Glycerol use in dairy diets: A systemic review

There is an increasing interest in the production of biodiesel as bio-renewable fuel source, with numerous biofuel byproducts becoming available. The annual productions of biodiesel and crude glycerol were 34.5 and 3.8 billion liters, respectively, in 2016 and that of biodiesel is expected to reach 41 billion liters in 2019. Glycerol is a sugar alcohol without a color or odor, but with a sweet taste and high solubility index in water. Experiments support the use of glycerol at low levels ranging from 5% to 8% of the diet dry matter as a transition cow therapy. Administration of glycerol increases serum glucose and decreases ketone bodies. Glycerol is very rapidly fermented in the rumen to propionate and butyrate, at the expense of acetate, resulting in a decreased milk fat. Because glycerol is highly fermented in the rumen, it requires an adaptation period at the beginning of feeding. Administration of glycerol in the diet of lactating animals was paralleled with a decreased or an unaffected feed intake in most experiments. Improved ruminal environment to enhance nutrient digestibility was observed in many experiments; however, others observed reduced digestion of dietary fiber with feeding glycerol. Enhanced, lowered, or unaffected milk production and composition were observed with the administration of glycerol in lactating animal diets; however, in most cases, glycerol decreased milk fat content. The inconsistencies between results of experiments are due to the level and the purity of glycerol, diets, production stage of the animals, and other factors. Therefore, further research should be conducted to establish the efficacy of different levels, purity and administration periods of glycerol, and production stage of dairy animals fed glycerol-based or supplemented diets.