• Isabel Vialoux

Breeding season growth can increase reproductive rate in Merino ewe lambs

Updated: Apr 26

I chose this weeks paper as we haven't covered much on Merino sheep and I also couldn't find a similar study in cross-bred sheep. So this week, let's take a look at the growth of the Merino ewe lamb across the breeding season and the effect this has on reproductive performance.


The reproductive performance of Merino ewe lambs is highly variable and generally poor when compared to their older counterparts. This is also a common issue in cross-bred sheep as outlined in a previous summary. It is well known that increasing nutrition and live weight during the breeding season can increase ovulation rate in adult ewes, however there are still large variations in reproductive rate in ewe lambs of a similar live weight. Therefore, this study also looks at the sire back fat depth to determine if that has an effect on reproductive performance.


Can growth to the end of the breeding season and sire breeding values influence the reproductive rate of merino hoggets?


Key Points

  1. Gaining an extra 100g/day of live weight during the breeding season increased reproductive rate by ~20%.

  2. Ewes from sires with a high BV for fat had greater reproductive rates.

  3. Using a combination of higher feeding levels and sires with high BV for fat showed the best reproductive rate in Merino ewe lambs overall.


This study looked at 500 ewe lambs with full pedigree records.

The Merino ewe lambs were managed under commercial conditions from weaning and weighed 43.5 kg at the start of the breeding period with an average age of 7.5 months.


Across the breeding period, the ewe lambs were offered a moderate or high feed allowance to achieve target growth rates of 100 or 200 g/day during a 46-day mating period.


They were then put back together for pregnancy scanning 60 days after the breeding period.



The reproductive rate was measured as the number of foetuses per 100 ewes mated. Ideally over 100% is desired in Merino ewes as that is equivalent to each ewe lambing at-least one lamb.


The graph below shows the reproductive rate on the y-axis (left side) and the live weight at mating (kg) on the x-axis (across the bottom). The different lines represent the different growth rates achieved. The reproductive rate is shown as a result of the combination of live weight and change in live weight across the breeding season.


Ewe lambs that had greater live weight at breeding had greater reproductive rate. Lambs with greater live weight have been associated with reaching puberty at a younger age, therefore, this result is not surprising.


However, if the lower live weight ewe lambs made up for this with high growth rates (250g/day) across the breeding season then they had comparable reproductive rate to those ewe lambs with greater live weight but lower growth rates (0-100 g/day).


So regardless of their live weight at the start of the mating period, gaining an extra 100 g/day of live weight during the mating period further increased their reproductive rate by about 20%.


Graph 1. The effect of live weight at the start of the breeding period on reproductive rate (foetuses per 100 ewes mated) for Merino ewe lambs that gained live weight at different rates during the breeding period [0 (bottom), 50, 100, 150, 200 and 200 g/day (top)].

At the individual animal level, a 5 kg greater live weight at the start of the breeding period increased the reproductive rate by about 20%.


The lines on the above graph for each growth rate follow a very similar pattern to one another. This potentially shows that there is a similar relationship between reproductive rate and growth rate, however, you would expect the greater live weight and growth rate relationship to begin to plateau at some point. Perhaps this is not the case as reproductive rate in Merinos is already low, so can continue to increase above 150%.



The next graph show the reproductive rate on the y-axis and the Australian Sheep Breeding Value (ASBV) for fat at post-weaning on the x-axis. The two feeding treatments are shown on the graph, moderate in purple and high feed in blue.


Ewe lambs from sires with higher Australian Sheep Breeding Values for fat measured post-weaning achieved a higher reproductive rate regardless of feed allowance treatment.


Graph 2. The effect of Australian Sheep Breeding Values (ASBV) for fat depth measured post-weaning (PFAT) of the sire on the reproductive rate (foetuses per 100 ewes mated) of Merino ewe lambs offered high (blue line) or moderate (purple line) feed allowances during the breeding period.

The effects of managing growth pattern and sire genetics were additive, so improving the reproductive performance of Merino ewe lambs bred as a hogget requires improving their feed allowance and rate of live weight gain until the end of the breeding period and using sires with higher breeding values for fatness.


The effects of sire fatness was significant even when the sire breeding values for live weight measured post-weaning were included in the model. This means that using rams with greater breeding values for fat depth can help with their offsprings reproductive performance.


Are you enjoying this summary? Have a read of one of these while you're here!


Live weight at mating and growth across the breeding season has an influence on reproductive rate in Merino ewe lambs. Having Merino ewe lambs at a greater live weight at mating and increasing in live weight resulted in the greatest reproductive rate. The results of this study show that growth rate is just as important as the starting live weight when breeding ewe lambs. Feeding ewe lambs well, allowing them to achieve growth rates above 200 g/day can result in positive reproductive rates.

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Full Paper:

Thompson A, Bairstow C, Ferguson M, Kearney G, Macleay C, Thompson H, & Paganoni B. (2019). Growth pattern to the end of the mating period influences the reproductive performance of Merino ewe lambs mated at 7 to 8 months of age. Small Ruminant Research, 179, 1-6.