An Overview of the Response to Selection to Increase Fertility in Fall Lambing
D. R. Notter
Department of Animal and Poultry Sciences, Virginia Tech
Selection to improve fertility in a fall-lambing system was begun in 1988 using a population of
50% Dorset, 25% Rambouillet, and 25% Finnsheep breeding. A selection line of 125 ewes was
established. Ewes were mated to 10 rams per year, with approximately one third of the ewes and
one half to two thirds of the rams replaced in each year. From 1988 through 1993 (Phase 1),
culling of ewes was based on mean fertility in the fall-lambing system and selection of
replacements was based on the fertility of the dam. After 1993 (Phase 2), estimated breeding
values (EBV) were calculated for all animals in the project using BLUP procedures with an
animal model, and selection was based on EBV for fall fertility.
Two separate control populations were used to produce unselected animals and to compare them
to animals in the selection line. An environmental control (EC) flock of 55 ewes was maintained
contemporary to the selection line in a fall-lambing system. Ewes in the EC flock were culled at
random to maintain a ewe age distribution comparable to that of the selection line. Replacement
animals for the EC flock were derived from a separate genetic control (GC) flock of 45 ewes
maintained in spring lambing. Animals born in GC were transferred to EC to provide a
contemporary, unselected control population for evaluation of selection response. In Phase 1,
EC ewes were mated at random to five GC rams, whereas in Phase 2, EC ewes were mated to the
same 10 rams used in the selection line to allow direct comparison between selected and EC
ewes in the same breeding pastures. No replacement animals from EC ewes were retained.
Table 1. Mean Ewe Fertility (%) for Select (S)
and Environmental Control (EC) Ewes by
Age Class in the Period 1996-1998
Flock
Ewe age (yr)
S
EC
1
9
24
2
68
57
3
82
68
The formal selection experiment was terminated after the fall, 1998 lambing and the EC and GC
lines were terminated at that time. Means for ewes of each age group and line in the period 1996
through 1998 are shown in Table 1. The selection line has been maintained and selection
continued, albeit at somewhat relaxed levels. Figure 1 shows estimates of genetic change in
breeding values for fall fertility in select, EC, and GC ewes using data through the fall, 2003
lambing. The trends for EC and GC terminate in 1998 when these lines were discontinued.
Genetic trends are commonly based on the mean EBV of lambs born in each year, but for this
project, the lambs born in any year represent a selected group, since they only represent offspring
of ewes that were capable of lambing in fall. Therefore values in Figure 1 are based on the mean
EBV of the ewes mated in each year.
Through 1998, the average increase in EBV in the selection line was 1.42 ± 0.05 %/yr. After
termination of the formal selection project in 1998, mean EBV declined in 2001 and 2002, in
part because of the use of high EBV ewes in semi-intensive projects. However, after the return
of many of these ewes to the flock in 2003, mean EBV returned to its previous level.
A substantial environmental trend in fertility was also observed, as shown by the pattern of
change in year effects (adjusted for mean EBV) in Figure 2, although the variation among years
was substantial. In the first year of the study, fertility was very low, likely in response to moving
the animals from spring lambing to fall lambing. After excluding the observation for 1988, the
average change in fertility due to nongenetic factors was 1.32 ± 0.91 %/yr (P = 0.18). We
suspect that there is an element of adaptation involved in out-of-season breeding, as well as
possible social effects whereby the presence of a critical proportion of cycling ewes in the flock
may stimulate remaining anestrous ewes to cycle.
Ten years of combined genetic and environmental trends, with cumulative positive effects of
approximately 14 and 13%, respectively, can explain the increase in mean fertility of adult ewes
from about 60% for this composite line in 1989 to the current mean fertility of around 87%. Part
of the rationale for termination of the line was that further increases will be difficult given the
categorical expression of this trait. While the theoretical ceiling would be 100% fertility, the
reality of the situation is that the current fertility is approaching that which could reasonably be
expected for single-sire matings in the normal fall breeding season.
Figure 1. Changes in mean fall fertility EBV over time for ewes mated in the selected (S),
environmental control (E), and genetic control (G) lines.
S
E
G
Fall fertility EBV, %
16
12
8
4
0
-4
-8
1988
1990
1992
1994
1996
Year
1998
2000
2002
2004
Figure 2. Changes in environmental effects (year constants) for fall fertility.
Year constant, %
Year effect
5
0
-5
-10
-15
-20
-25
-30
-35
-40
1988
1990
1992
1994
1996
1998
2000
2002
2004
Year
Effect of Lamb Separation on Rebreeding of Lactating Ewes
D. R. Notter and H. B. Vanimisetti
Department of Animal and Poultry Sciences, Virginia Tech
Over 2 yr (2003 and 2004), 71 ewes (61from the Virginia Tech out-of-season lambing flock and
10 purebred Dorsets) were evaluated for ability to cycle in March while lactating. Treatment
schedules are shown in Table 2. Ewes lambed between December 30 and January 23; mean
lambing dates were January 6 in 2003 and January 11 in 2004. At the start of the experiment in
early March, two blood samples were collected 7 to 10 days apart from each ewe and assayed for
progesterone to identify ewes that had an active CL. Ovulation was assumed to have occurred if
circulating progesterone levels exceeded 1 ng/ml on one of the 2 d. Dorset ewes were
represented only in 2004.
