- Author: Mark Lundy
With the summer season wrapping up, people are starting to look ahead to the wheat season. I talked to a grower recently about the timing of N demand for irrigated wheat grown in the Sacramento Valley. Based on the N rate and timing studies we began last year (and will continue this year), I put together a few figures that may be of interest in planning for the upcoming season.
The first is a depiction of the cumulative and daily demand for N by a high-yielding (7500 lb/acre) wheat crop with 11.5% protein.
This was based on measurements taken from last year's multiple N-rate, multiple N-timing study, where we grew Patwin (a hard white wheat) and Cal Rojo (a hard red wheat). It's important to note that the absolute demand will change depending on how much residual soil nitrate there is available to the crop. If you don't have an idea about how much residual N you have, you might want to consider a pre-plant soil sample to inform your fertility decisions and, potentially, save some fertilizer costs.
The next figure shows how yield, protein, protein yield and N use efficiency change according to when N fertilizer is applied.
This was measured at the UCD research fields where the yield potential was high and the management of water was optimal. So the absolute yields are probably less relevant than the way the yield, protein, and efficiency change according to when N is applied. The take home messages from this trial were that: 1) applications at tillering are more valuable in terms of N use efficiency than preplant applications and 2) late-season applications can boost protein content if the crop has sufficient protein yield potential. Of course, these biophysical results don't take into account important logistical consideration (Can you get the fertilizer on the field at these times? Will sufficient water follow it to get it into the root zone?). But, if it's feasible, considering N applications at these times may prove worthwhile.
We have also been calibrating decision support thresholds for several in-field tools. We are cautiously optimistic that they may help to determine, in real-time, whether a crop is likely to see a protein yield response to a late-season application. But we need to repeat the work this year under a wider set of conditions to improve our confidence. We took some measurements in growers' fields last year around boot, heading and flowering to help validate the work we were conducting on the research station. I'd like to increase the number of 'real-world' data points that inform our calibrations. So, if you are growing wheat and are interested in seeing how these tools work, get in touch with me and I'll come take some measurements at your field and give a hands-on demonstration of how they work.