Response of irrigated lucerne to defoliation and nitrogen fertiliser

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Abstract
A 3-year-old stand of lucerne (Medicago media Per.) cv. ‘Wairau’ was spray irrigated and subjected to three defohatlon frequencies from spring to. autumn, after which all plots received a common management. Nitrogen fertiliser was applied to half the plots after each defoliation. Total dry-matter Yields calculated for a 12-month period were 24.0 and 28.2 tonnes DM/ha, and herbage N yields for control plots and those receivino N were 730 and 860 kg/ha respectively when the lucerne was defoliated at first flower. The highest mean growth rates (150 kg DM ha/day) occurred in midsummer, coinciding with maximum temperatures and radiation. Maximum response to N fertiliser also occurred at this time. Nitrogen fertiliser depressed spring lucerne yields. Defoliation management did not affect short-term N yields, but frequent defoliation reduced dry-matter yields and allowed extensive weed invasion. Nitrogen fertiliser did not affect weed yields.