Two experiments were conducted to determine whether alien weed seeds mixed in imported fodder crops could become established in forage crop fields via cattle excreta. Horsenettle (Solanum carolinense L.) seeds were fed to cows which received 2 diets with different concentrates, i. e. flaked barley and corn, and the seeds recovered from excreta were tested for germination and viability. The 2 diets had no effect on the seed recovery, or on the germination or viability of recovered seeds. The mean percentages of seed recovery, and germination and viability of recovered seeds were 83, 61 and 76%, respectively. Among the seeds estimated to be contained in excreta placed outdoors, 15% emerged. Likewise, the germination percentage of 15 weed species exposed to compost incubation was determined. Weed seeds were placed in 39 composting tubes (52.1 L) for 7-25 days, and the regression equation was calculated between the maximum compost temperature and the percentage of species which retained their germinability. The effects were not conspicuous until the maximum temperature reached 46℃. Thereafter there was a rapid decline in the percentage of germinable species, and none of the species germinated when the temperature was above 57℃. The results showed that imported fodder crops containing weed seeds could become a source of weed infestation unless animal excreta were treated adequately.