Dispersal of seeds of giant Sporobolus spp. after ingestion by grazing cattle

Abstract

Samples of fresh cattle faeces were taken from 2 grazing paddocks on the North Coast of New South Wales. One paddock was heavily infested with giant Parramatta grass (GPG, Sporobolus indicus var. major) and the other paddock had a medium-light infestation. Fifty-two and 14 viable GPG seeds/kg were recovered from the dung samples collected in May from these paddocks, respectively. Seeds of GPG and giant rat's tail grass (GRTG, S. pyramidalis) were fed to caged Friesian heifers in 2 experiments, and the time taken for all of the seeds to be excreted was measured: 4 days in experiment 1 and 7 days in experiment 2. Consequently, a 7-day withholding period is suggested for cattle grazing pastures infested with giant sporobolus. The number of viable seeds excreted was estimated as a proportion of seeds fed to each heifer using average viability of seeds recovered, concentration of viable seeds collected daily, and daily manure weights. Based on this, the number of seeds ingested by gazing stock was estimated to be 8300 and 2200 seeds/head.day for the paddocks with heavy and medium-light infestation, respectively. Manure collected 2 and 3 days after feeding had the highest concentrations of viable seeds. This manure was placed outdoors in styrofoam boxes and left for 7 months. No GPG seedlings or plants were present in the manure after this time, and further subsampling of the manure indicated that no viable seeds remained. Processes that disperse fresh manure, such as heavy rain or the hosing down of transport vehicles, may allow successful germination and emergence of giant sporobolus seeds. However, where dung pats are left intact for some months, establishment of giant sporobolus plants is unlikely. Adhesion to machinery and the hair of stock are likely to be more important causes of dispersal of giant sporobolus than ingestion by stock.