Effects of irrigation, nitrogen and gypsum on yield, nitrogen accumulation and water use by wheat

Abstract

Responses of wheat grown on a heavy clay soil in the Goulburn-Murray Irrigation Region of south-eastern Australia to a factorial combination of three irrigation treatments and nitrogen and gypsum application were investigated.Irrigation treatments included a rainfed control (treatment R F) and irrigation on either a weekly (treatment I w) or fortnightly (treatmnt I F) basis beginning in spring and maintained until physiological maturity. Nitrogen was applied at 0 and 150 kg N ha −1 (treatments N 0 and N 150, respectively) and gypsum at 0 and 5 t ha −1. Nitrogen and gypsum treatments were applied at sowing. Yield increased from a mean of 4 t ha −1 treatment R F to 6.6 t ha −1 in treatments I F and I W, largely because of promotive effects of irrigation on kernel weight (increase from 31 mg to 42 mg kernel −1 and kernel spikelet −1 (1.4 as compared with 1.7). Seasonal conditions and the relative fertility of the site were sufficient to maximise spike number and spikelet spike −1. Nitrogen increased kernel spikelet −1 but effects on yield were not significant because of a decrease in kernel weight. Effects of gypsum on yield were not significant. Water-use efficiency of both rainfed and irrigated treatments was ca. 1.25 g grain kg −1 H 2O. However, transpirational water-use efficiency, calculated after allowing 110 mm water for soil evaporation, fell from 2 g kg −1 in treatment R F to 1.7 and 1.5 g kg −1 in treatments I F and I W, respectively. The decrease was ascribed, in part, to lodging and soil evaporative losses may have been in excess of 110 mm with more frequent irrigation. Effects of N on water use could not be distinguished, again because of the initial fertility of the site, which supported rapid growth and resulted in complete canopy closure. Nitrogen and irrigation treatments had independent effects on the concentration of N in the grain (% N G ) which increased by a mean of 0.6% with N treatment despite a decrease in N harvest index (HI N) from 0.77 to .70. Irrigation decreased % N G by approximately 0.5%. Approximately 90 kg pN ha −1 was found in the grain of treatments R FN 0, I WN 0, I wN 0 and R FN 150 and differences in % N G in these treatments attributed to a ‘dilution’ effect mediated by the increase in yield effected by irrigation. The grain accounted for approximately 115 kg N ha −1 in treatments I FN 150 and I WN 150, countering the inverse relationship between % N g and yield despite the increase in HI N index caused by N application.