The effect of replacing barley with glycerol in the diet of dairy cows on rumen parameters and milk fatty acid profile

• High amounts (up to 3 kg/d) of glycerol was fed to cows in mid-lactation.
• Dietary glycerol did not change rumen pH but affected volatile fatty acid proportions.
• Dietary glycerol increased the proportions of odd chain fatty acids in milk fat.

The objective of this paper was to evaluate the changes in rumen parameters (pH, NH3-N and volatile fatty acids (VFA) proportions), caused by large amounts of dietary glycerol fed to dairy cows, and whether those changes are associated with differences in the milk fatty acid (FA) composition. For this study eight primiparous Estonian Holstein cows (134 ± 5 DIM at the beginning of the experiment) were paired and used in a 4 × 4 Latin square experimental design. Cows were fed a total mixed ration ad libitum, consisting of grass silage, barley meal, soybean meal and minerals (control) with crude glycerol isoenergetically replacing barley meal in experimental diets to inclusion levels of 52 g/kg, 104 g/kg and 156 g/kg of crude glycerol in the dry matter (DM) of the diets. The effect of the diets was studied with the analyses of variance followed by the tests of linear and quadratic contrasts. Pearson correlation was used to assess the suitability of models predicting rumen VFA proportions based on milk FA, when glycerol-containing diets were fed. Results showed that dietary glycerol did not affect ruminal pH but linearly increased the relative proportions of propionate and butyrate in rumen fluid, at the expense of acetate. The ratios of acetate to propionate, and lipogenic VFA to propionate, showed both linear and quadratic changes. The proportion of 2-methyl butyrate changed both linearly and quadratically; highest when the diet with 52 g/kg in DM of crude glycerol was fed. Rumen total VFA content decreased linearly according to the crude glycerol content in the diets. Replacing barley meal with glycerol in the diets affected the concentration of most of the de novo synthesized milk FA, as well as FA in different groups according to chain length or degree of saturation. The proportions of even-chain saturated FA C6:0–C10:0 increased linearly in milk fat of cows in the dietary glycerol treatments, whereas the proportions of C14:0 and C16:0 decreased quadratically. The proportion of saturated odd-chain FA (C5–C17) increased linearly and quadratically with increasing dietary glycerol treatments. Correlation analysis indicated that the increase in the proportion of propionate in total VFA was accordance with the changes in the milk FA profile, increasing the odd-chain (C5–C17) FA concentration.