Soluble Non-Starch Polysaccharides Decrease Nutrient Digestibility and Ammonia Emission While Increasing Manure Odour

The use of soluble non-starch polysaccharide (NSP; guar gum) reduced ammonia emission in aged manure but decreased nutrient digestibility and increased odour emissions, thus limiting its environmental benefits. These are the conclusions from Eric van Heugten in an article published in the July 2009 issue of Swine News from North Carolina State University.
calendar icon 28 July 2009
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Fibre is resistant to digestion by endogenous enzymes in mammals, and it will decrease the digestibility of feed nutrients and increase endogenous protein and fat losses (Noblet & Perez, 1993; De Lange et al., 1989). Dietary fibre, particularly soluble non-starch polysaccharides (NSP), has high water-binding capacity (Bach Knudsen, 2001; Antoniou & Marquard, 1981; Jensen & Jørgensen, 1994). Therefore, soluble NSP may hold more water in the colon, stimulate microbial activity and extend hindgut fermentation time. As a result, more substrate, in the form of NSP, undigested protein, and endogenous losses, will be available for fermentation in the caecum and large intestine. This fermentation is expected to increase the production of volatile organic compounds and will contribute to unpleasant odours. On the other hand, dietary NSP can shift nitrogen (N) from urine to faeces in the form of bacteria protein, thereby reducing ammonia emission (Shriver et al., 2003). Non-starch polysaccharides increase faecal bulk (Moeser et al., 2002), which needs to be considered when evaluating the effects of fibre on overall odour.

Studies that directly measure the impact of soluble NSP on ammonia and odour production in swine are limited. Thus, we conducted a study to determine the impact of soluble NSP on nutrient digestibility, and emission of ammonia and odour from manure.

Study Design

Diets consisted of a low-fibre control (degermed, dehulled corn plus soy-protein isolate) and the control with two, four or eight per cent added guar gum (>75 per cent galactomannan).

The researchers aimed to formulate a diet with a fibre content well below that in a standard diet based on corn plus soybean meal. They increased dietary fibre content by supplementation of guar gum, with the highest fibre level being much greater than that of a standard corn plus soybean meal diet. Pigs (n=28; body weight=26.8 ± 1.4 kg) were fed diets for four weeks to ensure stabilisation of their intestinal microflora. Subsequently, faeces and urine were collected quantitatively and measured for three days. The pigs were then sacrificed for collection of digesta from the end of the small intestine (ileum), caecum, and large intestine (colon). Fresh manure was created by mixing faeces and urine for each pig in the ratios at which they were produced. Aged manure was obtained by anaerobically aging this mixture for 21 days.

Sample Analyses

Faecal and ileal digesta samples as well as urine samples were analysed for chemical composition to determine nutrient digestibility and excretion. Short-chain fatty acid (SCFA) concentrations of samples from the caecum and faeces were determined using gas chromatography.

Fresh and aged manure were sub-sampled (10 ml) and sent to West Texas A&M University for odour hedonic tone and intensity evaluation by a professional panel. The panelists were asked to smell each sample individually and designate a degree of pleasantness or unpleasantness according to a -10 to +10 hedonic scale, with 0 being neutral and -10 being most unpleasant. They also were asked to assign a score for odour intensity by smelling each sample and comparing it to a set of standards. The intensity standards were prepared with n-butanol at various concentrations to achieve a 1 to 5 scale, with 1 representing very faint, 2 faint, 3 moderate, 4 strong, and 5 very strong (Guo et al., 2001). The panelists smelled each sample, compared it to the standards for odour intensity, and assigned a standard number that matched the sample’s intensity. odour compounds from headspace air of manure samples were adsorbed by solid phase microextraction (SPME) fibres for 30 minutes and were analysed by gas chromatography and mass spectrometry immediately.

Ammonia emission of each manure sample was determined by placing 400 ml of the manure mixture in a rectangular container and drawing air through this container of manure at a flow rate of 1.4 litres per minute. Ammonia released from the manure was then trapped in sulfuric acid solution, which was then analyzed at 12, 24, 36, 48, 72 and 96 hours.

Results and Discussion

Guar gum did not affect faecal DM output, but linearly decreased (P<0.001) ileal and faecal DM content and thus increased daily faecal output (Table 1; P<0.02). These results are consistent with the observation that soluble NSP have a high water-binding capacity (Bach Knudsen, 2001), thus reducing the DM content of faeces.

Apparent ileal N digestibility (P<0.01), faecal N and GE digestibility (P<0.001), average daily feed intake (ADFI), average daily gain (ADG) as well as N intake and retention (P<0.03) linearly decreased with increasing guar gum (Table 2). Similarly, Owusu-Asiedu et al. (2006) observed decreased CP and energy digestibility in pigs fed diets containing seven per cent guar gum and attributed these effects to decreased digesta passage rate, increased digesta viscosity, and increased growth of bacterial populations in the gastrointestinal tract.

The reduction in CP digestibility in pigs fed guar gum in the present study resulted in a tendency (P=0.07) for increased N excretion in faeces. No significant differences were found, however, in urinary N excretion. This is in contrast to our expectation that increased dietary NSP would result in a shift in N excretion from urine to faeces as reported by Canh et al. (1997). The reduced N intake in pigs fed guar gum and the reduced ileal digestibility of N would have been expected to decrease urinary N excretion. Nitrogen retention linearly decreased (P=0.03) with increasing guar gum inclusion, which is consistent with observed reductions in growth rate.

The pH of the colon content, but not ileum or caecum, decreased linearly with increasing guar gum. Faecal concentrations of acetic acid, propionic acid, butyric acid, valeric acid and total SCFA increased linearly (P<0.05) with increasing guar gum (data not shown). These data suggest that the primary site of fermentation of guar gum was the colon.

Increasing guar gum had no effect on odourants and pH in fresh manure but it linearly (P<0.05) increased dimethyldisulfide, dimethyltrisulfide, and phenol in head space and acidity of aged manure (Table 3). Odour intensity tended (P=0.08) to increase (2.96, 3.27, 3.40 and 3.35) with increasing guar gum in aged manure, but not fresh manure. Thus, the increased concentrations of dimethyldisulfide, dimethyltrisulfide and phenol in the headspace of aged manure samples with increasing levels of guar gum did not result in clear differences in odour perception by a professional panel.

Cumulative manure ammonia emission increased in fresh manure with increasing guar gum but only up to 24 hours (P<0.05). This is contrary to previous observations that suggest decreased ammonia emission with NSP supplementation (Canh et al., 1998). In aged manure, ammonia emission decreased linearly (up to 47 per cent at 12 hours and seven per cent at 96 hours; P<0.05) with increasing guar gum. Increased fermentation of manure from pigs supplemented with guar gum may have reduced slurry pH and subsequently reduced ammonia volatilisation.

In conclusion, the use of soluble NSP reduced ammonia emission in aged manure but decreased nutrient digestibility and increased odour emission, thus limiting its environmental benefits.

Literature Cited

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July 2009

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