Ungrazed, lightly grazed, and heavily grazed intensity treatments for 50 years, in short-grass steppe of Colorado, were assessed in terms of forage nitrogen concentrations, nitrogen yields, and in vitro digestibilities. Current-year defoliation had positive effects, and long-term grazing and supplemental water had negative effects on forage nitrogen concentrations and digestibility. Defoliation may or may not increase leaf nitrogen concentrations, uptake of nitrogen by roots, aboveground yields, or pools of nitrogen. This study indicates that past grazing history interacts with current grazing treatment in determining nitrogen and energy availability to consumers. The positive-feedback of increased forage quality with defoliation was diminished by long-term heavy grazing. There were no direct effects of defoliation on aboveground primary production from this study, indicating that the quantity of forage produced in response to defoliation is not indicative of the response, in terms of the quality of forage produced.
Authors assessed forage nitrogen concentrations, nitrogen yields, and in vitro digestibilities from a short-grass steppe, that had been ungrazed, lightly, or heavily grazed for 50 years. In general, current-year defoliation had positive effects, and long-term grazing and supplemental water had negative effects on forage nitrogen concentrations and digestibilities. Nitrogen concentration and digestibility increased with defoliation in lightly, but not in heavily grazed treatments. The negative effects of long-term grazing on forage quality were small, equally or more than compensated for by defoliation in a year of average precipitation, but more pronounced in the simulated wet year. For both nitrogen and digestibility, yields were greater in grazed than ungrazed treatments in the year of average precipitation, but less in the simulated wet year. The authors suggest that optimizing quantity and year-to-year stability of nitrogen and digestible forage yield may best be achieved with light grazing rather than no or heavy grazing. Quantity (aboveground primary production, ANPP), quantity of quality (digestible and N yields), and quality (concentrations) do not necessarily respond similarly in interactions between current-year defoliation, long-term grazing history, and level of water resource.
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