FORAGE QUALITY IN RELATION TO LONG-TERM GRAZING HISTORY, CURRENT-YEARDEFOLIATION, AND WATER-RESOURCE

Forage nitrogen concentrations, nitrogen yields, and in vitro digestib ilities were assessed in shortgrass steppe that had been ungrazed, lig htly, or heavily grazed for 50 years. Caged plots were defoliated in a mounts based upon removals observed in naturally-grazed reference plot s or not defoliated. This was done in a year of average precipitation and with a supplemental water treatment to simulate a wet year. In gen eral, current-year defoliation had positive effects, and longterm graz ing and supplemental water had negative effects, on forage nitrogen co ncentrations and digestibilities. However, defoliation interacted with long-term grazing in determning forage nitrogen concentrations, and w ith grazing and with watering in determining digestibilities. Nitrogen concentration and digestibility increased with defoliation in lightly , but not in heavily, grazed treatments. The dilution effect of supple mental water an digestibilities through increased plant growth was off set by defoliation. The negative effects of longterm grazing on forage quality were small, equally or more than compensated for by defoliati on in a year of average precipitation, but more pronounced in the simu lated wet year. Nitrogen yields and digestible forage production were usually increased by defoliation, but this depended upon grazing and w atering treatments. Increased nitrogen and digestible forage yields an d concentrations in response to defoliation were greater than the biom ass response in lightly grazed grassland. For both nitrogen and digest ibility, yields were greater in grazed than ungrazed treatments in the year of average precipitation, but less in the simulated wet year. Op timizing quantity and year-to-year stability of nitrogen and digestibl e forage yield may best be achieved with light grazing rather than no or heavy grazing. Clipping was conducted in a manner closely resemblin g the natural pattern and intensity of defoliation by the cattle, and confirms the potential for a positive feedback of increased forage qua lity with defoliation observed in pot experiments. Long-term heavy gra zing can diminish this response. Quantity (aboveground primary product ion, 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 leve l of water resource.