Ronald P. Beyaert

Forage sorghum–sudangrass [Sorghum bicolor (L.) Moench] is a relatively new crop to eastern Canada and the effects of additions of fertilizer N on yield, N accumulation, and N use efficiency are not available for this region. In 1998, 1999, and 2000 the response of forage sorghum–sudangrass to additions of fertilizer N rates (0, 50, 100, 150, 200, and 250 kg N ha⁻¹) either applied as a single sidedress application or split into two sidedress applications was evaluated on a Fox loamy sand (Psammentic Hapludalf). Although timing of N application had little effect on DM production, splitting the N application into two equal applications may be of benefit by enhancing the NUE and ANR of individual cuts. Maximum yield was estimated at 5.95 Mg ha⁻¹ at an N rate of 125 kg N ha⁻¹ and the most economic rates of N ranged from 83 to 107 kg N ha⁻¹ dependent on the cost of N fertilizer and value of hay. Nitrogen concentration increased linearly with increasing N application and the maximum N accumulation was 161 kg N ha⁻¹ at an N rate of 196 kg N ha⁻¹. Total N use efficiency and apparent N recovery decreased with increasing N rates ranging from 36 to 11 kg DM kg⁻¹ and 90 to 24%, respectively. Optimum yield and N efficiency occurred when 100 kg N ha⁻¹ was applied as a split application. Producers in southern Ontario require N fertilizer additions to optimize sorghum–sudangrass yields but need to avoid overfertilization with N to maximize N use efficiency and apparent N recovery.