Ewes were stratified by lambing date, age, and breed and assigned to a control group or a group
that would be separated from their lambs for 72 h at the time of ram introduction. In mid-March,
all ewes began a 5-d regimen of twice-daily subcutaneous injections of 25 mg of progesterone
dissolved in corn oil. Ewes induced to ovulate by ram introduction often ovulate but do not
express estrus, and can be induced to exhibit estrus by progesterone pretreatment. The last
progesterone injection was given intramuscularly for faster clearance and, therefore, a more
abrupt progesterone withdrawal. Ewes in the lamb-separation group were removed from their
lambs the next morning, and two fertile rams were placed with each treatment group on that
afternoon. Ewes and lambs from the two treatment groups, and all four breeding rams, were
Table 2. Treatment schedules for lamb separation experiment in 2003 and 2004
Activity
Year
2003
2004
Mean lambing date
January 6
January 11
First pretreatment measure of circulating progesterone
February 27
March 3
Second pretreatment measure of circulating progesterone
March 10
March 10
Begin progesterone injections
March 12
March 10
Lambs removed and rams introduced
March 17
March 15
First blood sample
March 18
March 16
Lambs returned
March 20
March 18
Rams removed
April 22
April 21
Ultrasonic pregnancy diagnosis
May 29
June 14
recombined 3 d after lamb separation. Jugular blood samples were collected thrice weekly
(Tuesday, Thursday, and Saturday) from all ewes for 5 wk beginning the day after lamb
separation. Rams were equipped with marking harnesses, and marks were recorded at each
bleeding date. Rams were removed in late April. Ewes were thus an average of 64 d postpartum
at the start of breeding and an average of 104 d postpartum at the end of breeding.
No obvious undesirable effects of lamb separation were observed. Lambs were consuming creep
feed ad libitum before separation from their mothers and remained in the same pens with control
ewes and lambs during the period of separation. Excessive engorgement of the udders was not
observed in ewes separated from their lambs for 3 d. Reproductive performance of ewes in the
two treatment groups is shown in Table 3. Results were consistent across years. No differences
in mating behavior were observed between ewes that were separated from their lambs and
control ewes. Over 70% of the ewes had elevated progesterone levels suggestive of ovulation
prior to the start of progesterone pretreatment at an average of 52 to 63 d postpartum.
An average of 85% of the ewes in each treatment group mated within 2 wk of ram introduction,
and an additional 6% mated over the next 3 wk. These results suggest that most ewes exposed to
rams at this time either mate promptly or do not mate at all. Ultrasonic pregnancy diagnosis in
early June resulted in detection of pregnancy in 85% of the ewes. Thus, many of the ewes in this
study did not appear to be anestrus prior to April 1, even when lactating. Actual lambing rates in
August, however, were somewhat less encouraging than results of June pregnancy tests.
Although results were complicated somewhat by a dog attach that resulted in the death of seven
ewes and are only available for 2003, the average lambing rate of approximately 65% was
considerably lower than the ultrasonically determined June pregnancy rate of 77%. These results
suggest that lamb losses in late pregnancy may have been substantial in these ewes.
Dorset ewes in the Virginia Tech flock are normally mated in April for September lambing and
generally have lower fertility rates than selected crossbred ewes mated in May. However, the
Dorset ewes evaluated in 2004 gave no indication of poorer fertility while lactating compared to
crossbred ewes. The good performance of these Dorset ewes raises several questions about
whether the somewhat mediocre fall-lambing performance of Dorset ewes in our flock is related
to lamb losses in late pregnancy, or could perhaps be improved by running Dorset ewes with
either crossbred teaser rams or cycling crossbred ewes.
Table 3. Reproductive performance of ewes exposed to rams while lactating and effects of lamb separation by years
Year 1
Item
Number of ewes
Percentage with circulating progesterone
above 1.0 ng/ml before treatment
Percentage mated in first 2 weeks
Percentage mated within 5 weeks
Percentage not diagnosed as open based on
circulating progesterone and mating records
Percentage diagnosed pregnant in early June
Percentage that lambedc
a
Year 2
Combined
Lamb
Separation
20
Control
19
Lamb
Separation
16
Control
16
Lamb
Separation
36
Control
35
Combined
71
70
80
80
79
84
89
75
88
100
69
88
94
73
84
90
74
86
92
74
85
91
80
80
60 (56)
74
74
74 (67)
100
93a
80
94
94b
88
90
87
69
84
84
80
87
85
74
One ewe in the lamb separation group was killed by dogs before pregnancy diagnosis.
One ewe diagnosed as “early pregnant” was categorized as “open” since she could have conceived to a later April mating.
c
In year one ewe died in July and an additional seven ewes were killed by dogs on August 7 before lambing. Tabular values utilize
pregnancy rates to assess if the ewes that died would have lambed. Values in parentheses are based only on the ewes remaining alive
at lambing.
b