'Haying-off', the negative grain yield response of dryland wheat to nitrogen fertiliser. I. Biomass, grain yield, and water use

"Haying-off" was studied by comparing wheat responses to applied nitrogen (N) at 3 sites in southern New South Wales, which differed in the amount and timing of rainfall during crop growth. At a site where the crops encountered little water deficit, dry grain yield increased from 607 g/m2 for a low-N control crop to 798 g/m2 for a high-N crop. At a site with severe terminal drought, dry grain yield decreased 24% from 374 g/m2 for the control, to 284 g/m2 for the highest N crop. At the third site, yields increased with small applications of N, whereas greater applications resulted in a negative yield response. At the 2 latter sites, the crops that showed decreased yield with applied N had clearly hayed-off. At all sites, irrespective of water status, N application resulted in increases in biomass at anthesis, spike density, kernels per spike, and kernel number. Kernel weight decreased in response to additional N at all sites, but most markedly at the haying-off sites, where it decreased by up to 38%. Harvest index increased in response to N at the high-rainfall site, but decreased in crops that hayed-off. Grain protein increased in response to N at all sites, with a range from 9% to 18% at the haying-off sites. The apparent retranslocation of assimilates to grain contributed 37-39% of grain yield (depending on N supply) at the high-rainfall site, compared with 75-100% at the haying-off sites. In contrast, when apparent retranslocation was expressed in relation to biomass at anthesis, it remained relatively constant, amounting to 23-26% at the high-rainfall site and 24-28% when crops hayed-off. By anthesis, high-N crops extracted more soil water than the low-N crops. By maturity the most severely hayed-off crop had extracted 10 mm less soil water than a low-N crop, but at the high rainfall site the high-N crops extracted 20 mm more soil water than the control crops. The weather conditions between anthesis and physiological maturity were relatively mild, with no daily maximum temperatures above 30oC and no sudden increases in evaporative demand. Thus, there appeared to be 3 processes leading to haying-off. Firstly, the results confirm previous studies showing that haying-off was associated with reduced post-anthesis assimilation in response to a lack of soil water. The water deffcit was due to vigorous vegetative growth stimulated by a high level of soil N and was not associated with heat shocks or sudden increases in evaporation. Secondly, the most severely hayed-off crop failed to extract soil water fully, leading to a further reduction in post-anthesis assimilation. Thirdly, there was inadequate apparent retranslocation of pre-anthesis reserves to compensate for the lack of post-anthesis assimilation.