The number of lambs born per ewe joined to rams is a key driver of potential farm profitability. But is there an upper litter size limit that if exceeded becomes counterproductive?

This question was addressed by Teagasc geneticist Noirín McHugh at the ICBF and Sheep Ireland conference. Noirín showed that at present, the number of lambs born accounts for 18% of an animal’s replacement index, with the other traits being lambing (23%), growth (19%), ewe liveweight (16%), milk yield (12%), meat (11%) and health (1%).

In the current scenario, there is a linear economic value used whereby for every increase in economic breeding value for number of lambs born, it is worth €9.58. This is irrespective of whether the lambing percentage is increasing from a base of 1.3 lambs born per ewe joined or two lambs born.

Analysis

This means sheep with an excessive lambing percentage have the potential of being overvalued or excessively rewarded in genetic evaluations. An investigation was carried out with two components; firstly to establish if there is a genetic component to the number of lambs born and secondly to identify if an economic cut-off is needed.

The number of lambs born per ewe joined to rams is a key driver of potential farm profitability

An analysis was carried out on more than 45,000 ewe records in the database, the results of which are detailed in the table below. It showed there is a large genetic component contributing to the number of lambs born, which was recorded at a mean value of 1.67 including live and dead lambs.

The heritability of a trait was explained as the proportion of difference between animals due to genetics. Noirín said a heritability of 11% is pretty good, meaning significant use can be made of the trait. She said there is also a good correlation between the number of lambs born and pregnancy scan of 0.77 which means if pregnancy scan information is available, the number of lambs born can be predicted.

Optimum lamb number

To establish if an economic cut-off was needed, each 0.1 increase in number of lambs born was modelled using a Teagasc programme developed by Alan Bohan, who now works with Sheep Ireland. It looked at the effect each 0.1 increase in lambs had on mortality, the number of lambs reared per ewe (cross fostered), the number sold as pets, the labour requirement and ultimately number of lambs weaned. It found the optimum upper limit for number of lambs born is 2.18 lambs.

Impact on indexes

The outcome of the investigation means a new non-linear economic value will be used in genetic evaluations for the number of lambs born. Noirín says ewes or rams will not be penalised if their predicted performance for the number of lambs born is above 2.18 lambs, rather that the economic value will be capped at that level. The change in the index and incorporation of new data will also increase the accuracy of number of lambs born by about 6% while pregnancy scanning information will now become a key part of a flock’s data quality index (DQI).