Relationship between milk odd and branched-chain fatty acids and urinary purine derivatives in dairy cows supplemented with quebracho tannins—A study to test milk fatty acids as predictors of rumen microbial protein synthesis

• Quebracho tannin decreased most odd- and branched-chain fatty acids (OBCFA) proportions and yields in milk fat of cows.
• Milk OBCFA did not correlated with purine derivative excretion in urine.
• Milk OBCFA are not reliable markers for rumen microbial protein synthesis in cows.
• Shifts in chemical composition of microbial mass hamper accuracy of indirect markers.

The purpose of this study was to explore the potential of milk odd and branched-chain fatty acids (OBCFA) as biomarkers for rumen microbial protein synthesis in dairy cows, and to study the effects of diets supplemented with different levels of quebracho tannin extract (QTE) on milk OBCFA.The experiment included 50 lactating German Holstein dairy cows divided into two groups of 25 animals each and lasted for six consecutive periods of 21 days (13 days of adaptation and 8 days of feed, milk, and urine sampling). The basal diet was a total mixed ration with a constant forage to concentrate ratio of 65:35 (Control, without QTE). A commercial QTE product was added to the basal diet at 15 g and 30 g/kg dry matter to create diets QTE15 and QTE30, respectively. Milk fat samples were analysed for OBCFA contents and urine for purine derivatives (PD), nitrogen (N), and creatinine concentrations.The QTE addition increased the concentrations of creatinine and the ratio between total PD and N in urine, whereas the concentrations of allantoin, total PD, and N in urine decreased with QTE addition. Similarly, addition of QTE decreased the proportions (g/100 g fat) and yields (g/d) of milk OBCFA, with the exception of anteiso-C15:0 and anteiso-C17:0.Multivariate models to predict concentrations and ratios of PD, N, and creatinine in urine from milk OBCFA explained between 10% and 40% of the variance in the dataset of each predicted parameter. However, the explanation of that variance attributed to milk OBCFA was only between 2% and 26%. For most parameters, models developed from milk OBCFA proportions explained more variance in the dataset than those developed from OBCFA yields. Our findings suggest that milk OBCFA are poor indicators of rumen microbial protein synthesis in dairy cows.