Abstract The phytotoxicity of spent litter collected from pig pens employing the ‘[pig-on-litter’ system at various times was evaluated using seed germination and root elongation techniques. The percentage seed germination of four plant species (lettuce, Chinese cabbage, tomato and green beans) was not affected by the water extracts of spent litter samples collected in the first 30 weeks of production. Seed germination was significantly retarded by litter extracts from 34 weeks onwards. The percentages of seed germination at the end of the 45 weeks study were 1% for lettuce, 16% for cabbage, 21% for tomato, and 44% for green beans. Compared with seed germination, root elongation was more sensitive to the toxicity of the spent litter. The root lengths of all seedlings except green beans were less than 50% of the control (deionized water) throughout the experiment. The inhibitory effects of spent litter on root elongation increased with the age of the litter. The final root lengths of lettuce, Chinese cabbage and tomato seedlings were 14%, 24% and 28% of the control, respectively. Green beans behaved very differently from the other species; spent litter extracts stimulated root growth throughout the study, except the last week. The elevated concentrations of heavy metals (in particular extractable Cu) and nutrients (especially NH 4 +N) present in spent litter were the main factors responsible for the phytotocicty of the spent litter. The aged spent litter had accumulated more salts, nutrients and heavy metals and imposed more toxic effects on seed germination than did the young spent litter.