Accelerated Lambing and Out-of-Season Breeding With Hair Sheep
Virginia State University
Sheep have the theoretical potential to produce at a "biological
ceiling" of 3-4 lambs at 6 months intervals. A major constraint
towards achieving such an idealized level production is the seasonal
nature of breeding of sheep in temperate environments. Increasing
the frequency of lambing and shifting towards year-round production,
however, can be a means of utilizing facilities and labor more effectively,
and taking advantage of favorable markets.
Hair sheep originate in the tropical regions of the world
with limited changes in day length throughout the year. Under these
conditions they breed year-round, or have breeding seasons influenced
by rainfall and forage availability, rather than photoperiod. If
these tropical breeds are transferred to temperate climatic conditions
with more distinct seasonal variation in day length, they begin
to show periods of seasonal anestrus. In St. Croix ewes in the Virgin
Islands (17ºN) 100% of ewes displayed estrus and ovulated throughout
the year, whereas St. Croix ewes in a temperate environment in Utah
(41ºN), showed a distinct anestrous period in June and July,
with transitional seasons in April and May and again August (Figure
1). Similarly, ovulation rates at the tropical location remained
constant at 2.0 eggs/ovulation throughout the year, but declined
between April and June at the temperate location.
of seasonal breeding in St. Croix sheep under temperate conditions
was also provided in Arkansas (35ºN) where groups of ewes exposed
to breeding in two months intervals showed a distinct seasonal pattern
with the highest lambing percentage in October and December (>90%),
and a reduced percentage (<20%) in April and June (Brown and
Jackson, 1995). Similar observations were made in Ohio (40ºN)
where both St. Croix and Barbados Blackbelly ewes mated in July
through September had a higher lambing rate (95 and 80%, respectively)
and litter size (1.97 and 1.65, respectively) than those in February
(25% and 20%; and 1.67 and 1.20, respectively) (Parker et al., 1991).
With the information from these earlier studies in mind the use
of hair sheep in an accelerated mating project was evaluated at
Virginia State University (37°N). Three of the hair sheep breeds
currently available in the U.S. were used in the project (Barbados
Blackbelly, St. Croix, and Katahdin). Flocks of Katahdin and St.
Croix were established from a diverse cross-section of each breed,
representing ewes and rams from at least five breeders per breed,
and were maintained as purebred populations with several sire lines.
Polled Barbados Blackbelly in the U.S. are rare and mostly trace
their origin to a single flock located in North Carolina. Barbados
Blackbelly ewes used in this project originated from this polled
flock, and were mated to rams imported from a flock maintained at
the University of the Virgin Islands to establish the current research
flock. Animals used in the project should be considered representative
of breed populations in the southern U.S.
In a preliminary experiment, a Katahdin flock of 25 ewes was used
and exposed to two or more rams in single-sire mating groups in
42 day breeding seasons starting the first of November, July and
March. Mating periods were selected to avoid breeding ewes during
periods of likely seasonal anestrus (April through June). Rams used
for breeding had passed a breeding soundness examination 2-3 weeks
prior to mating that involved both an evaluation of semen quality
and libido. Ewes were managed extensively on pasture, and were maintained
as a single flock, except during breeding. During winter, forage
was supplied as grass hay on pasture. Ewes were supplemented with
concentrate feeds during the late stages pregnancy and lactation.
Lambs were born on pasture and assistance was provided only to ewes
in severe distress (i.e. abnormal presentation of lamb). Lambs were
handled merely to record birth weights and apply identification.
Lambs had access to supplement provided to ewes, but did not receive
creep feed, and were weaned at 63 days of age.
In this initial trial pregnancy rates were similar in the three
mating periods (87-90%). Following the November and March mating
periods lambing rate corresponded to pregnancy rate, but lambing
rate was reduced to 65% following the July mating. Also, during
July mating the average time to first mating was later (25 days)
than in November (9 days) and March (12 days). Findings from the
preliminary trial indicated that Katahdin hair sheep appear suited
for use in an accelerated mating scheme, but also suggest that summer
heat stress may have impaired embryo survival in ewes mated in July,
and that seasonal breeding patterns may have delayed mating in July.
A more comprehensive project evaluated the performance of all three
hair sheep breeds under a similar accelerated mating scheme over
two, 2-year production cycles. In this project 25-30 ewes per breed
were mated in 30-day mating periods in November, July, and March,
using two single sire mating groups per breed per mating period.
All other management was similar to that described for the preliminary
Pregnancy rates (30-50 days post mating) and lambing rates were
higher for all breeds following November mating (>90%) during
the peak of seasonal breeding, but exceeded 80% in the other two
mating periods (Figure 2). Pre-natal losses were generally
limited (0-2%) except for Katahdin in the first July mating period
(12%). In line with the higher pregnancy rate, weaning rate was
higher following November (89%) than July (70%) and March (74%)
mating. There were differences between breeds in average annual
lambing rate which was higher in Barbados Blackbelly (93%) than
St. Croix (77%), with Katahdin intermediate (85%).
Litter size at birth (no. lambs/ewe lambing) was also higher following
November (1.91), than July (1.64) and March (1.52) mating. Pre-weaning
losses reduced average litter size by 0.2-0.3 lambs, but were not
affected by mating period. Breeds differed in average litter size,
which was higher in St. Croix (1.85) than in Barbados (1.56) and
Katahdin (1.66). The weight of lamb weaned as percentage of ewe
body weight (production efficiency) was higher following November
(51.3%) than July (41.5%) and March (43.0%) mating, but was similar
for the three breeds.
Overall lambing and weaning rate was similar in the first and second
two-year production cycle (85% and 77%, respectively), while litter
size increased from 1.61 to 1.76 lambs at birth, and 1.32 to 1.54
lambs at weaning. Production efficiency also increased from 43.1
to 47.0% from the first to the second cycle, suggesting no negative
long-term effects of accelerated mating on ewe productivity.
1. Estimated Performance of Ewes in a 8-month Accelerated Lambing
Lambs born per ewe lambing
Lambs weaned per ewe lambing
60-day adjusted weaning weight, lbs.
Percent litter weight weaned
The accelerated mating system implemented here produced an annual
lamb crop of 2.36-2.77 lambs/ewe lambing depending on breed, with
a production efficiency of 62-63% (Table 1). This represents
an increase of 28-38% in annual lamb crop, and 20-22% increase in
production efficiency compared to once annual November mating. Costs
of additional feed resources for lactating ewes and additional labor
during lambing have to be evaluated to estimate the feasibility
of implementing an accelerated mating under specific production
Hair sheep breeds appear suited for use in accelerated mating systems
due to an extended breeding season and their ability to be productive
with limited inputs. Reduced pregnancy rates during the transitional
mating periods (March and July) were mostly the result of yearling
replacements ewes failing to conceive, but litter size at birth
was reduced regardless of ewe age. Care needs to be expanded in
the selection of fertile rams with adequate libido for use in the
transitional mating periods.
Brown, M. A. and W. G. Jackson, 1995. Ewe productivity and subsequent
preweaning lamb performance in St. Croix sheep bred at different
times during the year. J. Anim Sci. 73: 1258-1263.
Parker, C.F., K.E. McClure, and R.P. Herd,
1991. Hair sheep potential for specific environmental conditions
and production systems in North America. In: Proceedings Hair Sheep
Research Symposium, S. Wildeus (ed.), St. Croix, V.I., pp. 321-327.
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Accelerated Lambing and Out-of-Season Breeding With Hair Sheep