Early reproductive losses are a major factor contributing to the poor reproductive performance of Merino ewe lambs mated at 8–10 months of age

The reproductive performance of Merino ewe lambs is lower than that achieved by mature ewes and is highly variable. It is likely that embryo loss represents a major source of reproductive wastage in Merino ewe lambs, but to our knowledge no studies have attempted to determine when the major reproductive losses occur or identify predisposing factors that are likely to lead to high rates of pregnancy failures in ewe lambs. After characterising where reproductive losses occurred in Merino ewe lambs mated at 8–10 months of age, we tested the hypothesis that pregnancy failure in ewe lambs is influenced by nutrition and liveweight change around conception and the genetic potential for growth of the ewe lamb. Two-hundred and twenty-four Merino ewe lambs born from 10 sires with Australian Sheep Breeding Values were teased for 14 days using vasectomised rams, and then fed two different diets for 68 days during mating. All ewe lambs were weighed and the appearance of crayon marks on their rump was recorded most days. Blood samples for progesterone assay were collected on Days 5, 12 and 17 after the first record of a crayon mark. Ultrasonography using a trans-rectal probe was used to measure the number of corpora lutea present, as a proxy for ovulation rate, 9 days after the first record of a crayon mark during the mating period. A further trans-rectal ultrasonography was undertaken 30 days after marking from the entire ram to determine pregnancy status and count the number of embryos. Over the entire mating period 54% of ewe lambs were pregnant with 66 fetuses per 100 ewes mated. The average ovulation rate was 150% however up to 84% of this potential was lost by weaning and the major contributor to this apparent deficit was the loss that occurred during the first 17 days after mating. Pregnancy failure was not significantly related to nutrition or liveweight change during mating however, there were significant differences in pregnancy failure between different sire groups. Pregnancy failure was significantly less for ewe lambs from sires with higher breeding values for weight and fat at post-weaning age (8–10 months). Only 60% of ewe lambs had achieved puberty when rams were introduced and only 83% by 35 days after mating. In addition, almost half of the ewe lambs that were mated for the first time during the first 35 days after rams were introduced, but failed to get pregnant, then seemed to skip a cycle or did not cycle again before the end of the 68-day mating period. Liveweight at introduction of entire rams was positively related to fertility, ovulation rate and reproductive rate. This study confirms that selection of sires with higher breeding values for post-weaning weight and fat will increase the fertility and reproductive rate of Merino ewe lambs mated at 8–10 months. This response is due in part to enhancing the onset of puberty and increasing the proportion of ewe lambs cycling at the start of mating and reducing pregnancy failure.