Senescence, digestibility and carbohydrate content of buffel grass and green panic leaves in swards

Abstract

Seasonal changes in the rate of production, senescence and change in dry matter digestibility (DMD), cell wall content (CWC) and non-structural carbohydrate content (TNC) of leaves of buffel grass and green panic were followed over a period of 2 years in grazed swards in a dry, subtropical environment in southern Queensland. The swards were buffel grass sown alone with (buffel plus-nitrogen) and without (buffel minus-nitrogen) nitrogen fertilizer, buffel grass/Siratro mixture and green paniclsiratro mixture.

New, fully expanded leaves developed at the rate of c. one every 9–10 days in summer, 11 days in spring and 14–20 days in autumn. The overall rate of senescence of these leaves was fastest during the summer growing season and slowest during the cooler, drier conditions of autumn. Leaves in the buffel plus-nitrogen and buffel/Siratro swards senesced more rapidly than those on the buffel minus-nitrogen treatment. Leaf senescence was particularly accelerated by frost in the dry season and by intermittent periods of water stress during otherwise good moisture conditions in the growing season.

The DMD of recently developed leaves was generally higher in spring than in summer and autumn, and CWC showed the reverse trend. The decline in DMD of leaves as they aged was considerably higher in spring and summer (c. 0.5 unit/day) than in autumn (0.1–0.3 unit/day); these changes were proportionately greater than expected from the increase in CWC. Frosts caused a precipitous decline in DMD. Leaves of buffel plus-nitrogen were 2–4 units higher in DMD than buffel minus-nitrogen leaves when young, but the former leaves declined in DMD more rapidly as they aged; the DMD of leaves from the buffel—Siratro swards was intermediate.

The level of TNC was highest, up to c. 12%, in spring (largely due to an increase in starch) and lowest in late summer-autumn (c. 8%). Diurnal variation in TNC was small (2 percentage units from 0600 to 1800 hours) and did not affect the estimation of DMD.