GLOBAL ENVIRONMENT CHANGE AND SIMULATED FORAGE QUALITY OF WHEAT .2. WATER AND NITROGEN STRESS

Forage crops are frequently subjected to stress conditions resulting f rom inadequate supplies of water and N. Because forages grown under th ese stress conditions constitute an important resource in animal agric ulture, this study was undertaken to assess possible changes in the nu tritive value and productivity of forage crops as a consequence of glo bal environment change. A relatively simple, mechanistic model of whea t was extended to simulate growth and important determinants of feed q uality ([N], leaf:stem, dry matter digestibility) in an annual, temper ate climate C-3 forage grass. Weather data for a semiarid region and d ifferent levels of applied N were used to examine the response of fora ge productivity to various levels of water and N availability. Not sur prisingly, responses to global environment change were highly dependen t on the availability of both water and N. When either resource was av ailable at low levels, production of digestible dry matter was nearly unchanged by elevated [CO2] or increased temperature. When compared at equivalent development stages, small increases in forage quality were simulated, mainly because higher temperature resulted in achievement of the initiation of grain fill at an earlier date. As N availability increased, differences in forage characteristics and productivity beca me more prominent. Elevated ambient [CO2] increased vegetative mass, d igestible dry matter, and concentration of digestible dry matter but d ecreased leaf:stem and [N]. Increased temperature generally had an eff ect on forage traits that was opposite to the elevated [CO2] response. The combined effects of both factors sometimes cancelled each other, but usually one of the factors was dominant. Negative effects of tempe rature tended to be aggravated by dry conditions. At crop maturity, po sitive effects of elevated atmospheric [CO2] on forage productivity an d quality were severely decreased by nutrient and physiological constr aints. These simulations indicate that when forage crops are grown und er irrigation in semiarid regions, there may be substantial and comple x changes in productivity and feed quality as a consequence of warmer temperature and elevated atmospheric [CO2]. Under rainfed conditions, these differences could be quite erratic and virtually unpredictable w ithin the current range of interannual variation in forage productivit y and quality.