EFFECTS OF ELEVATED PARTIAL-PRESSURE OF CARBON-DIOXIDE AND SEASON OF THE YEAR ON FORAGE QUALITY AND CYANIDE CONCENTRATION OF TRIFOLIUM-REPENS L. FROM A FACE EXPERIMENT

Differently managed (cutting frequency and N fertilization) Trifolium repens monocultures were grown at 60 Pa and 35 Pa of pCO(2) (partial p ressure of CO2) in a Free Air Carbon dioxide Enrichment (FACE) array. The concentrations of cyanide, digestible organic matter, crude protei n and net energy for lactation were measured at different harvests thr oughout the growing season. The average cyanide concentrations differe d significantly in the years and the seasons within the year; however, the concentrations were not affected by CO2. Digestible organic matte r, crude protein and net energy for lactation differed significantly w ith the seasons of the year and cutting frequencies. While digestible organic matter and net energy for lactation were not affected by eleva ted pCO(2), the concentration of crude protein decreased from 288 g kg (-1) at ambient to 251 g kg(-1) at elevated pCO(2). Since the crude pr otein concentration in herbage from Trifolium monocultures was very hi gh even at elevated CO2, it is suggested that this decrease in crude p rotein concentration does not negatively affect forage quality. We con clude that, in Trifolium herbage, the seasons of the year and manageme nt practices are more decisive for forage quality than changes in pCO( 2). We shall discuss how forage quality and cyanide intake by ruminant s may, however, be affected by CO2-induced shifts in the proportion of species in mixed plant communities.