Buried hard seed from seed returned to the soil during harvesting and incorporated by cultivation, poses the greatest threat of contamination. When a different white clover (Trifolium repens L.) is sown, contamination will occur when hard seeds within the germinating zone soften and emerge. Harvest losses can return 40 to 210 kg/ha of hard seeds to the soil. Flowering management and efficient harvesting are necessary to minimise quantities of hard seed returned to the soil. For a buried seed load of 845/m* in the germinating zone of a cultivated seed bed at sowing in March, 8% of that seed load gave rise to establishing plants of which one-third (19/m') were surviving in mid-October. At closing the percentage crop contamination within the sown row was directly proportional to the within-row seeding rate of the sown cultivar. At wider-than-normal row spaces, (45 cm compared with 15 cm) either inter-row cultivation or spraying for removal of contaminants was necessary at closing to reduce contaminants to a density similar to that in 15 cm spacing treatments. Physical contamination of the harvested seedline, compared with levels present at closing, had doubled for normal, but trebled for wider-than-normal spacings. Lowest percentage physical contamination of a seedline gained was 13% for a 6 kg/ha seeding at 15 cm spacings. The results indicate that successful cultivar change on many areas will be difficult. A buried seed count is considered an essential guideline to aid choice of paddock. Future management systems now being developed, include the use of herbicides and direct drilling, and should help to minimise this problem. Keywords: White clover, Trifolium repens, seed production, contamination, cultivar change, harvest losses, hard seed, cultural practices.
Contamination of white clover seed crops by buried seed