Analysis of factors predisposing dairy Assaf ewes to marine lipid-induced milk fat depression

In sheep, diet-induced milk fat depression (MFD) results from ruminal alterations when ewes are fed marine lipids to modulate milk fatty acid profile and secretion. However, there is little available information on the relationship between characteristics existing before the consumption of MFD-inducing diets and response to those diets. Understanding predisposing factors for this condition may provide additional insight into the mechanisms of MFD and allow better predictions of its occurrence. Thus, a meta-analytical approach was used to identify traits that may predetermine the extent of marine lipid-induced MFD in individual Assaf sheep (a dairy breed with high genetic potential for milk production). Data were collected from 10 trials in which 160 ewes were fed 16 diets that included fish oil or marine algae until sustained MFD was observed. The decrease in milk fat concentration and yield was calculated using three approaches: absolute change (final - initial), relative change as a percent of pre-trial value, and potential change relative to the maximal expected MFD (assuming a maximal decrease to 3% milk fat). First, using bivariate analyses, there was a relationship between initial milk fat concentration and the absolute (R² = 0.46; partial R² of 0.08), relative (R² = 0.39; partial R² of 0.06), and potential changes in milk fat concentration during MFD (R² = 0.17; partial R² = 0.03). This finding was supported by a second approach that categorised ewes by initial milk fat concentration and yield and additionally by multivariate analyses. In addition, bivariate and multivariate analyses suggested that high MFD responsiveness in Assaf ewes was related to pre-trial milk yield and protein concentration, and to milk concentrations of candidate antilipogenic metabolites and the sum of preformed fatty acids. However, relationships were weaker than for initial milk fat concentration. Overall, when Assaf ewes were fed marine lipids, the higher the initial milk fat concentration, the greater the extent of MFD. The role of other performance traits and milk fatty acids as predisposing factors for marine lipid-induced MFD remains unclear.