The Environmental Impact of Lowering Dietary Crude Protein in Finishing Pig Diets—The Effect on Ammonia, Odour and Slurry Production

Round 1

Reviewer 1 Report

The manuscript entitled " The environmental impact of lowering dietary crude protein

in finishing pig diets – the effect on ammonia, odour and slurry production” " is well organized. The idea and design of this study is good although it is not clear.

However, I suggest the authors should address the following comments for further consideration of the manuscript for publication.
Re-write the focal aim section in Abstract.

The introduction to the manuscript is difficult to read and understand the existing knowledge gap and the relevance of the current work. Instead of focusing solely on odor, the author discusses greenhouse gases.

The authors need to enhance the introduction about the state of the art on manure additive for odor mitigation, including application costs and cross-pollutant effects. That frame may serve to justify the contributions of this work.

Author should be include more details about experimental design, how many animals were used for each treatment?
A general comment regards the methodology used as it is not always clear and rises some doubts on the results obtained. Therefore, should be clarified. Moreover, the characteristics of the slurry and the faces are not reported. Consequently, the discussion has few information to explain the mechanism behind the results. Also the statistical approach used does not seems the best approach to represent the experiment performed.

The authors should clearly define the protocol use in this experiment and this is applied to the results part.

 

Results: all descriptions of the results should be reworked. The following parts are required, 1) the comparison of the significant difference among the values of three treatments; The result descriptions should be presented in subsections based on the type of indicators.
Discussion: too many errors (especially in discussion) and need the author's full attention.
Conclusion: write the conclusion again and be more concise to the major findings and suggestions.

Author Response

Reviewer 1

Thank you for your comments and suggestions. Please see below the response to your comments.

1. Re-write the focal aim section in Abstract.

Abstract has been changed.

2. The introduction to the manuscript is difficult to read and understand the existing knowledge gap and the relevance of the current work. Instead of focusing solely on odor, the author discusses greenhouse gases.

The study aimed to see the effect of varying dietary CP on ammonia and odour and as such the introduction focuses on ammonia and odour. Ammonia is a source of nitrous oxide and hence the link to greenhouse gases.

3. The authors need to enhance the introduction about the state of the art on manure additive for odor mitigation, including application costs and cross-pollutant effects. That frame may serve to justify the contributions of this work.

The study did not involve the use of any manure additives to reduce odour and therefore a discussion about them would not be relevant or provide any justification for the work. There is a lack of information in the literature regarding the quantitative effect of reducing dietary CP on ammonia and odour emissions and therefore the trial was conducted.

4. Authors should be include more details about experimental design, how many animals were used for each treatment?

Pig numbers are included in the text but I have inserted more details where appropriate.

5. A general comment regards the methodology used as it is not always clear and rises some doubts on the results obtained. Therefore, should be clarified. 

I am unsure about what part of the methodology is being referred to.  

6. Moreover, the characteristics of the slurry and the faces are not reported. Consequently, the discussion has few information to explain the mechanism behind the results. Also the statistical approach used does not seems the best approach to represent the experiment performed.

The nutrient profile of the slurry is presented in Table 5.

7. Also the statistical approach used does not seems the best approach to represent the experiment performed

We believe that REML, Fisher’s LSD, and F-tests for linear and quadratic effects were appropriate for this type of trial.

8. The authors should clearly define the protocol use in this experiment and this is applied to the results part

Apologies, but we are unsure as to what is meant by this.

9. Results: all descriptions of the results should be reworked. The following parts are required, 1) the comparison of the significant difference among the values of three treatments; The result descriptions should be presented in subsections based on the type of indicators.

Based on our response to point 7 and the opinion of the other two Reviewers, we feel that this is not necessary.

10. Discussion: too many errors (especially in discussion) and need the author's full attention.

The manuscript (esp the discussion) has been checked for errors and changed as appropriate. .

11. Conclusion: write the conclusion again and be more concise to the major findings and suggestions

The conclusion has been modified.

Reviewer 2 Report

Please see attachment.

Comments for author File: Comments.pdf

Author Response

Reviewer 2

Thank you for your comments and suggestions. Please see below the response to your comments.

1. In the abstract author should concise the method description and know how to present in the abstract section

Abstract has been changed.

2. Check the reference of agricultural contribution to global emission. That is not correct. 90% emission by agriculture?

I have checked the specific reference and others and it relates to agricultural production including fertiliser production.

3. The author can also mention the recent approaches to lower CO2 footprint in pig farming by applying alternative diets.

This has been addressed in point 5 below.

4. In section 2.1, line 95 “Treatment 3 was formulated to 120g/kg CP but 95 when analysed

Not sure what is meant here… the dietary treatment was contained 130g/kg when analysed..

5. In section 4.1 authors could estimate the final impact of each diet. Calculating the impacts from grow-to-finishing pig with the all the formulated diets. Likewise in line 234, the impact of all ingredients can be estimated that accumulate to a final diet. That way author can assess the environmental impact of the formulated diet. Author can refer to the following articles to have an idea to assess the diet footprint calculation: https://doi.org/10.3390/su14031161

We have calculated the global warming potential of the diet formulations and the values have now been included in the manuscript.

