Dairy Sheep Symposium - the Department of Animal Sciences ...

Ewes were randomly assigned to one of six pens (A to F), balanced with respect to milk
production, for use in a 6 χ 6 Latin square design. At the morning milking (0630) immediately
prior to treatment administration (0 h), 2 IU of oxytocin was injected intravenously to empty the
udder as completely as possible. One of six milking interval treatments (4, 8, 12, 16, 20, or 24 h)
was then randomly applied to each pen for the first milking of six 3-d periods (1 to 6). Thus, a
pen of ewes was milked at 1030, 1430, 1830, 2230, 0230, or 0630 for the six treatments, respectively.
Following treatment administration, the next machine milking occurred at normal milking
times (0630 or 1830) and continued twice daily at these times for the remainder of milkings in
each 3-d period. At the termination of one 3-d period, oxytocin was administered at the 0630
milking (0 h), and the next series of treatments were applied; the experiment continued until all
pens had received all the treatments (18 d).
Cisternal and alveolar milk fractions were determined only at the treatment-milking with the
aid of an oxytocin receptor antagonist (Atosiban, Ferring Research Institute Inc., San Diego,
CA). Atosiban (1.0 mg/ewe) was administered intravenously to each ewe within a pen immediately
prior to entering the parlor. As a result of oxytocin receptor antagonism, milk ejection
during machine milking could not occur, and only the cisternal milk fraction was obtained.
Cisternal milk volume was recorded and sampled for each ewe. Ewes were then injected intravenously
with 2 IU of oxytocin to re-establish milk ejection and allow the alveolar milk fraction to
be measured and sampled. Individual ewe cisternal and alveolar milk samples were analyzed for
percentages of fat and protein, and for Fossomatic somatic cell count (SCC) by a State of Wisconsin
certified laboratory. Somatic cell count was transformed to logarithms of base ten. Milk
fat and protein yield was calculated with the following formula, which includes the specific
gravity for sheep milk expressed in g/ml (1.036, Jandal, 1996):
Fat or protein yield = milk volume x 1.036 x(fat or protein percentage/100).
Total milk yield, and milk fat and protein yield at a treatment were calculated by adding
cisternal and alveolar milk together; total percentages of milk fat and protein at a treatment were
calculated by dividing total milk fat or protein yield by total milk yield; and total SCC at a
treatment was calculated by averaging the cisternal and alveolar SCC. Milk yield was also
measured 12 h prior to time 0, and at all subsequent twice-daily milkings following treatment
within each 3-d period. Total 3-d milk yield was calculated by adding together all milk yields
obtained during the 3-d period.
Analyses of variance were conducted with the general linear models procedure of SAS
(1999) for a 6 x 6 Latin square design. Alveolar and cisternal fraction traits analyzed were milk
volume, milk fat and protein percentage, milk fat and protein yield, and SCC for following
independent variables and their interactions: fraction (alveolar or cisternal), treatment (4, 8, 12,
16, 20, or 24 h), pen (A to E), ewe within pen, 3-d period (1 to 6), fraction x treatment, fraction x
pen, and fraction x 3-d period. For the analyses of total treatment milk yield (alveolar + cisternal
milk), 3-d milk yield, total milk fat and protein yield, total milk fat and protein percentage, and
total SCC, fraction and its interactions were removed from the model. A separate analysis was
performed for milk yield produced prior to and after treatment and included the following independent
variables and interactions: time (12-h milk yield prior to treatment or milk yield during
the first complete 12-h interval after treatment), treatment, pen, ewe within pen, 3-d period, time
x treatment, time x pen, and time x 3-d period.