Grain protein as a post-harvest index of N sufficiency for hard red spring wheat in the semiarid prairies

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Abstract
Results from fertilizer trials with hard red spring wheat (Triticum aestivum L.) conducted throughout southwestern Saskatchewan under fallow and cereal stubble cropping conditions were used to determine if grain prote in concentration (GPC) could be used as an index of N sufficiency to the crop. Critical GPC were determined using the Cate-Nelson R2 procedure. Grain yield and protein concentration were negatively correlated under stubble and for fallow cropping when yields were below 2858 kg ha–1 ± 179, with grain protein decreasing by 15 mg g–1 for every 1000kg ha–1 yield increase. In these two groups of observations, water and N availability, N yield and grain produced per unit N available suggested that water availability was the dominant factor limiting grain yield. For the portion of fallow observations in which grain yields were higher than 2858 kg ha–1, water availability was not limiting, and N availability controlled grain yield and protein concentration. In this group, a GPC of 128 mg g–1 (range of 123 to 135 mg g–1) marked the transition between N deficiency and sufficiency. Under stubble cropping and for the lower-yielding portion of the fallow cropping system, where water stress was predominant, the Cate-Nelson analysis identified critical protein concentrations of 160 and 154 mg g–1, respectively. However, these critical concentrations separated populations into moderately and severely water-stressed crops, rather than providing a separation based on N availability. We concluded that GPC as a post-harvest index of N sufficiency must be used with caution in southwestern Saskatchewan. Grain protein concentration below the critical limit of 128 mg g–1 is a reliable indicator of low N sufficiency, but high grain protein does not necessarily imply N sufficiency because, frequently, grain yield and protein concentration are negatively correlated due to water stress. Key words: Yield, protein, N availability, critical levels, water stress

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