Rice Yield Gaps in Smallholder Systems of the Kilombero Floodplain in Tanzania

Round 1

Reviewer 1 Report

Review of “Rice yield gaps in smallholder systems of the Kilombero floodplain in Tanzania” by Kwesiga et al. submitted to Agronomy.

General comments:

This paper reports the results of yield gap analyses at different hydrological positions in Kilombero floodplain, utilizing the data from field trial on private farms and the simulated yield by APSIM-Oryza. The content of this paper is well within the scope of Agronomy, and the methods, strategy and results of the yield gap analyses are rational and interesting. I recommend the publication from Agronomy after minor revisions as follows:

One of my biggest concern is the Methods I couldn’t access, because they have not been published yet. (L113, “[22] in this issue” and L151, “[24] in prep”). If this paper shows the original data from field trials, the authors should describe more about the field experiments. The method for the model calibration is also be of interest. This issue affects the paper originality, so please describe the method clearly.

The second concern is numbers presented in results. For example, in manuscript, we can find “The resulting total yield gap (YGT)… was accordingly very large, ranging between years from 8.5 t ha-1 in 2015 to 9.1 t ha-1 in 2017 and between positions from 8.1 t ha-1 in the fringe to 9.2 t ha-1 in the middle(Table 1).” (L203-205) I couldn’t find these values in Table 1 and Figs. 2 and 3 (calculated the difference between dashed lines). The same is happening in other parameters! Please check the submitted results again.

Specific comments:

L152: The actual famers’ yield (Y_AF) might be Y_FAC. If there is the difference in definition, please describe clearly.

Similar to Y_AF, Y_AT or Y_Att? (L153 and L157) Y_P or Y_Pot ?(L154 and L166)

L200, L232-233: There is no data to support that 2017 was wet year and 2016 was dry year.

Author Response

Suggested responses to Reviewer one

Comment 1: This paper reports the results of yield gap analyses at different hydrological positions in Kilombero floodplain, utilizing the data from field trial on private farms and the simulated yield by APSIM-Oryza. The content of this paper is well within the scope of Agronomy, and the methods, strategy and results of the yield gap analyses are rational and interesting. I recommend the publication from Agronomy after minor revisions as follows:

Response: We thank the reviewer for this positive assessment.

Comment 2: One of my biggest concern is the Methods I couldn’t access, because they have not been published yet. (L113, “[22] in this issue” and L151, “[24] in prep”). If this paper shows the original data from field trials, the authors should describe more about the field experiments. The method for the model calibration is also be of interest. This issue affects the paper originality, so please describe the method clearly.

Response: We agree with the reviewer that the methods section needed to be improved.  We have clarified this issue by (among others) including a paper citation that has already been published [33]. The other paper, formerly [22] now [38] in the revised manuscript has also been considered for publication in the same Journal. This will help to increase the methodology detail. We have also added a paragraph of the climate and parameter measurements as suggested by the reviewer” see (lines 135-146 of the revised manuscript).

Model calibration. We have also added a paragraph as suggested. (See lines 189-200 of the revised manuscript).

Comment 3: The second concern is numbers presented in results. For example, in manuscript, we can find “The resulting total yield gap (YGT)… was accordingly very large, ranging between years from 8.5 t ha-1 in 2015 to 9.1 t ha-1 in 2017 and between positions from 8.1 t ha-1 in the fringe to 9.2 t ha-1 in the middle(Table 1).” (L203-205) I couldn’t find these values in Table 1 and Figs. 2 and 3 (calculated the difference between dashed lines). The same is happening in other parameters! Please check the submitted results again.

Response: We thank the reviewer for pointing out these inconsistencies.  We have clarified the issue in the results section. See (lines 254-255, 262-268, 270-271, 274-275, 282-287, 289-296 and 302-304. Table 1, Figures 1 and 2 of the revised manuscript).

Comment 4: L152: The actual famers’ yield (YAF) might be YFAC. If there is the difference in definition, please describe clearly.

Response: As suggested by the reviewer, we have revised (See lines 201 of the revised manuscript).

