Interaction between C (methane) and N (ammonia, nitrous oxide) emissions
Instructions: Please answer the following questions as a study guide for the paper of Dijkstra et al., (2011).
INTRODUCTION
1) Contrast the fate of ammonia when applied to the soil compared with the fate of ruminal ammonia. Explain some of the causes for the differences.
Ruminal ammonia serve to synthesize microbial protein in an anaerobic environment. In contrast, the ammonia applied to soil undergoes aerobic nitrification by soil micro-organisms resulting in nitrite (No2-) and nitrate (NO3-) and some nitrous oxide (N2O). Other anaerobic micro-organisms can subsequently denitrify the nitrate to produce N2 with again some release of N2O.
2) Name the following compounds and indicate their biological relevance in livestock systems.
NO2- : Nitrite is an intermediary in the process of nitrification (conversion of ammonia into nitrate in soil).
NO3- : Nitrate is a fertilizer. It is the end product of nitrification in soil. It is relevant because it can accumulate in forage under cold growing conditions and cause toxicity in grazing ruminant. Nitrate was also discussed earlier in class as a hydrogen sink, reducing methane emission when included in the diet.
N2O : Nitrous oxide is a greenhouse gas 280 times more potent than carbon dioxide. Nitrous oxide emissions associated with the fertilization of the land used to produce feed / forage for dairy cattle and associated with certain types of manure management, contribute substantially to milk carbon foot print.
DIETARY OPTIONS TO REDUCE N EXCRETION AND METHANE EMISSION
3) Why would feces with a high C:N ratio have a relatively large potential for C sequestration in soils? Because of its recalcitrance to decomposition (which would yield carbon dioxide) and mineralization (which would contribute to nitrous oxide emission).
4) According to the authors what are the main dietary strategies to reduce urinary urea-N losses (as described throughout the paper)?
1) Lower dietary CP
2) Lower N fertilization of grass to lower CP and increase highly fermentable CHO levels at the same time. The authors reported work that suggested that high sugar levels in grass at the expense of protein improved N efficiency and reduced in particular N excretion in urine whereas high sugar levels at the expense of fiber reduced fecal N excretion but not urinary N excretion and only a small change in N efficiency.
3) Harvesting (grasses) at a more mature stage (pg 417).
4) Feeding corn silage (lower CP feed) and/or feeding low-N feed supplements (pg 418).
5) According to the authors what are the main dietary strategies to reduce enteric methane emission?
a) Inclusion in the diet of potentially digestible nutrients that escape ruminal fermentation.
b) Construct diets with a fermentation profile that favors production of propionic acid.
c) Replacing roughage with concentrate.
d) improved pasture management (many places around the world).
e) Feeding corn silage (because of its starch content) as a forage source.
NOTE: IPCC Tier 1, Tier 2 and Tier 3 – What’s that?
IPCC (Intergovernmental Panel on Climate Change) has adopted three general methods to determine greenhouse gases emissions across nations worldwide.
Tier 1 is essentially the simpliest method where emission is calculated as # of animals within a species multiplied by a fixe “emission factor” for that animal.
Tier 2: is a more advance method than Tier 1 in which local conditions are taken into account (feed digestibility, local temperature, precipitation, etc.) that allow to determine emission from livestock managed under contrasting production systems (pasture, confinement, etc.).
Tier 3 is the most advanced method in which model that have been proven as valid and reliable are used to predict (multiple) greenhouse gases simultaneously in space (e.g., different crops on the farm) and time (e.g., throughout the seasons).
6) What is the difference between empirical and mechanistic models?
Empirical = model based on observation (with no attempt to argue cause and effect).
Mechanistic = model based on cause and effect relationship between (among) variables.
