) or 500 g FA with mainly 16:0 (P treatment), 18:0 (S treatment) or cis-18:1(n-9) (O treatment). Significantly higher arterial concentrations of infused FA in arterial plasma nonesterified FA and triglycerides (NETGFA) were observed with P and O treatments, but the effect of the S treatment was much lower. Arterio-venous differences of NETGFA increased with arterial concentrations. The number of synthesized FA in the mammary gland was not significantly affected by duodenal infusion of FA. Mean chain length was significantly reduced by P and O treatments, suggesting an effect of mammary gland uptake of long-chain FA on the termination process of mammary gland synthesis of FA. Across all treatments, 4:0 mammary gland balance increased linearly (r = 0.67, P = 0.004) with mammary gland FA uptake. Mammary gland desaturation of 18:0 to cis-18:1(n-9) averaged 52% and was not significantly affected by treatments, but was reduced by trans-18:1 mammary gland uptake. Uptake, synthesis and desaturation of FA by the mammary gland of dairy cows are affected by arterial concentrations of 16:0, 18:0 and cis-18:1(n-9).