Survival of annual ryegrass (Lolium rigidum Gaud.) in a Mediterranean type environment. 1. Effect of summer grazing by sheep on seed numbers and seed germination in autumn

Abstract

Annual ryegrass–subterranean clover pastures that produced about 5000 kg total dry weight per hectare and 23,500 ryegrass seed per sq metre in spring were grazed by sheep at different stocking rates during summer. Intensive stocking equivalent to about 3000 sheep days ha-1 reduced seed numbers by 20%.

Under continuous grazing, about 70% of the seed produced in spring fell readily to the ground during summer. The remaining seed was firmly held in seed heads, and apparently sheep ate mainly this component. Less than 1% of the seed ingested was voided in the faeces. No significant changes in seed numbers over summer were observed in ungrazed pasture.

Subsequently, at the break of season in autumn, germination of seeds was examined in situ near the soil surface. The summer grazing history of pastures influenced the percentage of seeds that germinated; more seeds germinated in heavily than in leniently grazed pastures. Whether the pasture was leniently or heavily grazed, there was little effect on germination of shed seeds. Seeds in seed heads were found to germinate more slowly than seeds shed to the soil surface.

Seedling emergence in autumn was regulated mainly by the interrelationship between the germination rate of the seed population, depending on summer-early autumn rains, and the period for which favourable moisture conditions prevailed at the soil surface after rain began in autumn. In the field, temperature and light appeared to be unimportant in influencing germination at the break of season.

Dynamics of seed and seedling numbers in annual ryegrass pastures in a Mediterranean type environment, particularly at the break of season, are discussed.