The effect of stocking rate on the population dynamics of siratro in siratro (Macroptilium atropurpureum)-setaria (Setaria sphacelata) pastures in south-east Queensland. II. Seed set, soil seed reserves, seedling recruitment and seedling survival

Abstract

Measurements were made of the seed set, seed content in cattle faeces, soil seed reserves, seedling regeneration and seedling survival of Siratro (Macroptilurn atropurpureum) in south-east Queensland. Measurements were made from 1972 to 1985 in pastures of Siratro/setaria (Setaria sphacelata) sown in 1968 and stocked at 1.1, 1.7 and 2.3 and at 3.0 (1969-1973) reduced to 2.0 (1974-1985) heifers ha-1. Siratro seed set and seedling demography were markedly affected by stocking rate. At the highest stocking rate, Siratro yields were low and seed input ceased after 1972, yet appreciable seedling recruitment from the soil seed bank was maintained for a further 10 years. At the lightest stocking rate (1.1 heifers ha-1), seed set and soil seed reserves were highest, but seedling recruitment was of little importance as almost all seedlings died. Seedling recruitment and survival were best at an intermediate stocking rate (1.7 heifers ha-1), where they were the key to long-term persistence of Siratro. Soil seed reserves of Siratro ranged from 500 m-2 under light stocking to < 100 m-2 after several years of heavy stocking. Very little seed was recovered in cattle faeces. Likely factors responsible for seedling death (light, water, nutrients, grazing, frosting and insect damage) are discussed. The limitations of seed set and seedling regeneration in the persistence of Siratro are contrasted with some other perennial pasture legumes. Studies of this type can help in qualitatively predicting long-term persistence of pasture legumes under different conditions of management and climate.