Effect of previous cropping on irrigation and nitrogen responses in spring-sown ‘Karamu’ wheat

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Abstract
First and second-year crops of spring-sown ‘Karamu’ wheat were examined for their response to irrigation and fertiliser nitrogen in two adjoining experiments. Irrigation was applied by the border-strip method at nil, 25% and 50% available soil moisture and fertiliser N at the rate of 50 kg/ha N was applied at drilling and at shoot emergence. Grain yields were greatly increased by irrigation, and although yields generally increased further with irrigation frequency, differences between the irrigated treatments were not significant in either experiment. Fertiliser N increased the yield with but not without irrigation. The irrigation x fertiliser N interaction was significant only in the second-year crop. The mean irrigated yield was lower in the secondyear crop by 970 kg/ha without N, 700 kg/ha with N at drilling, and 470 kg/ha with N at shoot emergence. The timing of N application was more critical in the second-year crop. Yield composition was unaffected by previous cropping history. Irrigation increased ear numbers (in association with N at shoot emergence) and individual grain weight. Fertiliser N increased ear numbers, the number of spikelets per ear, and the number of grains per spikelet. Grain N % was heavily reduced by irrigation and improved by fertiliser N only in the absence of irrigation.