Prime lamb producers can afford to pay £180 more for a terminal sire with an index of 200 points than for one with an index of 100.

Figures from a large-scale, multi-agency project, which aims to develop a selection index for longwool breeds, indicate that each additional 25 index points produce an extra return of £45 during the average working life of a tup.

Dennis Chapple, senior livestock researcher at ADAS Rosemaund in Herefordshire, told a breeding conference in Wales that high-index rams used for the project sired lambs weighing an extra 1.1kg at sale.

Charollais, Suffolk and Texel rams with indexes of up to 180 were used on 900 Welsh Mules and 900 Scottish Mules over three seasons.

On average, terminal sires with indexes of more than 100 produced lambs worth £1.50 a head more than those by sires with indexes of less than 100.

Dissection of some 800 carcasses also showed that lambs out of high-index rams had more lean meat and less fat in every joint.

“Monitoring has demonstrated the production benefits of using high-index terminal sires and that producers can afford to pay more to get them,” Mr Chapple said.

Figures were collected as part of a nine-year DEFRA and MLC-funded initiative to develop a selection index for the longwool crossing rams used to produce Halfbred/Mule ewes that dominate lowland prime lamb production.

A total of 45 Bluefaced Leicester tups, 1800 Mule ewes out of 4500 hardy Speckled Face and Scottish Blackface ewes and 90 terminal sires have been used since the work started in 1997.

The project partners – the University of Wales, ADAS and SAC – plan to publish a new selection index in 2007, which they are confident will lead to improved carcass quality of Halfbred/Mule wether lambs and of terminal sire cross lambs bred from their sisters.

But the index will not compromise highly valued maternal traits, such as prolificacy, rearing ability, mothering ability, disease resistance and ewe longevity.

Mr Chapple said early results showed that using high-index Bluefaced Leicester rams to breed Mule ewes improved leanness of the male wether lambs produced and the terminal sire cross lambs out of the ewes themselves.

An increase of 100 points produced 0.4kg extra live weight, 2% more lean meat and 7% less fat. “The overall economic benefit of using Bluefaced Leicester rams in the top 5% on index score was £2.16 per hill ewe mated.

Generally, ewe lambs from high-index Bluefaced Leicester sires had better style and structural soundness, so selection for improved carcass traits should not adversely affect important type traits.

“To date there is no evidence that selection for improved carcass traits in crossing sires has a negative effect on maternal traits of Mule progeny,” said Mr Chapple.

“However, ewe longevity data is still being collected and final analyses will be undertaken in the autumn.”

Weight of lean in a carcass was the single most important factor affecting economic returns, but the new selection index would try to maximise economic returns by balancing carcass and maternal traits.