An appraisal of the concept of Rumen Unsaturated Fatty Acid Load and its relation to milk fat concentration using data from commercial dairy farms

The concept of Rumen Unsaturated Fatty Acid Load (RUFAL) has been proposed to reflect the supply of dietary unsaturated fatty acids in relation with their potential to disturb ruminal fermentation and trigger milk fat depression in lactating dairy cows. The objective of this study was to assess this concept, and its relationship with milk fat concentration, using data available in a DHI database. Data from Holstein cows recorded over 3 yr by Valacta (Sainte-Anne-de-Bellevue, QC, Canada) were used for the analysis. The fatty acid concentrations in feed were obtained from CNCPS V6.1, INRA-AFZ Tables of Feed Composition, and peer-reviewed articles. Multiple regression analyses were performed at 2 stages of lactation (early: 1-100 DIM; and established: 101-350 DIM) using the MIXED procedure of SAS. Estimated breeding value was also included to account for the effect of genetics on milk fat concentration. Results show that RUFAL has a negative and significant relationship with milk fat concentration at each stage of lactation (P < 0.01), which confirms the validity of this concept under commercial conditions. The negative effect of RUFAL was continuous and progressive, and data did not allow defining a threshold value above which the supply of dietary unsaturated fatty acids should be limited to avoid milk fat depression. Dietary 16:0 has also been retained in prediction models with positive coefficients and was therefore associated with greater milk fat concentration (P < 0.01). As the complete fatty acid profile of feedstuffs could be determined in a single laboratory analysis using GLC, we conclude that the RUFAL concept could be improved by considering an adjustment factor taking into account the concentration of dietary 16:0.