Effect of immunization against androstenedione on the reproductive performance of ewe lambs : assessment of dosing regimes : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Animal Science, School of Agriclture and Environment, Palmerston North, New Zealand
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Date
2019
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Massey University
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Abstract
In New Zealand, there are a number of impediments to the adoption of ewe lamb breeding,
for example, their poor reproductive performance compared to mature ewes. This low
reproductive performance is primarily due to a lower ovulation rate. A series of studies
such as improving nutrition pre-breeding and live weight at breeding have been conducted
in New Zealand to improve the reproductive performance of ewe lambs, but they have
not resulted in significant increases in the ewe lamb ovulation rate. In mature ewes,
immunization against androstenedione can increase ovulation rate. Immunized mature
ewes release three or more ova in each estrous cycle. To date, little data has been
generated for the response to immunization against androstenedione of ewe lambs. One
of the most commonly used androstenedione vaccines in New Zealand is Androvax®. A
study conducted in Ireland using Fecundin® reported that control ewe lambs showed
estrous behaviors at an earlier date than immunized ewes. This thesis, therefore, examines
the effect of immunization against androstenedione using Androvax® on the reproductive
performance of ewe lambs at two different dosing regimens.
The present study was conducted between February 2017 and January 2018 using 300
Romney ewe lambs. Ewe lambs were immunized with Androvax® using two dosing
regimens, 1) ten and six (A10 – 6) weeks and, eight and four (A8 – 4) weeks prior to the
introduction of the entire ram. Teaser rams fitted with mating harnesses were introduced
to the ewe lamb flock at a ratio of 1:100 and remained with the flock until the start of
breeding. During this time, ewe lambs were checked for teaser harness crayon marks as
an indicator of the onset of behavioral oestrus. After the removal of the teaser rams, entire
rams fitted with mating harnesses were introduced at the ratio of 1: 50 and remained with
the flock for two oestrous cycles (34 days). Ewe lambs were also checked for ram harness
crayon marks after every 17 days from the introduction of the entire ram to identify the
cycle in which they were bred. Pregnancy diagnosis was carried out 90 days after the start
of the breeding period. Ewe lambs bearing triplet fetuses were removed from the study.
Lambing began in October and ended in November. Lambs were weaned in January at
the average of 102 days of age.
The majority of ewe lambs in A10 – 6 and those in a control treatment were marked by
the teaser ram prior to the introduction of the entire ram compared with those in A8 – 4
treatment (62 % vs 56 vs 44% respectively, p<0.05). Ewe lambs that show signs of behavioral oestrus prior to the introduction of the entire ram have higher ovulation rates
leading to a high fecundity rate, however, in this study, ewe lambs in A8 - 4 treatment
had greater (p<0.05) fecundity rates even though they showed later onset of behavioral
oestrus. Ewe lambs in A8 – 4 treatment had higher (p<0.05) fecundity rates at pregnancy
diagnosis than the control treatment while those in A10 – 6 treatment did not differ from
either group (1.66 vs 1.30 vs 1.49 respectively; p>0.05). Ewe lambs in A8 – 4 treatment
had a greater (p<0.05) percentage of twin and triplet fetuses and fewer singles compared
with the control and A10 – 6 treatment. At lambing, the number of lambs born did not
differ between the three treatments, however, treated groups (A10 – 6 and A8 – 4), had
more twins than the control treatment. Generally, immunization against androstenedione
increased the fecundity rates but not the itter size and number of lambs weaned and the
recommended dosing regimen is eight and four weeks before breeding (A8 – 4). It is,
therefore, important for farmers to work on minimizing pregnancy losses and lamb
mortality when they used androvax in ewe lambs in order to increase both litter size and
number of lambs weaned.
Description
Figure 2.1 (=Edwards & Juengel, 2017 Fig 3) was removed for copyright reasons.
Keywords
Lambs, Breeding, New Zealand, Ewes, Vaccination, Androstenedione