An Iowa State University field trial paves the way for practical reduction of odor intensity in hog manure.

A three-year study by Iowa State University (ISU) scientists shows that when levels of soybean meal are lowered, odors from hog manure are significantly reduced, according to lead researcher David Stender, ISU Extension specialist based at Cherokee, IA.

In the study, three commercial farms with similar 1,100-head finishers and deep manure pits were sampled seasonally for odor and concentrations of hydrogen sulfide and ammonia. One farm had an additional 1,100-head barn for a total of 36 replicates in the trials.

Odor emissions were reduced by an average of 27%, while concentrations of hydrogen sulfide and ammonia were reduced, but the differences were not statistically significant, Stender says.

Trial Objectives, Results

Stender says it's common knowledge that the nutritional ingredients in swine diets impact the nutrient composition of hog manure. The question is — will differences in manure nutrient composition during pit storage alter decomposition patterns, indirectly impacting odor intensity?

The goal of the ISU field trial was to evaluate the impact of soybean meal inclusion rates on air quality emission parameters, he says.

All treatment diets at the three farms were initiated in late winter, and pigs were fed to market weights in large commercial finishing barns.

In the trials comparing control and treated groups, an average of about 100 lb. of soybean meal from the control diet was replaced with a combination of 100 lb. of corn and synthetic (crystalline) amino acids per ton of feed (see Tables 1 and 2). The soybean meal to corn/amino acid substitution ratios between the control and treated groups remained consistent throughout the trials based on pig requirements, Stender emphasizes.

Air samples were collected in tedlar bags at the manure pit fans of each barn and analyzed at the ISU olfactometry laboratory for odor threshold and gas concentrations.

Overall, the high-protein (HP) control group averaged 1,420 odor units, compared to 1,035 odor units for the low-protein (LP) treatment group, reducing odor levels by 27%, Stender points out.

The impact of reduced soybean meal levels was calculated specifically for three seasons of the year. No statistical differences were found in the seasonal data; however, the simple averages were interesting as the average reduction in odor was 47% in the fall, 22% in the summer and 14% in the spring. Numbers are not statistically significant because of a large variability in the data with relatively small numbers of seasonal samples, he explains. More research is needed.

Changes in protein levels in the diet resulted in reductions in hydrogen sulfide that were not statistically significant, averaging 0.92 ppm and 0.59 ppm for the HP and LP treatments, respectively.

Changes in the concentrations of ammonia were also statistically insignificant. Simple averages were 12.3 ppm and 9.1 ppm for the HP and LP treatment groups, respectively.

Stender says feeding higher protein levels to pigs results in unnecessarily high concentrations of nitrogen in swine manure pits.

“A 1,100-head finishing barn will consume close to 1.8 million lb. of feed a year. If that feed contains 2% more protein than needed, then 36,000 lb. of excess protein goes through the pig into the pit annually,” he explains.

Pig performance and carcass quality were unaffected by changes in the dietary formulations.

Stender drew three overall conclusions from his research:

  • Lowering the amount of nitrogen in manure storage can contribute to a reduction in odor.

  • More field research needs to be conducted to evaluate the impact of hog diets on odor and other gases from manure.

  • Changing diets to impact odor emissions might be important in meeting future regulatory emission standards.

Stender comments producers have been quick to put this research into use on the farm. “Producers are adopting this practice, not because of odor, but because recently the addition of synthetic amino acids cheapens the ration compared to traditional soybean meal diets.”

In fact, some producers are replacing up to 150 lb. of soybean meal with corn and synthetic amino acids, further reducing odor levels and ration costs, he says.

Stender presented research results at the Midwest Section of the American Society of Animal Science meeting held this spring in Des Moines, IA.

Table 1. An Example Grow-Finish Diet
Ingredients High-Protein Low-Protein
Corn 1,265 1,370
Soybean Meal, 46.5% 480 370
Distiller's Dried Grains with Solubles 200 200
Grow-Finish Distiller's Dried Grains with Solubles basemix 50 50
L-Lysine HCL 3 6.5
L-Threonine 0 1.2
Copper sulfate 2 2
Table 2. Sample of Grow-Finish Diet Formulation
Ingredients Control Group Treated Group
Dry matter (%) 88.38 88.24
Crude protein (%) 20.00 18.15
Lysine, total (%) 1.08 1.07
Metabolizable energy 1,526 1,532
Fat (%) 4.65 4.88
Calcium (%) 0.62 0.60
Phosphorus (%) 0.58 0.56
Phos. available (%) 0.30 0.29
Calcium:phosphorus 1.07 1.08
Salt (%) 0.42 0.42

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