Effect of closing date and irrigation on seed yield (and some yield components) of ‘Grasslands Kopu’ white clover

Abstract In 1981 at Lincoln, an autumn‐sown seed crop of the late‐flowering ‘Grasslands Kopu’ white clover ( Trifolium repens L.) was closed for flowering in either mid November or mid December. The crop, growing on a Wakanui silt loam of 32 mm permanent wilting point and 69 mm field capacity, was either irrigated or unirrigated from mid December. Delay in closing reduced seed yields from 699 to 354 kg/ha and halved inflorescence populations, but reduced unit seed weight by only 4%. Irrigation increased seed yield by 53% (from 417 to 637 kg/ha), and increased inflorescence populations by 22% and unit seed weight by 4%, and reduced ovule abortion by 27%. Irrigated crops closed in mid November yielded most (826 kg/ha), and unirrigated crops closed in mid December the least (261 kg/ha). The main effect of irrigation was to increase total seed numbers — and, thereby, yield — by prolonging growth. Loss in seed yield by delaying closing was related directly to the removal of the first‐formed inflorescences at final defoliation.