Levels of germinable seed in topsoil and cattle faeces in legume-grass and nitrogen-fertilized pastures in south-east Queensland

Abstract

Measurements were made of germinable seed of all species in the top 5 cm of soil beneath 9 paddocks in an 11-year-old pasture experiment in south-east Queensland. The experiment comprised two pasture types grazed by cattle: legume-grass (Macroptilium atropurpureum- Setaria sphacelata) pastures grazed at four stocking rates and nitrogen fertilized S. sphacelata pastures grazed at five stocking rates. Levels of germinable seed in the soil varied with pasture type and stocking rate and ranged from 6760 to 45480 seeds/m2. Seed reserves increased on the legume-grass pastures as stocking rate increased. Fifty-seven different plant species were identified from the seedlings grown from the soil seed reserve. For 11 of these species, which contributed 70% of the seed reserves, there were significant trends in relation to pasture type and/or stocking rate. The only species where seed reserves significantly decreased with increasing stocking rate were the two sown species. Cattle faeces were also collected from lightly and heavily stocked legume-grass and nitrogen-fertilized pastures in spring, summer, autumn and winter. Germinable seed contents of all species were measured. Seed content of faeces was higher in the summer and autumn than in the winter and spring samplings, and in the heavily stocked than in the lightly stocked pastures. The highest content recorded was 20.5 germinable seeds g-1 faecal dry matter. The species with the highest germinable seed content in faeces were Digitaria didactyla, Axonopus affinis and Eleusine indica, the latter being restricted to nitrogen-fertilized pastures. Seed dissemination in faeces is a factor contributing to the invasiveness of prostrate perennial grasses such as D. didactyla and A. affinis under heavy grazing.