P.J. Lammers of Iowa State University and co-authors from USDA-ARS at Ames, USDA-ARS Poultry Research Unit and Mississippi State University set out to evaluate the energy of crude glycerol (glycerine) for growing pigs. They published a paper on their results in Journal of Animal Science last year.

In their introduction, the scientists noted, "Crude glycerol is the principal co-product of biodiesel production with 79 g of crude glycerol generated for every litre of biodiesel produced." With current biodiesel production capacity, approximately 4.16 × 10⁵ tonnes of crude glycerol could be generated annually in 2007.

And production of biofuels is likely to increase as a result of rising energy prices, uncertain access to petroleum supplies, and recognition of the environmental impacts of fossil fuel use. This will make more biofuel by-products available to livestock producers and nutritionists may need to be more flexible in their choice of feed ingredients in future.

"Crude glycerol, a readily available energy source, may play an important role in meeting the energy needs of pigs as biodiesel production expands," note the researchers.

There have been several reviews published on the metabolic effects of glycerol, which has been found to be both well absorbed and utilised by non-ruminants.

"Studies examining the effects of supplementing crude glycerol to diets fed to swine and broilers have shown little to no effect on animal performance," they wrote, "and research determining the energy value of crude glycerol is limited."

So Lammers and co-authors set up the present study to determine the apparent digestible energy and metabolisable energy of crude glycerol at various levels of supplementation and to determine if the apparent energy values differed between starter and finisher pigs.

They carried out five experiments using crude glycerol from a biodiesel production facility, which used soybean oil as the initial feedstock. The crude glycerol contained 86.95 per cent glycerol.

Dietary treatments were as follows:

• 0, 5 or 10 per cent glycerol addition to basal diets in Experiment 1
• 0, 5, 10, or 20 per cent glycerol addition to basal diets in Experiment 2 and
• 0 and 10 per cent crude glycerol addition to the basal diets in Experiments 3, 4 and 5.

Three experiments (1, 3 and 4) used starter pigs with an average initial weight of 10.3 kg, whereas experiments 2 and 5 involved finishing pigs with an average initial weight of 104.7 kg.

Each diet was fed twice daily to pigs in individual metabolism crates. After a 10-day adjustment period, a five-day balance trial was conducted. During the collection period, faeces and urine were collected separately after each meal and stored at 0°C until analysis.

The gross energy (GE) of each dietary treatment and samples of urine and faeces from each pig were determined by isoperibol bomb calorimetry.

Digestible energy of the diet was calculated by subtracting faecal energy from the GE in the feed, whereas ME was calculated by subtracting the urinary energy from DE.

The DE and ME values of crude glycerol were estimated as the slope of the linear relationship between either DE or ME intake from the experimental diet and feed intake.

Across all the experiments, the crude glycerol examined in this study was shown to have a DE of 3,344 ± 8 kcal/kg and an ME of 3,207 ± 10 kcal/kg. The values from each experiment are shown in the table.

Apparent energy values of crude glycerol fed to pigs, as-fed basis¹

Based on previous work, the report notes, "We did not expect the level of crude glycerol to affect ME determination. However, when data from Experiment 1 were analysed separately, the ME of crude glycerol declined with increasing levels of supplementation, with estimated ME values of 3,601, 3,239 and 2,579 kcal/kg crude glycerol for 5, 10 and 20 per cent inclusion levels, respectively (quadratic, P=0.05)".

The ME values were not affected by the age of the pigs.

The researchers say that their results confirm that crude glycerol is well digested by pigs, and it was also supported previously published work in which crude glycerol was found to be well utilized by the pig as a source of energy.

They raised an important point regarding the high purity of the material they used in their experiments. Commercially available glycerol may contain potentially harmful impurities. The researchers recommend that concentrations of other compounds in crude glycerol, i.e. methanol, sodium chloride, potassium chloride and free fatty acids, must be monitored to prevent excessive amounts in pig diets and for potential impacts on ME determination of this feedstuff.

"With an ME of 3,207 ± 10 kcal/kg, crude glycerol can be used as an excellent source of energy for growing pigs," concluded Lammers and colleagues.

Reference

Lammers, P.J, B.J. Kerr, T.E. Weber, W.A. Dozier III, M.T. Kidd, K. Bregendahl and M.S. Honeyman. 2008. Digestible and metabolizable energy of crude glycerol for growing pigs. J. Anim. Sci. 2008. 86:602–608.

Further Reading

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