W.A. Dozier of USDA Agricultural Research Service in Mississippi and colleagues noted in a paper recently published in Poultry Science that glycerin is a by-product of biodiesel production, and that it represents about nine per cent of the starting feedstocks on a weight basis.

With the growth in biofuel production, particularly in the US, increasing quantities of glycerin have become available on the market. Glycerin is a valuable ingredient in food, soaps, cosmetics and pharmaceuticals, and previous researchers have observed that it has a high gross energy content, and is easily digested and absorbed.

In earlier work, glycerin has been found to be an acceptable feed ingredient for poultry: adding glycerin up to an inclusion level of five per cent showed no adverse effects on growth or carcass yield but above 10 per cent, problems were experienced with flowability and feed intake, which negatively impacted growth and meat yield of broilers.

Dozier and colleagues were unable to find publication of papers on the apparent metabolisable energy (AMEn) of glycerin, and so they set out to determine this, and whether the value was affected by age of the bird.

They conducted three energy balance experiments to determine AMEn of glycerin using broiler chickens of diverse ages.

In experiment 1, two dietary treatments were fed from four to 11 days of age. Dietary treatments consisted of a control diet (no added glycerin) and a diet containing 6 per cent glycerin (94 per cent control diet + 6 per cent glycerin).

Four dietary treatments were provided in experiment 2 (from 17 to 24 days of age) and 3 (from 38 to 45 days of age). Diets in experiment 2 and 3 were 1) control diet (no added glycerin); 2) 3 per cent added glycerin (97 per cent control diet + 3 per cent glycerin); 3) 6 per cent added glycerin (94 per cent control diet + 6 per cent glycerin); and 4) 9 per cent added glycerin (91 per cent control diet + 9 per cent glycerin).

Diets in experiment 1 and 2 were identical but the diet used in experiment 3 had reduced nutrient levels based on bird age. In experiments 2 and 3, broilers were fed 91, 94, 97 and 100 per cent of ad libitum intake so that differences in AMEn consumption wer only due to glycerin.

A single source of glycerin was used in all experiments. It was analyzed to contain 3,625 kcal per kg in gross energy, 86.95 per cent glycerol and 9.63 per cent moisture.

The average AMEn of glycerin across the three experiments was 3,434 kcal/kg. This is similar to its gross energy value and so the authors concluded, "AMEn of glycerin is utilised efficiently by broiler chickens".

The AMEn of glycerin was determined as 3,621, 3,331 and 3,349 kcal/kg in experiments 1, 2 and 3, respectively. The researchers commented that these small differences are more likely from experimental error than from any effect of age of bird on the AMEn of glycerin for broilers.

Reference

Dozier W.A. III, B.J. Kerr, A. Corzo, M.T. Kidd, T.E. Weber and K. Bregendal. 2008. Apparent metabolizable energy of glycerin for broiler chickens. Poultry Science 87:317–322.