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

dividing the amount of milk obtained by the machine milking time (not including machine
stripping milk yield or time). Milk composition analyses for percentages of fat and protein were
performed by a State of Wisconsin certified laboratory. Test-day milk fat and protein yield were
calculated by multiplying morning milk production by percentage of milk fat or protein. Individual
jugular blood samples for P4 and/or E2 immunoassay were collected at 0830 (d –2, 0, 2,
4, 6, 10, 14, 18, 21, 25) into vacutainer tubes, refrigerated at 4° C for 24 h, and then centrifuged
for 15 min at 3000 x g. The serum was harvested and frozen at –20° C. Serum P4 concentration
was determined by enzyme immunoassay in duplicate after double extraction in petroleum ether
according to Rasmussen et al. (1996). Intra- and inter-assay coefficients of variation were 9.9
and 11.6%, respectively. Serum samples for E2 concentration were pooled for each ewe within a
period (estrus: d –2; pre-treatment: d 0 to 5, treatment: d 6 to 18; and post-treatment: d 19 to 25).
Serum E2 concentration was determined by radioimmunoassay according to Kulick et al. (1999)
with an intra-assay coefficient of variation of 16.0%.
Analyses of variance were conducted with the general linear models procedure of SAS
(1999) for a split plot on time experimental design. Traits analyzed were milk yield, milk flow
rate, milk fat and protein content, and serum P4 and E2 concentration. The following independent
variables and their interactions were included in the model for each trait, except E2 concentration:
main plot effects- treatment (CLY or CLN), pen (A, B, C, or D), treatment x pen, and
ewe within treatment x pen; sub-plot effects- period (pre-treatment, treatment, and post-treatment),
treatment x period, pen x period, day (0 to 25) within period, treatment x day within
period, and pen x day within period. Because of unexpected pre-treatment differences noted a
posteriori among treatment groups in percentages of milk fat and protein, the d-4 test-day value
was used as a continuous covariable for analyses of milk composition during the treatment and
post-treatment periods. Differences among treatment x period combinations and treatment x day
within period combinations were tested for significance against residual error. Because serum
E2 concentration was obtained from samples pooled within a ewe and period, day within period
and all interactions with day within period were dropped from the model used to analyze E2
concentration.
Results
Ovulation and the presence or absence of CL for CLY and CLN ewes was affirmed by
laparoscopy on d 4 (3.4 ± 1.6 vs. 3.3 ± 1.3 CL per ewe, mean ± SD, respectively), and on d 11
during the treatment period (2.4 ± 1.1 vs. 0 CL per ewe, mean ± SD, respectively).
Data are summarized in Table 1 for the treatment x experimental period combinations; the
evolution of treatment differences over time is displayed in Figures 2 to 5. Following treatment
on d 5 with PGF 2α (CLN only) and CIDRs (both groups), average daily milk yield of CLY ewes
increased (P < 0.05) from their pre-treatment production level (from 1.42 to 1.56 kg/d) and was
greater (P < 0.05) than the milk yield of CLN ewes (1.44 kg/d) during the treatment period
(Table 1). Higher milk yield in CLY ewes first reached significance on d 9 (4 d after treatment,
Figure 2A) and averaged 10% greater than CLN milk yield throughout the treatment period (d 6
to18, Table 1). Peak milk yield occurred on d 10 (1.71 kg/d) for CLY ewes, but as early as d 6
(1.57 kg/d, 1 d after PGF 2α treatment) for CLN ewes (Figure 2A). After CIDR removal and
PGF 2α treatment on d 18 (post-treatment period), milk yield for CLY ewes returned to levels
lower than pre-treatment period production (1.35 kg/d), but averaged 8% greater (P < 0.05) than
for CLN ewes (1.25 kg/d, Table 1). Average daily milk flow rate was 17 and 25% greater (P