Fiber Content of Diet Affects Perceived Odor of Swine Manure

Ammonia and volatile organic compounds are major contributors to air quality problems associated with swine production. Odorants result from the fermentative degradation of carbohydrates, fats, and proteins in the large intestine and manure of swine and, therefore, can be manipulated by diet composition (Hobbs, et al., 1996; Sutton, et al., 1999). However, individual differences in odor perception make it difficult to demonstrate odor abatement strategies or validate odor nuisance problems (Moeser, et al., 2003).

We at North Carolina State University designed a study with two objectives - first, to determine the ability of two different odor panels (one trained and one untrained) to detect differences in odor perception; and second, to evaluate the differences in the odor of fresh manure or aged manure from swine fed three different levels of fiber.

Materials and Methods

We used 15 nursery-age barrows weighing 26 kg (± 3 kg) and housed them in individual metabolism cages (1.5 m x 0.9 m) in an environmentally controlled room. Upon arrival, the barrows were fed the nursery diet they were receiving prior to the move and had free access to water. On the morning of Day 3, the pigs were separated into blocks by weight (three pigs per block) and randomly assigned a low-, medium-, or high-fiber diet within the block. The low-fiber diet contained 6.5 percent NDF and was formulated using degermed, dehulled corn and soybean meal; the medium fiber diet contained 10.5 percent NDF and was based on corn and soybean meal; the high fiber diet contained 20.8 percent NDF and was formulated with corn, soybean meal, and soybean hulls. Diets were fed at different levels so that pigs consumed the same digestible protein and energy per unit of metabolic body weight. On Day 9, the feed amounts were adjusted based on body weights, and pigs started to receive water on a restricted basis, using a water:feed ratio of 3:1. On Day 14, separate collection of feces and urine was initiated for a 48-hour period. Urine collection bottles were kept on ice during the collection periods to minimize gaseous losses.

Throughout the 48-hour collection period, feces and urine were stored in a refrigerator. After the last collection, the feces and urine were weighed and sampled. Collected feces and urine were placed in a bucket corresponding to the experimental diet they were collected from and homogenized to produce samples of manure suitable for the odor tests. For odor assessments, aliquots corresponding to 1 percent of the collected manure were transferred into brown Nalgene bottles. Half of the bottles from each of the three fiber groups were capped and frozen, representing the fresh manure (low fiber fresh = LF, medium fiber fresh = MF, high fiber fresh = HF). The other half was left to age undisturbed and uncapped at room temperature for three weeks to represent the aged manure (low fiber aged = LA, medium fiber aged = MA, high fiber aged = HA). These samples were then frozen until tested by the odor panels.

An untrained odor panel consisting of 52 residents of the North Carolina village of Cameron and surrounding area (a locale with a substantial pig population) evaluated the manure samples in a room that was set up in three different areas and with seven different stations. At the first station, participants were asked to rank the medium fiber, fresh sample (MF) on a scale of 1 to 10 for odor intensity, irritation, and unpleasantness. At the second station, they were asked to rank all of the samples for irritation, intensity, and unpleasantness compared to the MF sample, which was assigned an absolute score of 5. The third through seventh stations were set up for triangle tests in which the participants were asked to identify the odd sample in a group of three (two samples were the same and one was different). In addition, participants were asked if the sample identified as different smelled better or worse than the other two. The comparisons were MF, MF, LF for station 3; MA, HA, MA for station 4; MA, MF, MF for station 5; HF, MF, HF for station 6; and LA, MA, LA for station 7. Finally, these panelists completed a survey in which general statistics and exposure to odors from swine were assessed.

A professional panel, consisting of 10 trained odor specialists from Duke University scored the same manure samples and provided absolute values for each of the six samples for odor intensity, irritation, and pleasantness on a scale of 1 to 8. In addition, they conducted the same triangular tests as the untrained panel. They had to identify which of three samples was different and to say if this sample smelled better or worse than the other two.

Results and Discussion

The trained panel indicated that for fresh manure, a low-fiber diet resulted in manure with a relatively higher irritation rating, compared to manure from pigs fed a high-fiber diet (Figure 1).

 

 

For aged manure, a high-fiber diet resulted in an increased intensity and unpleasantness of manure samples. In contrast to the trained panel, the untrained panel perceived that fresh manure from pigs fed the high-fiber diet was more irritating and more unpleasant than aged manure from pigs fed the high-fiber diets (Figure 2).

 

 

In the triangular tests, only the trained panel was able to distinguish the odd sample when evaluating fresh manure samples and comparing medium fiber or high fiber (Figure 3).

 

For the aged samples, both panels were able to distinguish high-fiber from medium-fiber manure. Both panels were able to distinguish fresh from aged medium-fiber manure. Among the participants who correctly identified the odd sample, neither panel had any significant better or worse ratings.

Conclusions

Data from this trial confirm that dietary fiber levels can be used to affect odor from manure. The results also demonstrate that the response by the general public to such changes in odor is difficult to predict by using a trained odor panel. In the end, the study illustrates how complicated it will be to solve the problem of perception of swine odor.