Sheep Reared on Sewage Sludge-Treated Pasture: Flawed Conclusions

Referencing: Cellular and Hormonal Disruption of Fetal Testis Development in Sheep Reared on Pasture Treated with Sewage Sludge

I read the article by Paul et al. (2005) with interest. Although the authors devoted considerable energy and resources to their study, I believe that the experimental design is fundamentally flawed and the authors' conclusions are not supported by the facts. The flaw in this grazing experiment is that the control treatment, a pasture not treated with sewage sludge, is not a valid control. A valid experimental control should be as close to identical to the treatment(s) as possible, except for the factor under investigation.

In the study of Paul et al. (2005), the pastures received 250 kg nitrogen/ha from sewage sludge or from mineral fertilizer. Under the climatic conditions of the experiment, the sludge nitrogen would be equivalent to about 70 kg nitrogen from mineral fertilizer, with much of the rest of the nitrogen (and carbon) going into soil organic matter stocks. Thus, the "control" received three times as much plant-available nitrogen as the sludge pasture, and the herbage yield would have been greater. The lower herbage yield and more restricted diet on the sludge plot is borne out by the lighter weights of the ewes and the fetuses.

In addition to the difference in nitrogen, there was almost certainly a difference in the phosphorus supply. Paul et al. (2005) did not describe fertilizer applications apart from nitrogen, but I doubt that they added as much phosphorus to the control plot as they did to the treated plot in the form of sludge content. There is also the question of the other nutrients that would have been added in the sludge (potassium, magnesium, sulfur, calcium, and minor nutrients).

The lesser amount of available nitrogen and the much greater phosphorus (and the other nutrients) over ≥ 7 years would have almost certainly changed the sward composition. For example, there would almost certainly be much more clover in the sludge plots. Clover and other legumes are rich in phytoestrogens; therefore, if the effects observed by Paul et al. (2005) were due to endocrine-active substances in the diet, these substances could well have been phytoestrogens.

Paul et al. (2005) noted that some authors have reported similar effects in sheep on restricted diets, but other authors have not found the effects; therefore, this appears inconclusive. Paul et al. (2005) found physiologic effects but did not prove causation.

From the results of Paul et al. (2005), one could conclude that mineral nitrogen increased the number of quadruplets (which is bad from a farmer's point of view because the ewe does not have enough milk for four lambs) and sludge gave consistently more triplets (good for farmers), but that would not be accurate. Based on their data, one could also say that statistically significantly more ewes escape from mineral nitrogen-fertilized fields than from sludge-treated ones, but that would be silly.

The diets of the two populations were different because the pastures were managed differently and, as a consequence, the animals responded differently; it would not be valid to say more.

The author declares he has no competing financial interests.

Tim Evans
Tim Evans Environment
Ashtead, Surrey, England
E-mail: tim@timevansenvironment.com

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Sheep Reared on Sewage Sludge-Treated Pasture: Sharpe Responds

We thank Evans for his interest in our article (Paul et al. 2005) and for his comments. He appears to have misunderstood the purpose of our study, which was to provide evidence as to whether or not the complex mixture of chemicals that are present in treated sewage sludge are able to exert any effects on the developing fetal testis in the sheep. This information might provide insights into current understanding about the impact of environmental chemicals on development and malfunction of the human testis. In this study we chose what we considered to be the most appropriate control treatment, which was to maintain pasture in its normal format according to local environmental conditions with the addition of appropriate amounts of inorganic nitrogenous fertilizer. The aim was not to control exactly for the relative amounts of all other organic and/or inorganic materials because this would be almost impossible to do when considering the complex makeup of sewage sludge.

Possible differential effects of the control and sewage sludge treatment on growth of the sward in the two pastures and their consequent contribution to different nutritional effects in the ewes maintained on that pasture were controlled by varying the stocking levels according to the sward length. This was clearly indicated in the "Materials and Methods" of our article (Paul et al. 2005). Evidence that this approach was successful can be gleaned from the observation that there was no difference in body weight between the pregnant ewes maintained on the two types of pasture. Evans must have misunderstood our article because he seems to think that there was a difference in weight of the pregnant ewes.

Another point raised by Evans is the potential of clovers and their endogenous phytoestrogens to contribute to the changes we observed in our study (Paul et al. 2005). In the pastures in which these studies were conducted (55°N), there were minimal amounts of clover, even after several years of treatment with sludge; therefore, any contribution of this source to our study is almost certainly minimal.

Finally, we point out again that there were no significant differences between the control animals and those reared on sewage sludge-fertilized pastures in terms of the frequency of multiple births, so it is not appropriate to consider Evans' speculation regarding contributions that the mineral nitrogen might have made to this occurrence.

In summary, although we accept that there may be differences between the two types of pasture that may have contributed in some way to the present studies--for which we have been unable to control--we believe that our study achieved its primary goal: We established that exposure of pregnant ewes to a complex cocktail of environmental chemicals (those present in treated sewage sludge) could selectively affect development of the testes of male fetuses. We have not identified which chemical or mixture of chemicals caused this change, and we emphasized in our article (Paul et al. 2005) that to do so is a complex and probably impossible task. The important point is to prove the principal that exposure to mixtures of environmental chemicals at "real world" levels has the potential to alter male reproductive development.

The author declares he has no competing financial interests.

Richard Sharpe
MRC Human Reproductive Sciences Unit
Centre for Reproductive Biology University of Edinburgh
Edinburgh, United Kingdom
E-mail: r.sharpe@hrsu.mrc.ac.uk

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