Comment 6: Similar to YAF, YAT or YAtt? (L153 and L157) YP or YPot ?(L154 and L166)

Response: The different abbreviations have been harmonised (See lines 202 and 204, 202 214, 357, 367, 392, 392, 403, and 421 of the revised manuscript).

Comment 7: L200, L232-233: There is no data to support that 2017 was wet year and 2016 was dry year.

Response: We agree with the reviewer. Although the information on weather has been added, it can also be found in ou previously from previously published paper [33] (see lines 135-136 of the revised manuscript). We have decided to delete the statements “dry and wet years” from the results section. (See line 30, 250 and  284-287 of the revised manuscript)

Reviewer 2 Report

The manuscript assessed the yield gap of rice in rainfed lowlands by conducting both model simulations and on-farm trials. The data provided is highly valuable to understand the present yield-constraints in the region and some potential measures or geographical positions to be focused to fill the yield gap. The manuscript as a whole is well written while the authors may add some general implications that can be delivered from this site-specific study in the abstract and discussion/conclusion section. In what kind of agro-environment or farmers socioeconomic conditions, the result in the Kilombero floodplain can be applied. Second suggestion is the authors can discuss more in detail what measures are most profitable and feasible for farmers to take in different geographical positions relative to their importance in yield gap (bunding, weeding, fertilizer, micronutrient supplementation and their combinations etc). Another concern is that you treated hydrological stresses as uncontrollable factor to determine YG1, but hydrological stresses should be controlled in best management practices and should be treated to calculate YG2, I suppose. Below specific comments may also help authors to improve the manuscript.

• The abstract should stand alone. Therefore, I recommend the authors clarify undefined variables within the abstract, e.g., differences among yield-reducing, yield-limiting and yield-defining factors; exploitable yield gap.
• For the paragraph of L66­-74 and Discussion 4.3., the below paper may be of the authors’ interest.

https://0-www-tandfonline-com.brum.beds.ac.uk/doi/full/10.1080/1343943X.2019.1617638

• L93: “..” -> “.”
• L128: Please provide if available the other nutrient contents of farmyard manure and fresh weight applied.
• How did you adjust to the yield at 14% grain moisture content? Grain moisture sensor used?
• Discussion 4-1: I consider this textbook type information should be removed or moved to the Introduction section.
• L306–310: You defined YG1 as the difference between potential yield (model simulated) and yields under best management without any hydrological stresses. If you observe any hydrological stresses even under best management practices plot, this should not be YG1 and cannot be compared with previous studies in Philippines and in West Africa. In this regard, 40% of YG1 should be definitely reanalyzed.
• L331–335: Any quantitative analysis that supports your argument about the factors of YG3. For instance, how the soil N differences among different positions (0.1%, 0.09%, and 0.17% in Fringe, Middle and Center, respectively) affect YG3?

Author Response

Suggested responses to Reviewer

Comment 1: The manuscript assessed the yield gap of rice in rainfed lowlands by conducting both model simulations and on-farm trials. The data provided is highly valuable to understand the present yield-constraints in the region and some potential measures or geographical positions to be focused to fill the yield gap.

Response: We thank the reviewer for this positive assessment

Comment 2: The manuscript as a whole is well written while the authors may add some general implications that can be delivered from this site-specific study in the abstract and discussion/conclusion section. In what kind of agro-environment or farmers socioeconomic conditions, the result in the Kilombero floodplain can be applied.

Response: We thank the reviewer for this positive comment.

With regards to the general implications, the reviewer is quite right. We have added a sentence in the abstract section. “Similar benefits may be obtained in other hydrologically-variable floodplain environments of the region and beyond”. (See lines 37-38 of the revised manuscript).

Comment 3: Second suggestion is the authors can discuss more in detail what measures are most profitable and feasible for farmers to take in different geographical positions relative to their importance in yield gap (bunding, weeding, fertilizer, micronutrient supplementation and their combinations etc.).

