White clover soil seed loads: effect on requirements and resultant success of cultivar-change crops

Abstract

A study was done at Canterbury Agriculture and Science Centre, Lincoln, New Zealand, to equate volunteer white clover plant establishment with the decay of the soil seed burden over time. A 5 season break without clover sowings, if coupled with either annual cultivation or herbicide removal of establishing volunteer plants, reduced contamination potential below that desired to meet certification requirements for first-generation seed crops. 'Buried-seed counts' taken after the last deep cultivation for the change crop should be in the O-3 seeds per 50 cores range for breeders' and basic crops and 4-6 per 50 cores for first-generation crops. Sowings to produce breeders' and basic seed must be in 45-cm row spacings to facilitate inter-row herbicide elimination of volunteers. Ministry of Agriculture and Fisheries Seed Testing Station data on 705 potential change crop sites over 5 seasons, indicated that for breeders basic and first generation, only 31% and 50% of paddocks respectively fell within these ranges, justifying the continued use of this measure as an indicator of potential for success. For the 1989-90 season, 17% of the 182 first-year autumn-sown cultivar-change certification entries were withdrawn because of poor establishment and/or weed problems, 3% for wrong sowing methods obviating inspection a n d 5 % w e r e r e j e c t e d b e c a u s e o f contamination. Currently, the major limitation to growing high yielding white clover change crops, particularly of small-leaved cultivars, is in making seed multipliers aware of available technology. Keywords: white clover, contamination, buried seed, volunteer plants, certification