|dc.description.abstract||The objective of this study was to evaluate ewe and lamb performance in an
accelerated lamb production system, and to compare the performance and lamb output
between a conventional and an accelerated lamb production system. In the
“Conventional” system, ewes were bred in March to lamb in August. The
“Accelerated” system was based on the “STAR” system (Lewis et al., 1996), in which
there were five breeding periods within each year. In the current experiment these were
14th January, 28th March, 9th June, 21st August and 2nd November. Progesterone was
used to synchronise the breeding periods and during the non-breeding season, eCG was
used to induce reproductive activity. Lambing began on each of these dates and
weaning was 73 days later, coinciding with the next breeding period. The experiment
ran over a three-year period beginning with breeding in March 2003 and was complete
with the weaning of lambs from the January 2006-bred ewes. This resulted in 15
lambing and breeding periods over the three years in the Accelerated system and three
lambing and breeding periods in the Conventional system.
Average pregnancy rates were lower in the Accelerated system than in the
Conventional system. Lamb growth rates were similar between the two systems,
although lamb live weights at weaning were lower in the Accelerated system due to the
age of the lambs at weaning (average = 69 vs 96 days). More lambs were born and
weaned, resulting in more kilograms of lamb weaned in the Accelerated system relative
to the Conventional system over the experimental period (26,200 vs 24,300 kg).
Labour input was 35% higher in the Accelerated system, or 13% higher per lamb
weaned. Average annual ewe energy requirements were 6% higher in the Accelerated
system. Ewe energy requirements per kilogram of lamb weaned was lower (6%) in the
Accelerated system due to more breeding and lambing periods per ewe per year.
Laparoscopic observation of ewes’ ovaries at each breeding period revealed that
most ewes had active ovaries and were therefore capable of successfully producing a
viable foetus. In a subsequent experiment, blood samples were collected for analysis of
progesterone concentrations from ewes bred during the spring and autumn breeding
periods. Observations of data indicated that a small number of ewes conceived and lost
their conceptus, or had abnormal corpora lutea. Results suggested that pregnancies
were failing due to a lack of an appropriate signal from the embryo to the dam/uterus.
Exposing Romney ewes to an artificial lighting regimen was unsuccessful for
inducing reproductive activity during spring. In another experiment, melatonin
implants administered to Romney ewes in spring and used in conjunction with eCG and
progesterone, resulted in 61% more lambs born per ewe treated, compared to eCG and
progesterone alone. This result indicated that melatonin implants, used with eCG and
progesterone may be a suitable method for improving reproductive performance in
sheep bred out of season in New Zealand.
Delaying weaning of lambs and breeding lactating ewes can be used to obtain
heavier lamb weaning weights in the Accelerated system. Spring-bred ewes had lambs
weaned at either 69 days post partum or 90 days post partum. Reproductive
performance was similar between the two groups of ewes, and lamb live weights in the
later weaned group were heavier when lambs were 90 and 120 days of age.
This research has shown that accelerated or out-of-season lamb production is an
option for some New Zealand sheep farmers. However, the mechanisms associated
with reproductive seasonality and methods of successfully circumventing this
seasonality require further attention in order to achieve optimum reproductive