The growth, senescence and ignitability of annual pastures

Two field experiments were carried out at the Waite Agricultural Research Institute during 1961/2 and 1962/3 to study the growth and senescence characteristics or annual swards and their increasing propensity to ignite during senescence. In Experiment A. (1961/2) an annual sward of Wimmera ryegrass (Lolium rigidum Gaud) and subterranean clover (Trifolium subterraneum L.) was subjected to three irrigation treatments (giving three different lengths of growing season) and three defoliation treatments (no defoliation, bi-monthly defoliation and one defoliation at the hay- stage) . The experiment was repeated in 1962/3 with the three defoliation treatments imposed on a sward grown under natural rainfall only. Monthly samples were taken for dry matter yield, moisture content and the ratio of green material/dry residues during the period or growth and senescence. Monthly samples of dry residues were taken during the summer when the swards were dry. Maximum yield of an undefoliated sward depended on the ability of that sward to exploit the environment at ceiling yield and on the time of maturity and disintegration of the component species in relation to the sward ceiling yield. Both the time of commencement and length of the growing season and the time and frequency of defoliation had a significant effect on the total yield of the sward and on its botanical composition. Between maximum yield (September/October) and January there was a loss in the amount of dry matter of from 18-51 per cent. 15 per cent. of this drop in 1962 was accounted for in seed and fragmented plant material. Maximum moisture content was reached from 1-2 months prior to maximum dry matter yield. Sward moisture content trends varied with the species composition of the sward, the season and with the degree of defoliation; defoliation delayed (with adequate soil moisture) maturity and senescence. The moisture content trends of two species (subterranean clover and Wimmera ryegrass) growing in a sward were compared. Spring irrigation will delay but not reverse the moisture content decline in a sward, while it is suggested that an 'early break' to the season will produce a deeper rooted sward, which retards spring desiccation. Moisture content of the whole sward during senescence was inversely related to the per cent. of dry residues. In Experiment B. (1962) five annual species, barley grass (Hordeum murinum L.), brome grass (Bromus mollis L. cv. Blando), silver grass (Vulpia myuros L.), Wimmera ryegrass and subterranean clover were grown in mono-specific swards. During senescence, measurements were taken of dry matter yield, moisture content and ratio of green material/dry residues. Ignition tests were carried out during senescence by dropping a lighted match onto the swards from a standard striking apparatus. Distinct species differences were shown in both the incidence and rate or desiccation. Among the grass species barley grass was the first species to decline in moisture content and Wimmera ryegrass was the last. The rate of desiccation of subterranean clover exceeded that of all the grass species. Species differences were also shown in the incidence and rate of formation of dry residues. Moisture content and the amount of dry residues were related to ignition probability. While the dry residues, both in quantity and spatial distribution, were considered to control ignition probability, moisture content was more conveniently and accurately measured. A good inverse relationship between moisture content and per cent. dry residues was again shown. Differences between total and sustained ignition were an index of the continuity of the dry residues as fuel. Height of lodging of the match affected the ignition probability of the grass species but not of subterranean clover. During senescence an increasing proportion of matches reached ground level in the grasses, while the reverse was true for subterranean clover. Further factors that are related to ignitability are discussed. These included leaf/stem/seed ratios, time of seed fall and the moisture content of the dry plant material as determined by the current climate.Thesis (MAgricSc) -- University of Adelaide, School of Agriculture, Food & Wine, 196