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The Nutritive Value of Herbage Grown in High-CO2 Air
Luscher, A., Daepp, M., Blum, H., Hartwig, U.A. and Nosberger, J.  2004.  Fertile temperate grassland under elevated atmospheric CO2 - role of feed-back mechanisms and availability of growth resources.  European Journal of Agronomy 21: 379-398.

Periodically, the nitrogen concentrations of plants grown in CO2-enriched atmospheres are found to be lower than those of plants grown in ambient air; and this observation, whenever it occurs, seems to stimulate the appearance of stories in the popular press about CO2-induced decreases in plant nutritive value.  With respect to grasslands upon which domesticated animals graze, for example, Luscher et al. note that "a lower concentration of nitrogen may significantly affect the nutritive value of the herbage and, thus, affect digestibility, forage intake and the productivity of ruminants," which implies a potential for the occurrence of negative consequences in a CO2-accreting atmosphere.

What was done
The authors report some interesting things that have been learned about this subject in a review of the responses of fertile temperate grasslands to atmospheric CO2 enrichment, based primarily on studies conducted at the Swiss FACE facility that hosts what has become the world's longest continuous atmospheric CO2 enrichment study of a naturally-occurring grassland.

What was learned
In response to an approximate two-thirds increase in the air's CO2 concentration, the leaf nitrogen (N) concentrations of white clover (Trifolium repens L.) and perennial ryegrass (Lolium perenne L.) were reduced by 7% and 18%, respectively, when they were grown separately in pure stands.  However, as Luscher et al. report, "the considerably lower concentration of N under elevated CO2, observed for L. perenne leaves in pure stands, was found to a much lesser extent for L. perenne leaves in the bi-species mixture with T. repens (Zanetti et al., 1997; Hartwig et al., 2000)."  Furthermore, as they continue, "under elevated CO2 the proportion of N-rich T. repens (40 mg N g-1 dry matter) increased in the mixture at the expense of the N-poor L. perenne (24 mg N g-1 dry matter when grown in monoculture)," the end result being that "the concentration of N in the harvested biomass of the mixture showed no significant reduction [our italics]."

On another note, Luscher et al. state that "the nutritive value of herbage from intensively managed grassland dominated by L. perenne and T. repens ... is well above the minimum range of the concentration of crude protein necessary for efficient digestion by ruminants (Barney et al. 1981)."  Hence, they conclude that "a small decrease in the concentration of crude protein in intensively managed forage production systems [which may never occur, as noted in the preceding paragraph] is not likely to have a negative effect on the nutritive value or on the intake of forage."

What it means
In real-world situations where white clover and perennial ryegrass are grown together, atmospheric CO2 enrichment may not have a negative impact on total herbage nitrogen concentration; and even if it did, the impact would likely not be large enough to negatively impact the growth and wellbeing of ruminants feeding upon the forage.  In addition, in a CO2-enriched world of the future there would be much more such forage produced per unit of land and water devoted to the enterprise.

Barney, D.J., Grieve, D.G., Macleod, G.K. and Young, L.G.  1981.  Response of cows to a reduction in dietary crude protein from 17 to 13% during early lactation.  Journal of Dairy Science 64: 25-33.

Hartwig, U.A., Luscher, A., Daepp, M., Blum, H., Soussana, J.F. and Nosberger, J.  2000.  Due to symbiotic N2 fixation, five years of elevated atmospheric pCO2 had no effect on litter N concentration in a fertile grassland ecosystem.  Plant and Soil 224: 43-50.

Zanetti, S., Hartwig, U.A., Van Kessel, C., Luscher, A., Bebeisen, T., Frehner, M., Fischer, B.U., Hendrey, G.R., Blum, G. and Nosberger, J.  1997.  Does nitrogen nutrition restrict the CO2 response of fertile grassland lacking legumes?  Oecologia 112: 17-25.

Reviewed 9 February 2005