Reviewer 3 Report

Here, authors described a study with the aim to quantify ammonia and odor emissions and slurry production for pigs fed reduced crude protein diets. The study, though not entirely novel, appears well designed and is of interest to the scientific community and the manuscript is well written. Please find my specific comments below.

 

 

L22 – dietary Lys should be listed as standardized ileal digestible (not total) + I assume the units are %?

L30 – indicate whether feed was offered in the chambers (I assume the mention of feed disappearance here is referring to the feed that was consumed while in the chamber?)

L39 – weak positive relationship

 

L57 – isn’t urease produced by the microbes present in the faeces? I think this distinction is important – especially considering other complementary nutrition strategies that could influence the fecal microbial profile to reduce urease-producing bacteria

L63 – ‘environmental problems’ is a vague phrase – suggest to be specific

L64 – less N excreted…in urine?

L74 – ‘…divert N from urine to faeces…’ the mechanism here is not clear. To me, N excreted in urine is indicative of post-absorptive AA metabolism and N excreted in faeces depends on digestibility of the diet/ingredients and microbial use of the indigesta. Suggest to clarify.

 

L87 – This is confusing – are ‘five separate batches’ referring to ‘pens’ or ‘blocks’? How were the pigs housed and fed? I assume from L98 that pigs were housed individually?

L91 + table 1 – as for the abstract, the Lys should also be presented on a standardized ileal digestible basis

L98 – ideal protein – I assume this to mean that the AA:Lys ratios were supplied at or above NRC recommendations? Or was another requirements model used?

L117 – is there some justification/rationale for these timeframes within the calorimetry chambers?

L128 – same comment as for the abstract, how were pigs fed while in the calorimetry chambers? What was the waterer design?

L163 – quadratic effects for 3 treatment groups is not meaningful + the quadratic effects are not significant (only for one outcome) – suggest to remove.

L175 – wouldn’t adding an AA product that is designed to slowly release AA and ‘optimize AA absorption’ only to two dietary treatment confound the results? Which AA were included in this product?  

L192 – A bit picky, but I don’t agree with ‘slurry excreted’ – slurry also includes any water that the animal wasted while in the chamber and thus, is not ‘excreted’ from the animal

Table 5 – N-output in slurry (g/d) these values seem very low to me! The animals are consuming ~ 70-90 g/d of N, indicating an apparent utilization efficiency of ~93%, which is well beyond the maximum utilization efficiency above maintenance for growing/finishing pigs of 73.5% (NRC, 2012). Can the difference be accounted for in the volatilization of ammonia during the calorimetry period? If I do some napkin math, it looks like ammonia production is between 231 and 434 mg/h, which equates to between 5.5 and 10.4 g/d of ammonia and ~4-8 g/d of N. Something does not add up here. …ah, I see this has been addressed in the discussion – but that means 75 to 97% of the N was lost from the sample during freeze-thaw, it seems like a lot.

Table 6 – these units confuse me – ng? should it be ng/mL or ng/g?

L222 – I think this constitutes a weak (very weak) relationship (R² < 0.30).

 

L267 – or on a per kg of pork basis

L293 – should be referring to amino acids here, not protein. It is the AA that are supplied in excess (of requirements) and the AA that are catabolized to produce urea.

L296 – I suppose the lack of difference in water usage between the two lower CP groups could indicate that the 150 g/kg group was not providing AA in excess, but this is a stretch. Water usage is an indirect and variable outcome. Suggest to remove.

L380 – Yes, the skatole differences are difficult to explain, but do you think the 0.03 ng difference between the treatment groups is biologically relevant?

 

Author Response

Reviewer 3

Thank you for your comments and suggestions. Please see below the response to your comments.

1. L22 – dietary Lys should be listed as standardized ileal digestible (not total) + I assume the units are %?

The diets were formulated on total amino acids and while it would be possible to calculate SID using an assumed SID level (i.e. 84%) this would not be strictly correct as each ingredient/amino acid will have its own (calculated) SID. We have left it as total lysine but if you feel strongly that the amino acids should all be changed to SID, this can be done by going back through the formulation matrix but it will be on a calculated basis. .

The units have been changed to g/kg to be consistent with CP

2. L30 – indicate whether feed was offered in the chambers (I assume the mention of feed disappearance here is referring to the feed that was consumed while in the chamber?).

Pigs were offered treatment diets ad lib. This has now been inserted in the text.

3. L39 – weak positive relationship

The word “weak” has been inserted throughout. .

4. L57 – isn’t urease produced by the microbes present in the faeces? I think this distinction is important – especially considering other complementary nutrition strategies that could influence the fecal microbial profile to reduce urease-producing bacteria

Agree. This has been inserted into the text.

5. L63 – ‘environmental problems’ is a vague phrase – suggest to be specific

The word “eutrophication” has been added.