7) Based on the information presented in the paper, decide whether the following statements are true of false?
| 1) Adding dietary buffer to maintain rumen pH as high as possible may reduce the beneficial effect of replacing fiber with starch on methane emission. | T | F |
| 2) The modeling presented here suggested that in general the correlation between simulated manure N excretion (g/d) and methane emission (g/d) was small but positive. | T | F |
| 3) The modeling presented here suggested that in general the correlation between simulated manure N excretion (g/kg FPCM) and methane emission (g/kg FPCM) was small but positive. | T | F |
| 4) The authors determined the ammonia emission potential of manure N based on the predicted ratio of fecal N to urinary N. | T | F |
| 5) Simulation results indicated that diets that would yield lower methane per kg of FPCM would yield manure that contains a greater proportion of urea N (hint: See Figure 4). | T | F |
| 6) According to the simulation, N-mitigation options at animal level aimed at reducing urinary N excretion may well results in elevated methane emission levels per kg of FPCM. However, (a) predicted change in methane per gram of urinary N decrease varied widely and (b) experimental data are needed to confirm / disprove these simulated prediction. | T | F |
| 7) The authors argued that current whole-farm (empirical) models of greenhouse gases emission have not predicted an interaction between ammonia volatilization and methane emission. However these predictions may be wrong (incorrect) in the light of the results of the (mechanistic) modeling of what happens at the cow-level. | T | F |
1) TRUE: Figure 2 illustrates that replacing cell wall with starch reduced emission, but the reduction was lower when pH is kept higher.
2) TRUE: See Figure 4a. The authors argued that it made sense because since feed intake is a major determinant of both manure N excretion and methane emission.
3) FALSE: See Figure 4b. The authors argued that it made sense because since feed intake is a major determinant of both manure N excretion and methane emission.
4) TRUE: See last paragraph page 419.
5) TRUE: See Figure 4c. This is a concern!.
6) TRUE: See first paragraph page 420.
7) TRUE: See second paragraph page 420.
8) According to the modeling results presented in the manuscript, which of the following predict the impact of the following strategies on manure N excretion and methane emission expressed per kg of fat-and-protein corrected milk (FPCM):
8.1. Reducing N fertilization of pasture:
It would lower manure N excretion (because of lower CP in feed/diet) but increase methane emission (because more fermentable CHO in feed/diet) per kg of FPCM.
8.2. Cutting grass silage to a later stage of maturity:
If late cut has a negative effect on DMI and milk production, manure N excretion and methane emission might be increased per kg of FPCM (because of a reduction in FPCM). Nevertheless, a more mature grass has less CP and more fiber and thus everything else equal, one would expect a reduction in manure N excretion but an increase in methane emission by delaying harvest of grass.
8.3. Supplementing the diets with corn silage:
Corn silage’s low CP and high starch content would contribute to both reducing manure N and methane emission.
GHG EMISSIONS DURING STORAGE
Below is Figure 1 from Aguerre et al., (2012) whop reported ammonia emission, nitrous oxide emission, methane emission and carbon dioxide emission of manure stored in large (55-gallon) barrels up to 77 days. Note that the authors reported a crust formation after about 21 days of storage. Drawn your conclusion in regard to changes in ammonia emission and greenhouse gases emission during storage.
Source: Aguerre et al., 2012. JDS 95:7409-7416.
Note: This graph will be discussed in class only if time allows.
THOUGH-PROVOKING CASE-SCENARIO
Imagine you are a consultant advising organic dairy producers who want to feed their cows such that both ammonia and methane emission are reduced. The currently use rotational grazing with 35 day interval in grazing episodes.
1. Would you recommend that they intensify grazing and shorten grazing episode to 25 day interval such that the cow consume a “younger” grass or would you recommend that they “ex-tensify” grazing and extent grazing episodes to 45 day interval such that the cows consume an “older” grass?
Younger grass is high protein low fiber. Thus it may enhance ammonia emission, but reduce methane emission. In contrast the opposite may be true for the older grass. Younger grass may have the advantage of supplying more energy and thus contribute to high cow performance.
2. What else would you recommend in order to lower methane and ammonia emission and possible improve lactation performance of the cows:
Feed corn silage or corn grain, which would put starch in the diet)
Include unsaturated fat in the concentrate mix