Yield response of forage grasses to N fertilizer as related to spring soilnitrate sorbed on anionic exchange membranes

Soil N availability is an important factor in forage grass production. Maxi mising N fertilizer efficiency is essential to improve profitability and to reduce the environmental risk associated with residual excess soil N. The objectives of this study were (i): to determine the effects of N fertilizer on yield, N uptake and NO3-N concentration of forage grasses produced in W estern Quebec; and (ii) to compare spring soil NO3- measured by anionic exc hange membranes (NO3AEMs) and by water extraction (NO3w) as a criterion to predict fertilizer N requirements of forage grasses. The yield response of gasses, especially timothy (Phleum pratense L.), to different rates of NH4N O3 (0 to 240 kg N ha(-1)) on heavy clay soils (Humic Gleysols) was studied from 1994 to 1996 at four sites in the Abitibi-Temiscamingue area, Quebec ( Canada). Nitrogen significantly (P < 0.001) increased forage yield, N uptak e, and NO3-N concentration. The economically optimum N fertilizer rate (Nop ) for forage yield varied from 25 to 240 kg N ha(-1) depending on sites and years, and averaged 125 kg N ha(-1). The Nop can be predicted more adequat ely by NO3AEMs (R-2 = 0.45) than by NO3w (R-2 = 0.09). Based only on the re lationship between the relative yield and spring soil nitrate, NO3AEMs coul d be used as a criterion for fertilizer N recommendation of forage grasses in this cool continental climate.