NCSU Extension Swine Husbandry 2009

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July, 2009 Volume 32, Number 06


SOLUBLE NON-STARCH POLYSACCHARIDES (GUAR GUM) DECREASE NUTRIENT DIGESTIBILITY AND AMMONIA EMISSION IN SWINE WHILE INCREASING MANURE ODOR

Fiber 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 fiber, 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 cecum and large intestine. This fermentation is expected to increase the production of volatile organic compounds and will contribute to unpleasant odors. On the other hand, dietary NSP can shift nitrogen (N) from urine to feces in the form of bacteria protein, thereby reducing ammonia emission (Shriver et al., 2003). Non-starch polysaccharides increase fecal bulk (Moeser et al., 2002), which needs to be considered when evaluating the effects of fiber on overall odor.

Studies that directly measure the impact of soluble NSP on ammonia and odor 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 odor from manure.

Study Design
Diets consisted of a low-fiber control (degermed, dehulled corn plus soy-protein isolate) and the control with 2, 4, or 8% added guar gum (> 75% galactomannan). We aimed to formulate a diet with a fiber content well below that in a standard diet based on corn plus soybean meal. We increased dietary fiber content by supplementation of guar gum, with the highest fiber 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 4 weeks to ensure stabilization of their intestinal microflora. Subsequently, feces and urine were collected quantitatively and measured for 3 days. The pigs were then sacrificed for collection of digesta from the end of the small intestine (ileum), cecum, and large intestine (colon). Fresh manure was created by mixing feces 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
Fecal and ileal digesta samples as well as urine samples were analyzed for chemical composition to determine nutrient digestibility and excretion. Short-chain fatty acid (SCFA) concentrations of samples from the cecum and feces were determined using gas chromatography. Fresh and aged manure were subsampled (10 ml) and sent to West Texas A&M University for odor 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 odor 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 odor intensity, and assigned a standard number that matched the sample’s intensity. Odor compounds from headspace air of manure samples were adsorbed by solid phase microextraction (SPME) fibers for 30 min and were analyzed 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 L/min. Ammonia released from the manure was then trapped in sulfuric acid solution, which was then analyzed at 12, 24, 36, 48, 72, and 96 h.

Results and Discussion
Guar gum did not affect fecal DM output, but linearly decreased (P < 0.001) ileal and fecal DM content and thus increased daily fecal 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 feces.

Apparent ileal N digestibility (P < 0.01), fecal N and GE digestibility (P < 0.001), ADFI, ADG, N intake, and N 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 7% guar gum and attributed these effects to decreased digesta passage rate, increased digesta viscocity, 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 feces. 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 feces 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 cecum, decreased linearly with increasing guar gum. Fecal 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 odorants 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). Odor 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 odor perception by a professional panel.

Cumulative manure ammonia emission increased in fresh manure with increasing guar gum but only up to 24 h (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% at 12 h and 7% at 96 h; 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 volatilization.

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

Literature Cited

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  • Shriver, J.A., S. D. Carter, A. L. Sutton, B. T. Richert, B. W. Senne and L. A. Pettey. 2003. Effects of adding fiber sources to reduced-crude protein, amino acid-supplemented diets on nitrogen excretion, growth performance, and carcass traits of finishing pigs. J. Anim. Sci.81: 492- 502.

-Submitted by Eric van Heugten




Last modified July 10, 2009.