Forage intake and dry matter digestibility were determined in
animals maintained in individual metabolic cages. Each isolation unit
contained 4 to 6 cages. The base of each metabolic cage had an iron
mesh floor. Separation of urine and faeces occurred automatically. The
urine flowed to the bottom of the tray where it was collected in flasks.
The faeces of each animal and the spilt food were trapped by the fine
wire mesh of the tray, and were collected and a sample stored for
chemical analysis (A.O.A.C., 1985).
All animals were fed, ad libitum, a low quality chopped hay (i.e. 28%
fiber, 4% crude protein), thus simulating the natural quality of forage
available during periods of drought. To ensure that most food was
consumed the hay was chopped into short pieces. Food and water
containers were located outside and in front of each metabolic cage to
minimize spillage. They were cleaned daily with de-mineralized water.
The animals fed on the same batch of hay throughout the entire
experimental period. Each animal adapted to, and fed, the low quality
forage for a period of threeweeks before being housed in the metabolic
cages. Food was provided in trays at the front of each cage and the daily
consumption recorded. During preliminary trials and experimental
periods, food was offered at twice the previous day's level of ad libitum
intake. The moisture content of the forage (generally less than 12%) was
measured daily. Thus, the amount of preformed water consumed was
calculated, and an accurate measure of dry matter intake determined.
Each animal was then further acclimated to their designated environmental
control chamber and forage diet for one additional week. After
the acclimation period, measurements of food intake and dry matter
digestibility were made daily for a further 18–21 days. The same
procedures were followed for animals subjected to a temperature of
22 °C and to the alternating heat load (22 °C and 40 °C,12-hour cycle).An
8–10 day resting period was allowed for each animal that came out of
one experimental treatment before being used in the next treatment.
Each animalwasweighed daily inside his or her respectivemetabolic
cage using a Slater 500 kg balance. A hydraulic jack connected to the
balance lifted them. This system was periodically calibrated with
weighed bags of sand and was found to have an accuracy of ±1.0 kg.
Animals were weighed at the same time daily (usually at 0900 h)
throughout the experimental treatments.
For the water deprivation (dehydration) phase of the experiments,
the animals were provided with graded restricted amounts of water
until they had lost 15% of their initial body mass. Itwas assumed that this
level of dehydration would cause maximal stimulation of physiological
mechanismforwater conservation.Waterwas then given at a level that