Response: We agree with the reviewer. However, giving more details on the different options would somewhat duplicate the already discussed benefits of bunding (see line 399-400), of fertilizer application See line 410-412) and of organic amendments, a sentence has been added to reflect their performance. ” The benefits of these organic N sources were largest in the wetter middle and center positions of the floodplain”. (see lines 421-422).    

Comment 4: Another concern is that you treated hydrological stresses as uncontrollable factor to determine YG1, but hydrological stresses should be controlled in best management practices and should be treated to calculate YG2, I suppose.

Response: The reviewer is quite right. This concern would apply for irrigated rice systems, where the potential yield is only influenced by; solar radiation, temperature, photoperiod, carbon dioxide and genotypic characteristics hence the attainable yield is not influenced by water. However, under rainfed conditions where the supply of water is not adequately controlled by the farmer, which is the case for our experiments. Water-limiting yield is considered as the maximum attainable yield for yield gap analysis.

In addition, the sheer size of Kilombero floodplain and the massive and unexpected submergence events that are determined by off-site rainfall in the upper watershed make any kind of water control outside of the drier fringe impossible, or at least only possible with massive investments in large-scale drainage and irrigation infrastructure.

Below specific comments may also help authors to improve the manuscript.

Comment 5: The abstract should stand alone. Therefore, I recommend the authors clarify undefined variables within the abstract, e.g., differences among yield-reducing, yield-limiting and yield-defining factors; exploitable yield gap.

Response: We agree with the reviewer. We have clarified on the issue in the abstract section. (See line 26-29 of the revised manuscript)

 Comment 6: For the paragraph of L66­-74 and Discussion 4.3., the below paper may be of the authors’ interest.

https://0-www-tandfonline-com.brum.beds.ac.uk/doi/full/10.1080/1343943X.2019.1617638

Response: We thank for reviewer for the above manuscript. “We have indeed adapted it to our manuscript”. (See lines 100-102 and 373 – 375 and 385-388 of the revised manuscript)  

Comment 7: L93: “..” -> “.”

Response: The extra full stop has been removed. (See lines 121 of the revised manuscript)  

Comment 8: L128: Please provide if available the other nutrient contents of farmyard manure and fresh weight applied.

Response: The fresh weight of the farmyard manure has been included. However, we did not measure other nutrient contents. (See lines 170-171 of the revised manuscript)    

Comment 9: How did you adjust to the yield at 14% grain moisture content? Grain moisture sensor used?

Response: The rice grains were dried under the sun after harvest later weighed before storage. Three samples were taken from each bag and measured using a digital grain moisture meter (Satake Moistex SS7). Later the 14% grain moisture content was adjusted from the average moisture of the three samples and the rice weight.

Comment 10: Discussion 4-1: I consider this textbook type information should be removed or moved to the Introduction section.

Response: We agree with the reviewer. “The text has been moved from the discussion section to the introduction section”. (See from lines 313-340 to lines  55-76 of the revised manuscript)  

Comment 11: L306–310: You defined YG1 as the difference between potential yield (model simulated) and yields under best management without any hydrological stresses. If you observe any hydrological stresses even under best management practices plot, this should not be YG1 and cannot be compared with previous studies in Philippines and in West Africa. In this regard, 40% of YG1 should be definitely reanalyzed.

Response: We would agree with the reviewer if only we were dealing with irrigated rice production systems. But under rainfed rice production systems, we fail to agree. We were able to control the drought stress by supplemental irrigation, but not the submergence stress.

Comment 12: L331–335: Any quantitative analysis that supports your argument about the factors of YG3. For instance, how the soil N differences among different positions (0.1%, 0.09%, and 0.17% in Fringe, Middle and Center, respectively) affect YG3?

Response: While soil N content is low in fringe and middle, it cannot be excluded that a relatively higher soil N may well have contributed to reducing YG3 in the center position. On the other hand the C:N-ratio was lower in the center and hence soil N supplying capacity may also have been reduced. This is however all rather hypothetical and we would like to refrain from listing such non-substantiated speculations and instead simply refer to more detailed soil properties, highlighted in the revised manuscript (See line 143)

Author Response File: Author Response.docx