Nitrogen management in wheat (Triticum aestivum L.): Grain yield and quality as influenced by topography and fertilization

Excessive N fertilization of soft red winter wheat ( Triticum aestivum L.) may lead to undesirable protein concentrations. The inter-relationships between wheat grain yield, grain protein concentration, soil nitrogen (N) levels and N fertilization on two variable landscapes (site 1 and 2) in Southwestern Ontario were examined. Six N rates (0 to 145 kg N/ha) were applied to plots (400 m long), arranged in a randomized complete split block design with four replicates. Along each plot, samples for soil N test and grain yield were collected on a 20-m interval resulting in a 3 x 20 m grid-sampling pattern (456 sampling points/site). Upper slope positions typically had lower yields and higher protein concentrations than the lower slope positions. For each field the most economic rate of N (MERN) for yield calculated from quadratic regression models was determined to be 103.1 and 105.2 kg N/ha at sites 1 and 2, respectively. The MERN varied with slope position at site 2 suggesting the potential to variably apply N. Protein concentrations followed a sigmoidal response to applied N. Although the response was similar for each slope position, there was a greater risk of exceeding the desirable protein concentration at the upper slope positions.