Evaluation of late nitrogen applications to achieve yield potential and increased protein content in wheat

Nitrogen timing, application rate, and fertilizer form are key management decisions for growers every season. In the high rainfall zone, low grain protein usually indicates that N management has limited crop yields. Eight farmer wheat paddocks were selected and 3 N sources (i.e., granular urea, liquid urea, UAN) were applied at either 25 or 50 kg N/ha during either stem elongation, flag leaf, head emergence, or at the end of anthesis.

Across all treatments, there were yield responses at 3 of 8 sites, and protein responses at 5 of 8 sites. The responses at each site differ and only one site showed no response to any combination of N source, rate or timing. The mean N fertilizer response was 0.3 t/ha and a protein increase of 0.5%. For the majority of sites, Growth State (GS) 39 was the key timing to influence yield, and there were no differences between product type. Application rate had a small yield effect, with the higher rate of 50 kg N/ha having slightly higher yield than 25 kg N/ha. This timing is slightly later than we have previously thought for applying N for yield, with GS32 being the recognized standard. The data in this trial needs to be interpreted with the 2013 season in mind, as a tighter finish in the spring could mean plant uptake of late applied N could be limited.

The most significant outcome from this series of trials was the very strong effect seen to increasing grain protein. In nearly all cases, the higher application rate of N (50 kg N/ha) had a more significant effect on protein, as did delaying the timing until GS55. Product type had very little effect on increasing grain protein, but this may well have been influenced more by the softer finish to the season (longer period to maturity) that most sites experienced. The late N application at GS70 was too late to impact grain protein, irrespective of rate or source.

Messages to growers are to apply N early (stem elongation) for yield and later (head emergence) for protein and make those fertilization decisions as the season unfolds because a good finish can make all the difference.