The effects of added glycerol or unprotected free fatty acids or a combination of the two on silage intake, milk production, rumen fermentation and diet digestibility in cows given grass silage based diets

The addition of glycerol or free fatty acids either alone or in combination to concentrate was studied for the effects on feed intake, milk production, rumen fermentation, blood metabolites and diet digestibility in dairy cows given grass silage ad libitum. The study was conducted on 12 mid-lactating cows, four of them ruminally cannulated. Barley-based concentrate (control diet, C) was given 7 kg/d as fed. In the other three diets, 36 g/kg of barley was replaced by glycerol (G) or a mixture of free fatty acids (FA) or by a combination of the two, making a total of 72 g/kg (GFA). The experimental design consisted of balanced 4 x 4 Latin squares with a 2 x 2 factorial arrangement of diets: the effects of G, FA and G*FA interaction. The FA diets significantly decreased silage intake, increased milk yield, decreased milk protein content, increased the concentrations of C18:0, C18:1, and C20:1 and decreased those of C8-16, and C18:3 fatty acids in milk fat. The FA diets also increased the concentration of nonesterified fatty acids in plasma, and decreased the digestibility of organic matter and neutral detergent fibre but increased that of fat. Glycerol decreased the molar proportion of acetate and increased the molar proportions of propionate and butyrate in the rumen, but the addition of glycerol did not have any effect on silage intake, milk yield or milk composition. Milk yield was highest when glycerol and free fatty acids were given together, showing a positive interaction