Approaches to optimizing nitrogen fertilization in a winter wheat-red clover (Trifolium pratense L.) relay cropping system

Diversifying rotations by including winter wheat and red clover in relay cropping provides several advantages to temperate cropping systems dominated by maize and soybean. To improve the environmental and economic benefits of introducing such relay systems, approaches toward wheat N fertilization that consider the negative impact of high N rates on the interseeded legume must be developed. We conducted experiments at 10 locations across Ontario over two years to quantify the effects of N fertilization on soft winter wheat-red clover relay cropping system yields and estimated the N rate at which maximum economic return to N can be achieved. Both crops responded inversely to N fertilization independently of site and year but the competition mechanisms involved were different between the two growing seasons. Whereas the balance between the two species was significantly adjusted to wheat growth response to N and the resulting decrease in radiation reaching the clover canopy in 2006, low precipitation in the summer of 2007 severely affected red clover biomass accumulation. Using economic assumptions to estimate N and forage values of red clover biomass, we show that relay cropping consistently increased the average return to N investment across the N fertility gradient. Under our conditions, N applications at the maximum economic rate (MERN) for wheat grain yield alone significantly decreased the economic benefits of red clover when the forage value of the clover biomass was considered. High N rates also increased the likelihood of non-uniform red clover stands, possibly decreasing red clover benefits and its value as a system component. We demonstrate that reduction in N rates can maximize economic returns from both wheat and red clover and that higher partial profits can be reached using lower N inputs.