Effects of water stress on in vitro dry matter digestibility and chemical composition of herbage of tropical pasture species

The effects of water stress of varying duration and intensity (Dry treatment) on the in vitro dry matter digestibility (DMD) of different plant fractions of three tropical grasses (green panic, buffel and spear grass) and a tropical legume (Siratro) grown in plots under semi-arid field conditions is reported. Other plots of these species were irrigated regularly for comparison (Wet treatment). Generally, the DMD of herbage from water-stressed grasses was either similar to or higher than that from plants of the Wet treatment. Where DMD of leaves was higher for the Dry treatment this was usually due to a slower decline in their DMD as they aged. Most comparisons of dead leaf tissue indicated a higher DMD for the Dry than the Wet treatment. The most recently expanded leaves usually did not differ in DMD between Wet and Dry treatment. Water stress slowed stem development in the grasses and, particularly in early spring when this effect was most evident, the DMD of stem was higher in the Dry than the Wet treatment. Water stress applied to buffel grass stems after they had elongated and started flowering did not affect their DMD compared with the Wet treatment. Cell wall and lignin content of herbage from the Dry treatment was similar to or lower than that from the Wet treatment. In contrast to the grasses, the stress-adapted small leaves of Siratro, which developed after some time under water stress conditions, were lower in DMD than the recently expanded Wet leaves; their lignin and cellulose contents were higher but hemicellulose markedly lower than those of Wet leaves.