Dietary DDGS and ammonia and hydrogen sulfide emissions

Alvaro Garcia

Greater ethanol production has increased the availability of one of its coproducts, distillers dried grains with solubles (DDGS). Once starch in corn is fermented to ethanol, the nutrients in the remaining mash that will end-up as DDGS, concentrate nearly three-fold. Research studies have explored the maximization of DDGS in dairy cow diets and its impacts on overall performance.

Dietary concentrations of DDGS up to 30% DM have been explored recently in lactating dairy cow diets, particularly as a result of the appearance in the market of competitively priced, reduced fat DDGS. Higher concentrations of DDGS have consistently resulted in increased feed efficiency and, as a result, reduced methane production. Higher concentrations of minerals in cow’s excreta can be an asset since they will be utilized by crops when manure is incorporated into the field.

There have been however fewer studies on the increased mineral concentrations on gaseous emissions [particularly ammonia (NH3) and hydrogen sulfate (H2S)], on water [nitrogen (N) and phosphorus (P) leaching and runoff), and on soil quality. When DDGS replace soybean meal (SBM) in dairy rations, there are changes in the manure characteristics and its emissions of NH3 and H2S. The proportions of rumen degradable protein (RDP) and rumen undegradable protein (RUP) also change (more RUP), potentially shifting nitrogen excretion from urine to feces, and decreasing NH3 emission from manure. Hydrogen sulfide emissions from manure on the other hand, may increase with higher concentrations of DDGS in the diet because of the high sulfur (S) content in DDGS.

A recent study (Lee et al., 2020) examined the effects of a diet with DDGS on NH3 and H2S emissions from dairy cow manure. Cows were assigned to one of two diets: a control with no DDGS and a treatment diet with 28.8 % DDGS on a DM basis. The control diet consisted on a DM basis of corn silage (41.6%), alfalfa (9.7%), alfalfa hay (5.0%), corn grain (12.9%), SBM (15.1%), soyhulls (12.3%), supplemental fat (1.34%), plus minerals and vitamins. In the treatment diet, DDGS replaced SBM, soybean hulls, fat source, and supplemental phosphorus.  Chemical composition of the diets was 17.6% protein; 30.5% fiber neutral detergent; 4.5% fat; 1.57 Mcal NEL/kg. The content of phosphorus and sulfur differed and were phosphorus 0.36 and 0.48% (control and treatment, respectively) and sulfur (0.21 and 0.41%, for control and treatment, respectively).

Cows produced less urine when fed DDGS compared to controls (27.7 vs. 31.3 kg/d). Feeding DDGS also tended to decrease (6.41 vs.  6.66) the pH of feces. Nitrogen concentration in feces also tended to be lower in DDGS fed cows (24.5 vs. 25.5 g/kg of DM), however these cows had greater S excretion in feces. Content of N and S in the manure were also lower in cows fed DDGS, while manure total output was not affected. While cumulative ammonia production was lower (27.5 vs. 46.1 g/cow) in the DDGS group, these cows also had greater H2S emission rates and cumulative H2S emissions from manure over 10 days.

Manure characteristics after 10-day incubation were also different between treatments with pH lower (7.24 vs. 7.47) for DDGS fed cows. In cows fed DDGS manure concentrations of N and S were greater. Bear in mind that although manure can constitute an asset to save on fertilizer, it has also the potential to affect the environment. It is important to test soils and apply just enough manure to amend soil nutrient deficiencies, while also using good management practices such as adequate rates and application methods.

The gas production potential through microbial fermentation of the manure needs to also be considered. Gases such as NH3, greenhouse gases, and H2S through, increase odor and decrease air quality. Gas emissions data form this experiment should be interpreted with caution since manure was incubated in vitro, and it may not exactly reflect what occurs in the farm. The DDGS diet in this experiment had greater RUP concentrations compared to the control; this decreased milk urea nitrogen (MUN) concentrations. Since MUN increases urine excretion the lower RDP concentration in the DDGS diet likely explained this reduced urine output.

This reduction in urine output obviously resulted in less urinary N excretion in DDGS fed cows, which contributed to reduced NH3 production. Overall, the authors found a about 5 percent points of N shift from urine to feces as a result of decreased urinary N excretion in DDGS fed cows.  Conversely, inclusion of DDGS generally increases dietary S concentration because DDGS are rich in this mineral.

Feeding DDGS in this trial lowered ammonia emission from manure like feeding a lower protein diet. Since it is known that acidifying manure reduces NH3 production, it is possible the lower urine pH in cows fed DDGS, decreased NH3 emissions from manure (when manure and urine are combined). Because of the high S concentration, DCAD (Na + K − Cl − S) in DDGS fed cows dropped from 192 to 65 mEq/kg of DM. Urine pH with the reduced DCAD (65 mEq/kg of DM) of the DDGS fed cows was 7.48. Feeding a negative DCAD diet reduces urine pH, which is a good approach to close-up cow nutrition to prevent hypocalcemia. The cows in this experiment however were in lactation, where DCAD should be positive.

Conclusion

In conclusion, manure from cows fed a high DDGS diet decreased urinary N contribution to manure N and lowered urine pH, which in turn decreased NH3 emission from manure. However, the DDGS diet increased dietary S concentration and increased S excretion in urine and feces.

At the present time these results support the ideal concentrations of DDGS in lactating cow diets are still below 30%. It also demonstrated the efficacy of using DDGS in close-up cows as a tool to reduce hypocalcemia early in lactation.

Reference:

Lee, C., Morris, D. L., Lefever, K.M., Dieter, P.A. 2020. Feeding a diet with corn distillers grain with solubles to dairy cows alters manure characteristics and ammonia and hydrogen sulfide emissions from manure. Journal of Dairy Science. 103(3): 2363-2372.

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