6. L64 – less N excreted…in urine?

“in urine” now included in text.

7. L74 – ‘…divert N from urine to faeces…’ the mechanism here is not clear. To me, N excreted in urine is indicative of post-absorptive AA metabolism and N excreted in faeces depends on digestibility of the diet/ingredients and microbial use of the in digesta. Suggest to clarify.

I have modified this in the text. I was trying to bring out that changing the dietary ingredients can change the microbial profile…but this is more to do with fibre/carbohydrate than protein per se.

8. L87 – This is confusing – are ‘five separate batches’ referring to ‘pens’ or ‘blocks’? How were the pigs housed and fed? I assume from L98 that pigs were housed individually?

The batches are pigs although “groups” would be a better description and I have inserted this. Pigs are housed individually throughout. (see line 88 after “…(Danish Duroc) were housed individual in….” . I have included  “ad libitum”. 

9. L91 + table 1 – as for the abstract, the Lys should also be presented on a standardized ileal digestible basis

See comments for point 1 above

10. L98 – ideal protein – I assume this to mean that the AA:Lys ratios were supplied at or above NRC recommendations? Or was another requirements model used?

Yes and according to BSAS 2003. .

11. L117 – is there some justification/rationale for these timeframes within the calorimetry chambers?

Yes, timescales and recordings within the chambers are according to previously published methodologies at AFBI

12. L128 – same comment as for the abstract, how were pigs fed while in the calorimetry chambers? What was the waterer design?

Text has been modified.

13. L163 – quadratic effects for 3 treatment groups is not meaningful + the quadratic effects are not significant (only for one outcome) – suggest to remove.

References to quadratic effects have been removed.

14. L175 – wouldn’t adding an AA product that is designed to slowly release AA and ‘optimize AA absorption’ only to two dietary treatment confound the results? Which AA were included in this product?

The diet that did not contain the product was designed to be an “historic” diet and the other two diets formulated with knowledge gained on amino acid nutrition by our research group. From a local perspective, one of our primary aims was to provide evidence to our Environmental Agency that the pig industry has reduced ammonia emissions through dietary strategies within Northern Ireland. To do this, we compared emissions from pigs offered the historic treatment, a more current type diet and a low CP diet to “see how low we could go”. Some of our previous work has shown that Devigain can replace 1% of amino acids and we incorporated this into the more current diets. As Devigain did not exist when the historic diet was used in commercial production, it was not included in that diet.  The results are not confounded as the reduction in dietary CP still stands across the three diets and the contribution of Devigain was accounted for in the formulation. Devigain contains lysine, methionine and threonine bound to sugars to form an early glycated product.

15. L192 – A bit picky, but I don’t agree with ‘slurry excreted’ – slurry also includes any water that the animal wasted while in the chamber and thus, is not ‘excreted’ from the animal

You are quite correct (and not picky!). I have changed it to output.

16. Table 5 – N-output in slurry (g/d) these values seem very low to me! The animals are consuming ~ 70-90 g/d of N, indicating an apparent utilization efficiency of ~93%, which is well beyond the maximum utilization efficiency above maintenance for growing/finishing pigs of 73.5% (NRC, 2012). Can the difference be accounted for in the volatilization of ammonia during the calorimetry period? If I do some napkin math, it looks like ammonia production is between 231 and 434 mg/h, which equates to between 5.5 and 10.4 g/d of ammonia and ~4-8 g/d of N. Something does not add up here. …ah, I see this has been addressed in the discussion – but that means 75 to 97% of the N was lost from the sample during freeze-thaw, it seems like a lot.

Again, you are correct in that the N output seems way too low and impossible given intake and N efficiency. The loss during freeze/thaw is substantial. Some N would also have been volatilised in the period between excretion and sample collection but the amount lost during freeze/thaw is surprising. It is certainly making us think about the value of absolute N analysis on samples that have been frozen and then analysed for other projects.

17. Table 6 – these units confuse me – ng? should it be ng/mL or ng/g?

The units are ng/g and this has been updated in the text.

18. L222 – I think this constitutes a weak (very weak) relationship (R²< 0.30).

The word “weak” has been inserted.

19. L267 – or on a per kg of pork basis

This has been changed in the text.

20. L293 – should be referring to amino acids here, not protein. It is the AA that are supplied in excess (of requirements) and the AA that are catabolized to produce urea.

This has been changed in the text.

21. L296 – I suppose the lack of difference in water usage between the two lower CP groups could indicate that the 150 g/kg group was not providing AA in excess, but this is a stretch. Water usage is an indirect and variable outcome. Suggest to remove

This has been removed.

22. L380 – Yes, the skatole differences are difficult to explain, but do you think the 0.03 ng difference between the treatment groups is biologically relevant?

Most likely not biologically relevant. I have reflected this in the text.

 

Round 2

Reviewer 1 Report

Dear authors

Thank you for addressing the comments. The correction was satisfied. So, I recommend the manuscript for publication.

 

Best regards