The n-alkane and chromium/in vitro procedures for
estimating herbage intake were compared in grazing ewes during late pregnancy,
early lactation, and mid-lactation. To ensure differences in herbage intake,
the ewes were grazed in 4 plots of phalaris-dominant pasture at 2 levels of
stocking: 17.1 ewes/ha and 30.8 ewes/ha. To investigate whether either
procedure for estimating herbage intake was influenced by supplement
consumption, half of the ewes at each stocking level received 500 g/day
air-dry of a pelletted supplement (1 : 1 milled oat grain : sunflower meal).
Supplement intakes were estimated using tritiated gypsum as a marker. During
intake measurement periods, ewes were dosed twice daily with both alkane
capsules and capsules containing chromium sesquioxide. For the last 6 days of
the 12-day dosing period, rectal faecal samples were taken twice daily,
immediately before the dosing. Over these same periods, wether sheep fitted
with faecal collection harnesses were similarly dosed and sampled, and their
total faecal output collected to establish the faecal recovery of chromium and
the alkanes.
Herbage intakes were estimated using the C27/C28, C29/C28,
C31/C32, and C33/C32 alkane pairs. Estimates of intake based on the
shorter alkane pairs were lower than those estimated with the C33/C32
alkane pair, by amounts which differed between the periods. Evidence is
presented that estimates based on the last pair of alkanes (C33/C32) are
the most accurate and are also more accurate than those based on the
chromium/in vitro procedure. The relationship
between these 2 methods for estimating intake was different in mid-pregnancy
compared with either stage of lactation. The consumption of supplement did not
interfere with any of the methods for estimating herbage intake.
Estimates of faecal output based on the use of chromium, C28 alkane, or C32 as
an external marker were statistically identical, indicating that the
difference between the 2 methods for estimating herbage intake was not related
to a failure to accommodate the incomplete recovery of any of the markers used
or to the failure of rectal grab samples to be representative of total faeces.
Our results indicate that herbage collected by oesophageally fistulated (OF)
sheep was representative of that grazed by the ewes and could thus be used to
provide the herbage alkane data needed to estimate herbage intake by the
alkane method. However, the in vitro digestibility
values obtained from the OF samples did not represent the digestibilities
actually occurring in vivo. This was the main cause of
the observed difference between the 2 methods for estimating intake. Possible
reasons for the differences between the in vitro and
in vivo estimates of digestibility are discussed.
Progesterone Control of the Initiation of Lactose Synthesis in the Rat