Critique of: Impacts of soil carbon sequestration on life cycle greenhouse gas emissions in Midwestern USA beef finishing systems, (2018) by Dr. Paige L. Stanley et al, Michigan State University

Canadian Cattlemen beef photo

Credit: Canadian Cattlemen

A number of recent research reports/reviews have concluded that, contrary to popular opinion, intensive feed-lot systems for finishing beef cattle result in lower greenhouse gas emissions per kg of beef carcass weight than grazing (“grass-fed”) systems (an example here). However, a recent paper from a group at Michigan State University found that, when the soil carbon sequestration benefit is included with a well-managed grazing system, the balance is reversed. Because of the dramatic nature of this finding and a long-term personal interest in soil carbon sequestration, I have reviewed their paper in detail. I conclude that their results are not realistic nor supported adequately by their research methodology.

Dr. Stanley et al describe research where beef pasture research fields in Michigan were converted from continuous grazing to managed-rotational grazing. After four years, they reported that the organic carbon (OM) content in the upper 30 cm of soil increased 40% from 34 to 48 tonnes/ha – or an average of 3.6 t/ha/year.

If it is assumed that soil organic matter is 58% carbon, this equates to an average OM addition of 6.2 t/ha/year. And if it’s assumed (generously, 2018 reference here) that about 25% of the organic matter (tops and roots), fixed by the forage crop during the four years after grazing, remains in the soil after the four years, this equates to about 25 t/ha/year of annual organic matter addition. Actual crop organic matter production must have been somewhat larger to account for the material removed and respired by grazing animals.

To put this in perspective, 25t/ha/year is about equivalent to the grain, stover, root and root exudates produced by a 190 bushel/acre corn crop, assuming grain represents 40% of total OM .

I don’t believe that this is credible.

So why the discrepancy? There is no suggestion that the authors fudged their data and no evidence for such, but here are some possible explanations.

Firstly, the authors give no accounting of how the manure produced in the non-grazing season was used – the manure coming from hay apparently purchased from off-station and used to feed cows and over-wintering calves. The authors state that no commercial fertilizer was used in this research so manure likely supplied the fertility need, especially for potassium for alfalfa (not clarified in the research description).  Manure addition would have also provided organic matter.

Secondly the authors measured the organic carbon content in the upper 30cm of soil at both the beginning and end of the four-year period, but apparently made no adjustment for possible changes in soil bulk density (g/cm3 of dry soil). A change in soil weight per volume could have affected the results – though very likely not nearly enough to trigger a 40% increase in measured soil OM.

Third, the authors appear to have only one valid replicate comparison for the change in soil organic matter content. They do present data for three sample locations but only one comparison involves the same soil type measured both before and after (i.e., a “sandy loam” soil).

The paper has some other weaknesses which cause concern. It’s essentially devoid of statistical analysis and most of the calculations are based on data from elsewhere including Michigan averages and global numbers provided by the International Panel on Climate Change or other sources. But yet the authors present their results to three (sometimes four) significant figures, implying unjustified precision. One significant-figure precision would be more appropriate.

In summarizing, this is not an attempt to attack the integrity of researchers or institution, or to understate the importance of the issue under consideration. However, I don’t believe that the results provide more than a hint that soil organic matter might be enhanced by well-managed grazing in a beef system; and this, in turn, could reduce the net greenhouse gas emissions in beef production.

Final note: I did try twice, without success, to contact the senior author before drafting the comments provided above.

 

 

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