Global environment change and simulated forage quality of wheat II. Water and nitrogen stress

Forage crops are frequently subjected to stress conditions resulting from inadequate supplies of water and N. Because forages grown under these stress conditions constitute an important resource in animal agriculture, this study was undertaken to assess possible changes in the nutritive value and productivity of forage crops as a consequence of global environment change. A relatively simple, mechanistic model of wheat was extended to simulate growth and important determinants of feed quality (N], leaf:stem, dry matter digestibility) in an annual, temperate climate C₃ forage grass. Weather data for a semiarid region and different levels of applied N were used to examine the response of forage productivity to various levels of water and N availability. Not surprisingly, responses to global environment change were highly dependent on the availability of both water and N. When either resource was available 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 became more prominent. Elevated ambient CO₂] increased vegetative mass, digestible dry matter, and concentration of digestible dry matter but decreased leaf:stem and N]. Increased temperature generally had an effect 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 temperature tended to be aggravated by dry conditions. At crop maturity, positive effects of elevated atmospheric CO₂] on forage productivity and quality were severely decreased by nutrient and physiological constraints. These simulations indicate that when forage crops are grown under irrigation in semiarid regions, there may be substantial and complex 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 within the current range of interannual variation in forage productivity and quality.