Seed contaminants in ryegrass and clover seed for sowing imported into New Zealand

About 40% of New Zealand’s land area is in pasture, with perennial ryegrass and white clover being favoured forages. Ryegrass and clover contribute $14.5 billion and $2.3 billion NZD to GDP annually. Seed companies develop varieties but also import seed for multiplication. Ryegrass and clover seed is sold to local and international markets, contributing ca. $125 million in sales. To control biosecurity risks, imported seed is inspected using the International Seed Testing Association Rules to determine physical purity and contamination by weed seeds. Enforcement measures against seed consignments based on the weed contaminants are rare. A quarantine list identifies ca. 1170 species of concern. We examined seed contaminants from inspections carried out by the Ministry for Primary Industries over five years (2014–2018). Eight seed companies contributed to most of the importations. Seed lots were sourced from 17 (ryegrass) and 19 (clover) countries. Of the 560 ryegrass consignments, 16% had weed contaminants and 0.5% contained the quarantine species Alopecurus myosuroides and Arctium minus. Of 381 clover consignments, 19% were contaminated but no quarantine weed seeds were detected. A total of 65 and 62 contaminating weed species were detected in ryegrass and clover respectively. The most common were Polygonum aviculare and Poa annua in ryegrass, and Lolium spp. and Chenopodium album in clover. Most weed seed detections do not cause a shipment to be re-dressed or destroyed. The frequency with which weed seed contaminants are missed during inspection is not known and there is concern that some seeds could be from herbicide-resistant genotypes. The New Zealand arable industry participates in an international seed network that supports arable and pasture farming worldwide, but this has important biosecurity risks.