Glycerol Supplementation in Dairy Cows and Calves

The production of biodiesel from rapeseed oil methyl ester leaves glycerol (synonym: glycerine, 1,2,3-propanetriol) as a valuable by-product and a promising feed supplement for farm animals. This thesis provides information about the supplemental feeding with glycerol to dairy cows in early lactation and to young calves, and describes the fate of glycerol entering the rumen and the impact of glycerol on enteric methane production and gastrointestinal microbial populations. This was achieved by investigating the effects of two glycerol products of different purity crude glycerol (88.1%) and glycerol (>99%) on milk production, feed intake and metabolic parameters in 42 dairy cows in early lactation. Furthermore, different routes of the disappearance of glycerol from the rumen were quantified. These were i) fermentation in the rumen, ii) absorption across the rumen epithelium and iii) rumen outflow through the omasal orifice. The effect of glycerol on enteric methane production, rumen volatile fatty acid profiles and microbial population in rumen fluid was investigated in a gas in vitro system. Finally, the effects of oral rehydration solutions (ORS) with added glycerol on metabolic parameters and intestinal microbiota were examined in young calves. Glycerol supplementation, irrespective of purity, did not significantly affect milk yield or composition, the total intake of dry matter or metabolic parameters in dairy cows in early lactation. It was estimated that approximately 70% of the glycerol was absorbed mainly from the rumen, but probably also from the small intestine. A smaller fraction disappeared from the rumen compartment by microbial digestion. These findings indicate that glycerol is an available gluconeogenic substrate which might efficiently contribute to glucose synthesis in the liver. Addition of glycerol in the gas in vitro system indicated no reduction in methane production. Furthermore, bacterial and archaeal community structures subjected to additional glycerol followed a similar pattern as the in vitro control with no feed additive. Glycerol was also shown to be a suitable component in ORS for young calves since it ameliorated the effects of dehydration on feed and fluid deprived calves. In calves, glycerol was rapidly absorbed, presumably in the small intestine, and thus most likely not available to the intestinal microbiota.