CHANGES IN THE AMOUNT AND NUTRITIVE CHARACTERISTICS OF ANNUAL PASTURES FROM LATE SPRING TO AUTUMN ON THE SOUTH COAST OF WESTERN-AUSTRALIA

We examined declines in annual pasture dry matter on offer (DMO) from around peak accumulation in spring through summer-autumn in 14 supplem entary feeding experiments across the south coast of Western Australia . Changes in dry matter digestibility (DMD) and concentrations of nitr ogen, phosphorus, potassium (K), calcium, magnesium and sulfur were al so measured. Plants which germinated in summer-autumn were manually se parated, as far as possible, by removing green components from the dry material, and it was assumed all dry pasture had grown during the pre vious growing season. Measurements were made under grazing (stocking r ates varying from 8 to 16.7 young wethers/ha) and in exclosures, and t he experiments were carried out at 5 locations over 3 years. Within an y experiment, there were no significant effects of supplementary feedi ng treatments on declines in DMO, DMD or digestible dry matter on offe r (DDMO). Highest measured DMO and DMD varied from 2600 to 8600 kg DM/ ha and from 63 to 81%, minimum measured values ranged from 100 to 1600 kg DM/ha and 45 to 54%, with amounts of DMO lost varying from 2000 to 8500 kg DM/ha. The declines in DMO, DMD and DDMO under grazing throug h summer-autumn were described by exponential decay curves. Losses in DMO and DDMO, as calculated from curve parameters, were positively cor related (P < 0.001) with maximum DMO or DDMO, respectively. Loss and a verage rate of loss in DMO were also correlated (P < 0.01) with total rainfall through the period of measurement. The number of rainy days, stocking rate and botanical composition of pastures in spring were not significantly correlated with these losses under the conditions of th ese experiments. The percentage rate of decline in DMO was not strongl y correlated with any of these variables and its variation across expe riments was small. Change and rate of change in DMD were positively co rrelated (P < 0.001) with maximum measured DMD and negatively correlat ed (P < 0.05) with the percentage of grass in the pasture. It was esti mated that most of the lost material was not consumed by sheep. The co ncentrations of some essential minerals in grazed dry pasture approach ed, or fell below, their critical concentrations in most experiments. Of the minerals measured, only changes in K concentrations in dry past ure were well described by exponential regressions.