The majority of ewes in New Zealand are bred for the first time as 2-tooths when they are approximately 18-20 months old. Mating ewe lambs so that they produce a lamb at one year of age (yearlings) provides a clear opportunity to improve farm profitability through increasing the lifetime production of each ewe. If ewes lamb, on average, 4 times during their lifetime, producing a litter in their first year of life (so that they lamb 5 times), has the potential to increase their lifetime production by 25% thereby improving efficiency.
On average, a ewe lamb put to the ram will produce only 0.6 lambs to weaning, compared to an average of 1.2 lambs in older ewes. If the preferential feeding required to successfully lamb yearlings is also considered, the potential gain can be quickly eroded. Understanding the cause of this low efficiency and designing methods to improve or mitigate these effects would likely increase adoption of lambing yearlings.
We examined two cohorts of ewes selected over 2 years and mated in order to lamb at 1 and 2 years of age. Onset of puberty, ovulation rate, mid-pregnancy litter size, number of lambs born and number of lambs weaned were measured. Lower ovulation rate (1.54±0.07 vs 2.00±0.09), failure to be mated by the fertile ram and lower embryo survival (66% vs 78%) contributed to the poor reproductive efficiency of the younger animals. In a second study, the mid-pregnancy litter size of ewes mated in order to lamb at 1 year of age was shown to be greater in those ewes with a higher ovulation rate suggesting that manipulation of ovulation rate could be used to improve the efficiency of lambing yearlings.