|dc.description.abstract||In May 1985, four cultivars of wheat (Triticum aestivum L.): Rongotea, Oroua, Weka and Otane were sown into a Templeton silt loam. Two levels of early nitrogen were applied at the mid-tillering stage; nil N or 112 kg N ha⁻¹ as sulphate of ammonia, this rate being calculated from the results of the soil incubation test which indicated low soil N levels. Late nitrogen, as a split plot treatment, was 0, 25, 50 or 100 kg N ha⁻¹ as sulphate of ammonia applied at late ear emergence.
The objectives of the experiment were to determine the degree of yield component compensation from the late N application, and to quantify the response of grain nitrogen content and MDD bake score to rate and timing of nitrogen application. Analysis of the protein composition of the grain was conducted to identify differences among cultivars and nitrogen treatments on grain protein deposition and whether this could be related to observed differences in baking characteristics. SDS sedimentation testing as well as MDD bake testing was used to assess baking quality.
Sequential harvesting was conducted throughout the growing season to measure changes in the accumulation of above ground dry matter and nitrogen. A very high level of pest and disease control was used as well as irrigation to minimise any stress the crop may have been subjected to. As a result the crop grew very well and an average rate of DM accumulation in excess of 190 kg ha⁻¹ day⁻¹ was measured in both nil and plus early N plots. Subsequent header harvested yields ranged from 6.5 to 9.2 t ha⁻¹, depending on cultivar and nitrogen input.
Late N application was too late to affect grains m⁻², but where no early N had been applied, late N was able to increase yield through an increase in kemel weight. This was achieved by nitrogen application delaying leaf senescence and therefore increasing source capacity as remobilisation of vegetative N to the developing peduncle at times of sub-optimal soil N uptake can accelerate leaf senescence. Early N application reduced mean kernel weight in all cultivars except Otane and late N had no effect on kernel weight where early N had been applied. Early N application appeared to limit potential and/or actual grain size even in the prescence of apparently adequate assimilate supplies. The mechanism by which this occured may be linked to the N % of the ear at anthesis, but requires further work.
Otane tillered poorly, even with early N application, and in spite of yield compensation from other components, yielded less than the other cultivars. Kernel weight changes in Otane were something of an enigma, not following any of the trends observed in any of the other cultivars. Otane had a massive grain size, with an average quadrat kernel weight of 65mg and some individual grains weighing over 80mg. Weka yielded well at over 8 t ha⁻¹, but was remarkably unresponsive to early N application except to reduce mean kernel weight. All cultivars except Weka gave a mean increase in yield up to about 21,000 grins m⁻² and a decrease in kernel weight with increasing grains m⁻², indicating that final yield was probably determined by the balance of source and sink of assimilates.
Bake scores were generally high as was mean grain protein content, but there was no relationship identified between MDD bake score and grain protein content. A much larger analysis, looking at this relationship, identified a positive, non linear, but poor relationship between these two parameters. The usefulness of continued work to measure bake score responses to increasing grain protein content is questioned due to the wide range of bake scores observed at anyone grain protein content.
Analysis of protein composition identified differences among cultivars in the partitioning of grain nitrogen between protein types. These differences in the partitioning of nitrogen among proteins indicates that some cultivars are more likely than others to give an increase in baking quality with increases in grain N % Grain N% increased linearly with late applied N, although the rate of response was reduced by early N application. Cultivar also affected the grain N %, with Rongotea and Otane having the highest and Weka and Oroua the lowest grain N% at anyone nitrogen level. High yields with high grain protein contents can be achieved provided the correct balance and timing of all inputs is achieved.||en